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    2021

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  1. G. Wang, Y.-X. Liu, Y. Zhu and P. Cappellaro
  2. "Nanoscale Vector AC Magnetometry with a Single Nitrogen-Vacancy Center in Diamond"
    Nano Lett. 21, 5143-5150 (2021)
    Abstract: Detection of AC magnetic fields at the nanoscale is critical in applications ranging from fundamental physics to materials science. Isolated quantum spin defects, such as the nitrogen-vacancy center in diamond, can achieve the desired spatial resolution with high sensitivity. Still, vector AC magnetometry currently relies on using different orientations of an ensemble of sensors, with degraded spatial resolution, and a protocol based on a single NV is lacking. Here we propose and experimentally demonstrate a protocol that exploits a single NV to reconstruct the vectorial components of an AC magnetic field by tuning a continuous driving to distinct resonance conditions. We map the spatial distribution of an AC field generated by a copper wire on the surface of the diamond. The proposed protocol combines high sensitivity, broad dynamic range, and sensitivity to both coherent and stochastic signals, with broad applications in condensed matter physics, such as probing spin fluctuations.
    BibTeX
    @article{Wang21,
      author = {Wang, Guoqing and Liu, Yi-Xiang and Zhu, Yuan and Cappellaro, Paola},
      title = {Nanoscale Vector AC Magnetometry with a Single Nitrogen-Vacancy Center in Diamond},
      journal = {Nano Lett.},
      publisher = {American Chemical Society},
      year = {2021},
      volume = {21},
      number = {12},
      pages = {5143--5150},
      doi = {10.1021/acs.nanolett.1c01165}
    }
    
  3. S. Hernández-Gómez, S. Gherardini, N. Staudenmaier, F. Poggiali, M. Campisi, A. Trombettoni, F. S. Cataliotti, P. Cappellaro and N. Fabbri
  4. "Non-thermal energy fluctuations of a diamond spin qutrit with feedback-controlled dissipative dynamics"
    arXiv:2105.14011 (2021)
    Abstract: Engineered dynamical maps that combine not only coherent, but also unital and dissipative transformations of quantum states, have demonstrated a number of technological applications, and promise to be a beneficial tool also in quantum thermodynamic processes. Here, we exploit control of a spin qutrit to investigate energy exchange fluctuations of an open quantum system. The qutrit engineer dynamics can be understood as an autonomous feedback process, where random measurement events condition the subsequent dissipative evolution. To analyze this dynamical process, we introduce a generalization of the Sagawa-Ueda-Tasaki relation for dissipative dynamics and verify it experimentally. Not only we characterize the efficacy of the autonomous feedback protocol, but also find that the characteristic function of energy variations G(eta) becomes insensitive to the process details at a single specific value of its argument. This allows us to demonstrate that a fluctuation theorem of the Jarzynski type holds for this general dissipative feedback dynamics, while previous relations were limited to unital dynamics. Moreover, in addition to the feedback efficacy, we find a witness of unitality associated with the fixed point of the dynamics.
    BibTeX:
    @article{Hernandez21x,
      author = {S. Hernández-Gómez and S. Gherardini and N. Staudenmaier and F. Poggiali and M. Campisi and A. Trombettoni and F. S. Cataliotti and P. Cappellaro and N. Fabbri},
      title = {Non-thermal energy fluctuations of a diamond spin qutrit with feedback-controlled dissipative dynamics},
      journal = {arXiv:2105.14011},
      year = {2021}
    }
    
  5. G. Wang, C. Li and P. Cappellaro
  6. "Observation of symmetry-protected selection rules in periodically driven quantum systems"
    arXiv:2105.12209 (2021)
    Abstract: Periodically driven quantum systems, known as Floquet systems, have been a focus of non-equilibrium physics in recent years, thanks to their rich dynamics. Not only time-periodic systems exhibit symmetries similar to those in spatially periodic systems, but they also display novel behavior due to symmetry breaking. Characterizing such dynamical symmetries is crucial, but the task is often challenging, due to limited driving strength and the lack of an experimentally accessible characterization protocol. Here, we show how to characterize dynamical symmetries including parity, rotation, and particle-hole symmetry by observing the symmetry-induced selection rules between Floquet states. Specifically, we exploit modulated quantum driving to reach the strong light-matter coupling regime and we introduce a protocol to experimentally extract the transition elements between Floquet states from the coherent evolution of the system. Using the nitrogen-vacancy center in diamond as an experimental testbed, we apply our methods to observe symmetry-protected dark states and dark bands, and the coherent destruction of tunneling effect. Our work shows how to exploit the quantum control toolkit to study dynamical symmetries that can arise in topological phases of strongly-driven Floquet systems.
    BibTeX:
    @article{Wang21x,
      author = {Guoqing Wang and Changhao Li and Paola Cappellaro},
      title = {Observation of symmetry-protected selection rules in periodically driven quantum systems},
      journal = {arXiv:2105.12209},
      year = {2021}
    }
    
  7. P. Peng, X. Huang, C. Yin, L. Joseph, C. Ramanathan and P. Cappellaro
  8. "Deep reinforcement learning for quantum Hamiltonian engineering"
    arXiv:2102.13161 (2021)
    Abstract: Engineering desired Hamiltonian in quantum many-body systems is essential for applications such as quantum simulation, computation and sensing. Conventional quantum Hamiltonian engineering sequences are designed using human intuition based on perturbation theory, which may not describe the optimal solution and is unable to accommodate complex experimental imperfections. Here we numerically search for Hamiltonian engineering sequences using deep reinforcement learning (DRL) techniques and experimentally demonstrate that they outperform celebrated sequences on a solid-state nuclear magnetic resonance quantum simulator. As an example, we aim at decoupling strongly-interacting spin-1/2 systems. We train DRL agents in the presence of different experimental imperfections and verify robustness of the output sequences both in simulations and experiments. Surprisingly, many of the learned sequences exhibit a common pattern that had not been discovered before, to our knowledge, but has an meaningful analytical description. We can thus restrict the searching space based on this control pattern, allowing to search for longer sequences, ultimately leading to sequences that are robust against dominant imperfections in our experiments. Our results not only demonstrate a general method for quantum Hamiltonian engineering, but also highlight the importance of combining black-box artificial intelligence with understanding of physical system in order to realize experimentally feasible applications.
    BibTeX:
    @article{Peng21x,
      author = {Pai Peng and Xiaoyang Huang and Chao Yin and Linta Joseph and Chandrasekhar Ramanathan and Paola Cappellaro},
      title = {Deep reinforcement learning for quantum Hamiltonian engineering},
      journal = {arXiv:2102.13161},
      year = {2021}
    }
    
  9. M. Chen, C. Li, G. Palumbo, Y.-Q. Zhu, N. Goldman and P. Cappellaro
  10. "Experimental characterization of the 4D tensor monopole and topological nodal rings"
    arXiv:2008.00596 (2020)
    Abstract: Quantum mechanics predicts the existence of the Dirac and the Yang monopoles. Although their direct experimental observation in high-energy physics is still lacking, these monopoles, together with their associated vector gauge fields, have been demonstrated in synthetic matter. On the other hand, monopoles in even-dimensional spaces have proven more elusive. A potential unifying framework--string theory--that encompasses quantum mechanics promotes the vector gauge fields to tensor gauge fields, and predicts the existence of more exotic tensor monopole in 4D space. Here we report the first experimental observation of a tensor monopole in a 4D parameter space synthesized by the spin degrees of freedom of a single solid-state defect in diamond. Using two complementary methods, we reveal the existence of the tensor monopole through measurements of its quantized topological invariant. By introducing a fictitious external field that breaks chiral symmetry, we further observe a novel phase transition to a topological nodal ring semimetal phase that is protected by mirror symmetries.
    BibTeX:
    @article{Chen20x,
      author = {Mo Chen and Changhao Li and Giandomenico Palumbo and Yan-Qing Zhu and Nathan Goldman and Paola Cappellaro},
      title = {Experimental characterization of the 4D tensor monopole and topological nodal rings},
      journal = {arXiv:2008.00596},
      year = {2020}
    }
    
  11. I. Rojkov, D. Layden, P. Cappellaro, J. Home and F. Reiter
  12. "Bias in error-corrected quantum sensing"
    arxiv:2101.05817 (2021)
    Abstract: The sensitivity afforded by quantum sensors is limited by decoherence. Quantum error correction (QEC) can enhance sensitivity by suppressing decoherence, but it has a side-effect: it biases a sensor's output in realistic settings. If unaccounted for, this bias can systematically reduce a sensor's performance in experiment, and also give misleading values for the minimum detectable signal in theory. We analyze this effect in the experimentally-motivated setting of continuous-time QEC, showing both how one can remedy it, and how incorrect results can arise when one does not.
    BibTeX:
    @article{Rojkov21x,
      author = {Ivan Rojkov and David Layden and Paola Cappellaro and Jonathan Home and Florentin Reiter},
      title = {Bias in error-corrected quantum sensing},
      journal = {arxiv:2101.05817},
      year = {2021}
    }
    
  13. P. Peng, C. Yin, X. Huang, C. Ramanathan and P. Cappellaro
  14. "Floquet prethermalization in dipolar spin chains"
    Nat. Phys. , s41567 (2021)
    Abstract: Periodically driven Floquet quantum systems could provide a promising platform to investigate novel physics out of equilibrium1, but the drive generically heats the system to a featureless infinite-temperature state2-4. Fortunately, for high driving frequency, the heat absorption rate has been theoretically predicted to be exponentially small, giving rise to a long-lived prethermal regime that exhibits all the intriguing properties of Floquet systems5-8. Here we experimentally observe Floquet prethermalization using NMR techniques and probe the heating rate. We first show the relaxation of a far-from-equilibrium initial state to a long-lived prethermal state, well described by a time-independent ‘prethermal’ Hamiltonian. By measuring the autocorrelation of this prethermal Hamiltonian we can further experimentally confirm the predicted exponentially slow heating rate. More strikingly, we find that, on the timescale at which the prethermal Hamiltonian picture breaks down, the Floquet system still possesses other quasiconservation laws. Our results demonstrate that it is possible to realize robust Floquet engineering, thus enabling the experimental observation of non-trivial Floquet phases of matter.
    BibTeX:
    @article{Peng21,
      author = {Peng, Pai and Yin, Chao and Huang, Xiaoyang and Ramanathan, Chandrasekhar and Cappellaro, Paola},
      title = {Floquet prethermalization in dipolar spin chains},
      journal = {Nat. Phys.},
      year = {2021},
      pages = {s41567},
      doi = {10.1038/s41567-020-01120-z}
    }
    
  15. C. Li, T. Li, Y.-X. Liu and P. Cappellaro
  16. "Effective routing design for remote entanglement generation on quantum networks"
    npj Q. Inf. 7, 10 (2021)
    Abstract: Quantum network is a promising platform for many ground-breaking applications that lie beyond the capability of its classical counterparts. Efficient entanglement generation on quantum networks with relatively limited resources such as quantum memories is essential to fully realize the network’s capabilities, the solution to which calls for delicate network design and is currently at the primitive stage. In this study we propose an effective routing scheme to enable automatic responses for multiple requests of entanglement generation between source-terminal stations on a quantum lattice network with finite edge capacities. Multiple connection paths are exploited for each connection request while entanglement fidelity is ensured for each path by performing entanglement purification. The routing scheme is highly modularized with a flexible nature, embedding quantum operations within the algorithmic workflow, whose performance is evaluated from multiple perspectives. In particular, three algorithms are proposed and compared for the scheduling of capacity allocation on the edges of quantum network. Embodying the ideas of proportional share and progressive filling that have been well-studied in classical routing problems, we design another scheduling algorithm, the propagatory update method, which in certain aspects overrides the two algorithms based on classical heuristics in scheduling performances. The general solution scheme paves the road for effective design of efficient routing and flow control protocols on applicational quantum networks.
    BibTeX:
    @article{Li21,
      author = {Li, Changhao and Li, Tianyi and Liu, Yi-Xiang and Cappellaro, Paola},
      title = {Effective routing design for remote entanglement generation on quantum networks},
      journal = {npj Q. Inf.},
      year = {2021},
      volume = {7},
      number = {1},
      pages = {10},
      doi = {10.1038/s41534-020-00344-4}
    }
    
  17. G. Wang, Y.-X. Liu and P. Cappellaro
  18. "Observation of the high-order Mollow triplet by quantum mode control with concatenated continuous driving"
    Phys. Rev. A 103, 022415 (2021)
    Abstract: The Mollow triplet is a fundamental signature of quantum optics and has been observed in numerous quantum systems. Although it arises in the “strong driving” regime of the quantized field, where the atoms undergo coherent oscillations, it can be typically analyzed within the rotating wave approximation. Here we report the first observation of high-order effects in the Mollow triplet structure due to strong driving. In experiments, we explore the regime beyond the rotating wave approximation using concatenated continuous driving that has less stringent requirements on the driving field power. We are then able to reveal additional transition frequencies, shifts in energy levels, and corrections to the transition amplitudes. In particular, we find that these amplitudes are more sensitive to high-order effects than the frequency shifts and that they still require an accurate determination in order to achieve high-fidelity quantum control. The experimental results are validated by Floquet theory, which enables the precise numerical simulation of the evolution and further provides an analytical form for an effective Hamiltonian that approximately predicts the spin dynamics beyond the rotating wave approximation.
    BibTeX:
    @article{Wang21a,
      author = {Wang, Guoqing and Liu, Yi-Xiang and Cappellaro, Paola},
      title = {Observation of the high-order Mollow triplet by quantum mode control with concatenated continuous driving},
      journal = {Phys. Rev. A},
      publisher = {American Physical Society},
      year = {2021},
      volume = {103},
      pages = {022415},
      doi = {10.1103/PhysRevA.103.022415}
    }
    
  19. C. Yin, P. Peng, X. Huang, C. Ramanathan and P. Cappellaro
  20. "Prethermal quasiconserved observables in Floquet quantum systems"
    Phys. Rev. B 103, 054305 (2021)
    Abstract: Prethermalization, by introducing emergent quasiconserved observables, plays a crucial role in protecting periodically driven (Floquet) many-body phases over an exponentially long time, while the ultimate fate of such quasiconserved operators can signal thermalization to infinite temperature. To elucidate the properties of prethermal quasiconservation in many-body Floquet systems, here we systematically analyze infinite-temperature correlations between observables. We numerically show that the late-time behavior of the autocorrelations unambiguously distinguishes quasiconserved observables from nonconserved ones, allowing one to single out a set of linearly independent quasiconserved observables. By investigating two Floquet spin models, we identify two different mechanisms underlying the quasiconservation law. First, we numerically verify energy quasiconservation when the driving frequency is large, so that the system dynamics is approximately described by a static prethermal Hamiltonian. More interestingly, under moderate driving frequency, another quasiconserved observable can still persist if the Floquet driving contains a large global rotation. We show theoretically how to calculate this conserved observable and provide numerical verification. Having systematically identified all quasiconserved observables, we can finally investigate their behavior in the infinite-time limit and thermodynamic limit, using autocorrelations obtained from both numerical simulation and experiments in solid-state nuclear magnetic resonance systems.
    BibTeX:
    @article{Yin21,
      author = {Yin, Chao and Peng, Pai and Huang, Xiaoyang and Ramanathan, Chandrasekhar and Cappellaro, Paola},
      title = {Prethermal quasiconserved observables in Floquet quantum systems},
      journal = {Phys. Rev. B},
      publisher = {American Physical Society},
      year = {2021},
      volume = {103},
      pages = {054305},
      doi = {10.1103/PhysRevB.103.054305}
    }
    

    2020

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  21. G. Wang, Y.-X. Liu and P. Cappellaro
  22. "Coherence protection and decay mechanism in qubit ensembles under concatenated continuous driving"
    New J. Phys. 22, 123045 (2020)
    Abstract: Dense ensembles of spin qubits are valuable for quantum applications, even though their coherence protection remains challenging. Continuous dynamical decoupling can protect ensemble qubits from noise while allowing gate operations, but it is hindered by the additional noise introduced by the driving. Concatenated continuous driving (CCD) techniques can, in principle, mitigate this problem. Here we provide deeper insights into the dynamics under CCD, based on Floquet theory, that lead to optimized state protection by adjusting driving parameters in the CCD scheme to induce mode evolution control. We experimentally demonstrate the improved control by simultaneously addressing a dense nitrogen-vacancy (NV) ensemble with 1010 spins. We achieve an experimental 15-fold improvement in coherence time for an arbitrary, unknown state, and a 500-fold improvement for an arbitrary, known state, corresponding to driving the sidebands and the center band of the resulting Mollow triplet, respectively. We can achieve such coherence time gains by optimizing the driving parameters to take into account the noise affecting our system. By extending the generalized Bloch equation approach to the CCD scenario, we identify the noise sources that dominate the decay mechanisms in NV ensembles, confirm our model by experimental results, and identify the driving strengths yielding optimal coherence. Our results can be directly used to optimize qubit coherence protection under continuous driving and bath driving, and enable applications in robust pulse design and quantum sensing.
    BibTeX:
    @article{Wang20n,
      author = {Wang, Guoqing and Liu, Yi-Xiang and Cappellaro, Paola},
      title = {Coherence protection and decay mechanism in qubit ensembles under concatenated continuous driving},
      journal = {New J. Phys.},
      publisher = {IOP Publishing},
      year = {2020},
      volume = {22},
      number = {12},
      pages = {123045},
      doi = {10.1088/1367-2630/abd2e5}
    }
    
  23. A. Sone, Y.-X. Liu and P. Cappellaro
  24. "Quantum Jarzynski Equality in Open Quantum Systems from the One-Time Measurement Scheme"
    Phys. Rev. Lett. 125, 060602 (2020)
    Abstract: In open quantum systems, a clear distinction between work and heat is often challenging, and extending the quantum Jarzynski equality to systems evolving under general quantum channels beyond unitality remains an open problem in quantum thermodynamics. In this letter, we introduce well-defined notions of guessed heat and guessed work, by exploiting the one-time measurement scheme, which only requires an initial energy measurement on the system alone. We derive a modified quantum Jarzynski equality and the principle of maximum work with respect to the guessed quantum work, which requires the knowledge of the system only. We further show the significance of guessed quantum heat and work by linking them to the problem of quantum hypothesis testing.
    BibTeX:
    @article{Sone20,
      author = {Sone, Akira and Liu, Yi-Xiang and Cappellaro, Paola},
      title = {Quantum Jarzynski Equality in Open Quantum Systems from the One-Time Measurement Scheme},
      journal = {Phys. Rev. Lett.},
      year = {2020},
      volume = {125},
      pages = {060602},
      doi = {10.1103/PhysRevLett.125.060602}
    }
    
  25. S. Hernández-Gómez, S. Gherardini, F. Poggiali, F. S. Cataliotti, A. Trombettoni, P. Cappellaro and N. Fabbri
  26. "Experimental test of exchange fluctuation relations in an open quantum system"
    Phys. Rev. Research 2, 023327 (2020)
    Abstract: Elucidating the energy transfer between a quantum system and a reservoir is a central issue in quantum non-equilibrium thermodynamics, which could provide novel tools to engineer quantum-enhanced heat engines. The lack of information on the reservoir inherently limits the practical insight that can be gained on the exchange process. Here, we investigate the energy transfer for an open quantum system in the framework of quantum fluctuation relations. As a novel toolbox, we employ a nitrogen-vacancy center spin qubit in diamond, subject to repeated quantum projective measurements accompanied by a tunable dissipation channel. When the system is tuned to be insensitive to dissipation, we verify the closed-system quantum Jarzynski equality. In the presence of competition between dissipation and quantum projective measurements, the experimental results suggest a formulation of the energy exchange fluctuation relation that incorporates the reservoir properties in the guise of an effective temperature of the final out-of-equilibrium steady-state. Our findings pave the way to investigate energy exchange mechanisms in higher-dimension open quantum systems.
    BibTeX:
    @article{Hernandez20,
      author = {Hernández-Gómez, S. and Gherardini, S. and Poggiali, F. and Cataliotti, F. S. and Trombettoni, A. and Cappellaro, P. and Fabbri, N.},
      title = {Experimental test of exchange fluctuation relations in an open quantum system},
      journal = {Phys. Rev. Research},
      year = {2020},
      volume = {2},
      pages = {023327},
      doi = {10.1103/PhysRevResearch.2.023327}
    }
    
  27. Y.-X. Liu, J. Hines, Z. Li, A. Ajoy and P. Cappellaro
  28. "High-fidelity Trotter formulas for digital quantum simulation"
    Phys. Rev. A 102, 010601 (2020)
    Abstract: Quantum simulation promises to address many challenges in fields ranging from quantum chemistry to material science and high-energy physics, and could be implemented in noisy intermediate-scale quantum devices. A challenge in building good digital quantum simulators is the fidelity of the engineered dynamics given a finite set of elementary operations. Here we present a framework for optimizing the order of operations based on a geometric picture, thus abstracting from the operation details and achieving computational efficiency. Based on this geometric framework, we provide two alternative second-order Trotter expansions: one with optimal fidelity at a short timescale, and the second robust at a long timescale. Thanks to the improved fidelity at different timescales, the two expansions we introduce can form the basis for experimental-constrained digital quantum simulation.
    BibTeX:
    @article{Liu20a,
      author = {Liu, Yi-Xiang and Hines, Jordan and Li, Zhi and Ajoy, Ashok and Cappellaro, Paola},
      title = {High-fidelity Trotter formulas for digital quantum simulation},
      journal = {Phys. Rev. A},
      publisher = {American Physical Society},
      year = {2020},
      volume = {102},
      pages = {010601},
      doi = {10.1103/PhysRevA.102.010601}
    }
    
  29. H. Zhou, J. Choi, S. Choi, R. Landig, A. M. Douglas, J. Isoya, F. Jelezko, S. Onoda, H. Sumiya, P. Cappellaro, H. S. Knowles, H. Park and M. D. Lukin
  30. "Quantum Metrology with Strongly Interacting Spin Systems"
    Phys. Rev. X 10, 031003 (2020)
    Abstract: Quantum metrology makes use of coherent superpositions to detect weak signals. While in principle the sensitivity can be improved by increasing the density of sensing particles, in practice this improvement is severely hindered by interactions between them. Using a dense ensemble of interacting electronic spins in diamond, we demonstrate a novel approach to quantum metrology. It is based on a new method of robust quantum control, which allows us to simultaneously eliminate the undesired effects associated with spin-spin interactions, disorder and control imperfections, enabling a five-fold enhancement in coherence time compared to conventional control sequences. Combined with optimal initialization and readout protocols, this allows us to break the limit for AC magnetic field sensing imposed by interactions, opening a promising avenue for the development of solid-state ensemble magnetometers with unprecedented sensitivity.
    BibTeX:
    @article{Zhou20,
      author = {Zhou, Hengyun and Choi, Joonhee and Choi, Soonwon and Landig, Renate and Douglas, Alexander M. and Isoya, Junichi and Jelezko, Fedor and Onoda, Shinobu and Sumiya, Hitoshi and Cappellaro, Paola and Knowles, Helena S. and Park, Hongkun and Lukin, Mikhail D.},
      title = {Quantum Metrology with Strongly Interacting Spin Systems},
      journal = {Phys. Rev. X},
      publisher = {American Physical Society},
      year = {2020},
      volume = {10},
      pages = {031003},
      doi = {10.1103/PhysRevX.10.031003}
    }
    
  31. S. Hernandez-Gomez, F. Poggiali, N. Fabbri, P. Cappellaro
  32. "Environment spectroscopy with an NV center in diamond"
    In Proceedings of the International School of Physics "Enrico Fermi": Nanoscale Quantum Optics 204, 245 - 249 (2020)
    Abstract: Nitrogen-vacancy (NV) centers in diamond have emerged in the last decade as a prominent platform for quantum technologies. As for any qubit system, a good understanding of their local environment is crucial to build quantum devices protected from detrimental noise. Here, we describe in detail a method to spectroscopically characterize the spin bath around an NV center, even when the NV coherence time is short, and identify the coherent coupling with the nearest nuclear spins. In the regime of weak qubit-bath coupling, the acquired knowledge of the bath reliably predicts the qubit dynamics under different controls.
    BibTeX:
    @inproceedings{Hernandez20p,
      author = {S. Hernandez-Gomez, F. Poggiali, N. Fabbri, P. Cappellaro},
      title = {Environment spectroscopy with an NV center in diamond},
      booktitle = {Proceedings of the International School of Physics "Enrico Fermi": Nanoscale Quantum Optics},
      year = {2020},
      volume = {204},
      pages = {245 - 249},
      doi = {10.3254/ENFI200027},
      url = {http://ebooks.iospress.nl/volumearticle/55677}
    }
    
  33. A. Cooper, W. K. C. Sun, J.-C. Jaskula and P. Cappellaro
  34. "Identification and Control of Electron-Nuclear Spin Defects in Diamond"
    Phys. Rev. Lett. 124, 083602 (2020)
    Abstract: We experimentally demonstrate an approach to scale up quantum devices by harnessing spin defects in the environment of a quantum probe. We follow this approach to identify, locate, and control two electron-nuclear spin defects in the environment of a single nitrogen-vacancy center in diamond. By performing spectroscopy at various orientations of the magnetic field, we extract the unknown parameters of the hyperfine and dipolar interaction tensors, which we use to locate the two spin defects and design control sequences to initialize, manipulate, and readout their quantum state. Finally, we create quantum coherence among the three electron spins, paving the way for the creation of genuine tripartite entanglement. This approach will be useful in assembling multispin quantum registers for applications in quantum sensing and quantum information processing.
    BibTeX:
    @article{Cooper20,
      author = {Cooper, Alexandre and Sun, Won Kyu Calvin and Jaskula, Jean-Christophe and Cappellaro, Paola},
      title = {Identification and Control of Electron-Nuclear Spin Defects in Diamond},
      journal = {Phys. Rev. Lett.},
      publisher = {American Physical Society},
      year = {2020},
      volume = {124},
      pages = {083602},
      doi = {10.1103/PhysRevLett.124.083602}
    }
    
  35. D. Layden, M. Chen and P. Cappellaro
  36. "Efficient Quantum Error Correction of Dephasing Induced by a Common Fluctuator"
    Phys. Rev. Lett. 124, 020504 (2020)
    Abstract: Quantum error correction is expected to be essential in large-scale quantum technologies. However, the substantial overhead of qubits it requires is thought to greatly limit its utility in smaller, near-term devices. Here we introduce a new family of special-purpose quantum error-correcting codes that offer an exponential reduction in overhead compared to the usual repetition code. They are tailored for a common and important source of decoherence in current experiments, whereby a register of qubits is subject to phase noise through coupling to a common fluctuator, such as a resonator or a spin defect. The smallest instance encodes one logical qubit into two physical qubits, and corrects decoherence to leading-order using a constant number of one- and two-qubit operations. More generally, while the repetition code on n qubits corrects errors to order t^O(n), with t the time between recoveries, our codes correct to order t^O(2^n). Moreover, they are robust to model imperfections in small- and intermediate-scale devices, where they already provide substantial gains in error suppression. As a result, these hardware-efficient codes open a potential avenue for useful quantum error correction in near-term, pre-fault tolerant devices.
    BibTeX:
    @article{Layden20,
      author = {Layden, David and Chen, Mo and Cappellaro, Paola},
      title = {Efficient Quantum Error Correction of Dephasing Induced by a Common Fluctuator},
      journal = {Phys. Rev. Lett.},
      publisher = {American Physical Society},
      year = {2020},
      volume = {124},
      pages = {020504},
      doi = {10.1103/PhysRevLett.124.020504}
    }
    
  37. C. M. Sánchez, A. K. Chattah, K. X. Wei, L. Buljubasich, P. Cappellaro and H. M. Pastawski
  38. "Perturbation Independent Decay of the Loschmidt Echo in a Many-Body System"
    Phys. Rev. Lett. 124, 030601 (2020)
    Abstract: Evaluating the role of perturbations versus the intrinsic coherent dynamics in driving to equilibrium is of fundamental interest to understand quantum many-body thermalization, in the quest to build ever complex quantum devices. Here we introduce a protocol that scales down the coupling strength in a quantum simulator based on a solid-state nuclear spin system, leading to a longer decay time T2, while keeping perturbations associated to control error constant. We can monitor quantum information scrambling by measuring two powerful metrics, out-of-time-ordered correlators (OTOCs) and Loschmidt Echoes (LEs). While OTOCs reveal quantum information scrambling involving hundreds of spins, the LE decay quantifies, via the time scale T3, how well the scrambled information can be recovered through time reversal. We find that when the interactions dominate the perturbation, the LE decay rate only depends on the interactions themselves, T3 ~ T2, and not on the perturbation. Then, in an unbounded many-spin system, decoherence can achieve a perturbation-independent regime, with a rate only related to the local second moment of the Hamiltonian.
    BibTeX:
    @article{Sanchez20,
      author = {Sánchez, C. M. and Chattah, A. K. and Wei, K. X. and Buljubasich, L. and Cappellaro, P. and Pastawski, H. M.},
      title = {Perturbation Independent Decay of the Loschmidt Echo in a Many-Body System},
      journal = {Phys. Rev. Lett.},
      publisher = {American Physical Society},
      year = {2020},
      volume = {124},
      pages = {030601},
      doi = {10.1103/PhysRevLett.124.030601}
    }
    
  39. W. K. C. Sun, A. Cooper and P. Cappellaro
  40. "Improved entanglement detection with subspace witnesses"
    Phys. Rev. A 101, 012319 (2020)
    Abstract: Entanglement, while being critical in many quantum applications, is difficult to characterize experimentally. While entanglement witnesses based on the fidelity to the target entangled state are efficient detectors of entanglement, they in general underestimate the amount of entanglement due to errors during state preparation and measurement. Here, to improve entanglement detection, we introduce a 'subspace' witness that can detect a broader class of entangled states than the conventional state-fidelity witnesses, while still remaining more efficient than state tomography. We experimentally demonstrate the advantages of the subspace witness by generating and detecting entanglement with a hybrid, two-qubit system composed of electronic spins in diamond. We further extend the notion of subspace witness to specific genuine multipartite entangled (GME) states such as GHZ, W, and Dicke states, and motivate the choice of the metric based on quantum information tasks such as entanglement-enhanced sensing. We expect the straightforward and efficient implementation of subspace witnesses would be beneficial in detecting specific GME states in noisy, intermediate-scale quantum processors with a hundred qubits.
    BibTeX:
    @article{Sun20,
      author = {Sun, Won Kyu Calvin and Cooper, Alexandre and Cappellaro, Paola},
      title = {Improved entanglement detection with subspace witnesses},
      journal = {Phys. Rev. A},
      publisher = {American Physical Society},
      year = {2020},
      volume = {101},
      pages = {012319},
      doi = {10.1103/PhysRevA.101.012319}
    }
    
  41. D. Layden, L. R. Huang and P. Cappellaro
  42. "Robustness-optimized quantum error correction"
    Quantum Science and Technology 5, 025004 (2020)
    Abstract: Quantum error correction (QEC) codes are usually designed to correct errors regardless of their physical origins. In large-scale devices, this is an essential feature. In smaller-scale devices, however, the main error sources are often understood, and this knowledge could be exploited for more efficient error correction. Optimizing the QEC protocol is therefore a promising strategy in smaller devices. Typically, this involves tailoring the protocol to a given decoherence channel by solving an appropriate optimization problem. Here we introduce a new optimization-based approach, which maximizes the robustness to faults in the recovery. Our approach is inspired by recent experiments, where such faults have been a significant source of logical errors. We illustrate this approach with a three-qubit model, and show how near-term experiments could benefit from more robust QEC protocols.
    BibTeX:
    @article{Layden20q,
      author = {David Layden and Louisa Ruixue Huang and Paola Cappellaro},
      title = {Robustness-optimized quantum error correction},
      journal = {Quantum Science and Technology},
      publisher = {IOP Publishing},
      year = {2020},
      volume = {5},
      number = {2},
      pages = {025004},
      doi = {10.1088/2058-9565/ab79b2}
    }
    
  43. G. Liu, M. Chen, Y.-X. Liu, D. Layden and P. Cappellaro
  44. "Repetitive readout enhanced by machine learning"
    Machine Learning: Science and Technology 1, 015003 (2020)
    Abstract: Single-shot readout is a key component for scalable quantum information processing. However, many solid-state qubits with favorable properties lack the single-shot readout capability. One solution is to use the repetitive quantum-non-demolition readout technique, where the qubit is correlated with an ancilla, which is subsequently read out. The readout fidelity is therefore limited by the back-action on the qubit from the measurement. Traditionally, a threshold method is taken, where only the total photon count is used to discriminate qubit state, discarding all the information of the back-action hidden in the time trace of repetitive readout measurement. Here we show by using machine learning (ML), one obtains higher readout fidelity by taking advantage of the time trace data. ML is able to identify when back-action happened, and correctly read out the original state. Since the information is already recorded (but usually discarded), this improvement in fidelity does not consume additional experimental time, and could be directly applied to preparation-by-measurement and quantum metrology applications involving repetitive readout.
    BibTeX:
    @article{Liu20,
      author = {Genyue Liu and Mo Chen and Yi-Xiang Liu and David Layden and Paola Cappellaro},
      title = {Repetitive readout enhanced by machine learning},
      journal = {Machine Learning: Science and Technology},
      publisher = {IOP Publishing},
      year = {2020},
      volume = {1},
      number = {1},
      pages = {015003},
      doi = {10.1088/2632-2153/ab4e24}
    }
    
  45. Y. Wang, D. Dong, A. Sone, I. R. Petersen, H. Yonezawa and P. Cappellaro
  46. "Quantum Hamiltonian Identifiability via a Similarity Transformation Approach and Beyond"
    IEEE Transactions on Automatic Control In IEEE Transactions on Automatic Control , 1 (2020)
    Abstract: The identifiability of a system is concerned with whether the unknown parameters in the system can be uniquely determined with all the possible data generated by a certain experimental setting. In this paper, we generalize the identifiability test based on the Similarity Transformation Approach (STA) in classical control theory and extend it to the domain of quantum Hamiltonian identification. We employ STA to prove the identifiability of spin-1/2 chain systems with arbitrary dimension assisted by single-qubit probes. We further extend the traditional STA method by proposing a Structure Preserving Transformation (SPT) method for non-minimal systems. We use the SPT method to introduce an indicator for the existence of economic quantum Hamiltonian identification algorithms, whose computational complexity directly depends on the number of unknown parameters (which could be much smaller than the system dimension). Finally, we give an example of such an economic Hamiltonian identification algorithm and perform simulations to demonstrate its effectiveness.
    BibTeX:
    @article{Wang20,
      author = {Wang, Y. and Dong, D. and Sone, A. and Petersen, I. R. and Yonezawa, H. and Cappellaro, P.},
      title = {Quantum Hamiltonian Identifiability via a Similarity Transformation Approach and Beyond},
      booktitle = {IEEE Transactions on Automatic Control},
      journal = {IEEE Transactions on Automatic Control},
      year = {2020},
      pages = {1},
      doi = {10.1109/TAC.2020.2973582}
    }
    

    2019

    UP ↑

  47. P. Peng, Z. Li, H. Yan, K. X. Wei and P. Cappellaro
  48. "Comparing many-body localization lengths via non-perturbative construction of local integrals of motion"
    Phys. Rev. B 100, 214203 (2019)
    Abstract: Many-body localization (MBL), characterized by the absence of thermalization and the violation of conventional thermodynamics, has elicited much interest both as a fundamental physical phenomenon and for practical applications in quantum information. A phenomenological model, which describes the system using a complete set of local integrals of motion (LIOMs), provides a powerful tool to understand MBL, but can be usually only computed approximately. Here we explicitly compute a complete set of LIOMs with a non-perturbative approach, by maximizing the overlap between LIOMs and physical spin operators in real space. The set of LIOMs satisfies the desired exponential decay of weight of LIOMs in real-space. This LIOM construction enables a direct mapping from the real space Hamiltonian to the phenomenological model and thus enables studying the localized Hamiltonian and the system dynamics. We can thus study and compare the localization lengths extracted from the LIOM weights, their interactions, and dephasing dynamics, revealing interesting aspects of many-body localization. Our scheme is immune to accidental resonances and can be applied even at phase transition point, providing a novel tool to study the microscopic features of the phenomenological model of MBL.
    BibTeX:
    @article{Peng19,
      author = {Pai Peng and Zeyang Li and Haoxiong Yan and Ken Xuan Wei and Paola Cappellaro},
      title = {Comparing many-body localization lengths via non-perturbative construction of local integrals of motion},
      journal = {Phys. Rev. B},
      year = {2019},
      volume = {100},
      pages = {214203},
      doi = {10.1103/PhysRevB.100.214203}
    }
    
  49. C. Li and P. Cappellaro
  50. "Telecom photon interface of solid-state quantum nodes"
    J. Phys. Comms. 3, 095016- (2019)
    Abstract: Solid-state spins such as nitrogen-vacancy (NV) center are promising platforms for large-scale quantum networks. Despite the optical interface of NV center system, however, the significant attenuation of its zero-phonon-line photon in optical fiber prevents the network extended to long distances. Therefore a telecom-wavelength photon interface would be essential to reduce the photon loss in transporting quantum information. Here we propose an efficient scheme for coupling telecom photon to NV center ensembles mediated by rare-earth doped crystal. Specifically, we proposed protocols for high fidelity quantum state transfer and entanglement generation with parameters within reach of current technologies. Such an interface would bring new insights into future implementations of long-range quantum network with NV centers in diamond acting as quantum nodes.
    BibTeX:
    @article{Li19,
      author = {Li, Changhao and Cappellaro, Paola},
      title = {Telecom photon interface of solid-state quantum nodes},
      journal = {J. Phys. Comms.},
      publisher = {IOP Publishing},
      year = {2019},
      volume = {3},
      number = {9},
      pages = {095016--}
    }
    
  51. C. Li, M. Chen, D. Lyzwa and P. Cappellaro
  52. "All-Optical Quantum Sensing of Rotational Brownian Motion of Magnetic Molecules"
    Nano Lett. 19, 7342-7348 (2019)
    Abstract: Sensing the local environment through the motional response of small molecules lays the foundation of many fundamental technologies. The information on local viscosity, for example, is contained in the random rotational Brownian motions of molecules. However, detection of the motions is challenging for molecules with sub-nanometer scale or high motional rates. Here we propose and experimentally demonstrate a novel method of detecting fast rotational Brownian motions of small magnetic molecules. With electronic spins as sensors, we are able to detect changes in motional rates, which yield different noise spectra and therefore different relaxation signals of the sensors. As a proof-of-principle demonstration, we experimentally implemented this method to detect the motions of gadolinium (Gd) complex molecules with nitrogen-vacancy (NV) centers in nanodiamonds. With all-optical measurements of the NV centers’ longitudinal relaxation, we distinguished binary solutions with varying viscosities. Our method paves a new way for detecting fast motions of sub-nanometer sized magnetic molecules with better spatial resolution than conventional optical methods. It also provides a new tool in designing better contrast agents in magnetic resonance imaging.
    Sensing the local environment through the motional response of small molecules lays the foundation of many fundamental technologies. The information on local viscosity, for example, is contained in the random rotational Brownian motions of molecules. However, detection of the motions is challenging for molecules with sub-nanometer scale or high motional rates. Here we propose and experimentally demonstrate a novel method of detecting fast rotational Brownian motions of small magnetic molecules. With electronic spins as sensors, we are able to detect changes in motional rates, which yield different noise spectra and therefore different relaxation signals of the sensors. As a proof-of-principle demonstration, we experimentally implemented this method to detect the motions of gadolinium (Gd) complex molecules with nitrogen-vacancy (NV) centers in nanodiamonds. With all-optical measurements of the NV centers’ longitudinal relaxation, we distinguished binary solutions with varying viscosities. Our method paves a new way for detecting fast motions of sub-nanometer sized magnetic molecules with better spatial resolution than conventional optical methods. It also provides a new tool in designing better contrast agents in magnetic resonance imaging.
    BibTeX:
    @article{Li19b,
      author = {Li, Changhao and Chen, Mo and Lyzwa, Dominika and Cappellaro, Paola},
      title = {All-Optical Quantum Sensing of Rotational Brownian Motion of Magnetic Molecules},
      journal = {Nano Lett.},
      year = {2019},
      volume = {19},
      number = {10},
      pages = {7342--7348},
      doi = {10.1021/acs.nanolett.9b02960}
    }
    
  53. A. Cooper, W. K. C. Sun, J.-C. Jaskula and P. Cappellaro
  54. "Environment-assisted Quantum-enhanced Sensing with Electronic Spins in Diamond"
    Phys. Rev. Applied 12, 044047 (2019)
    Abstract: The performance of solid-state quantum sensors based on electronic spin defects is often limited by the presence of environmental spin impurities that cause decoherence. A promising approach to improve these quantum sensors is to convert environment spins into useful resources for sensing, in particular, entangled states. However, the sensitivity enhancement that can be achieved from entangled states is limited by experimental constraints, such as control errors, decoherence, and time overheads. Here we experimentally demonstrate the efficient use of an unknown electronic spin defect in the proximity of a nitrogen-vacancy center in diamond to achieve both an entangled quantum sensor and a quantum memory for readout. We show that, whereas entanglement alone does not provide an enhancement in sensitivity, combining both entanglement and repetitive readout achieves an enhancement in performance over the use of a single-spin sensor, and more broadly we discuss regimes where sensitivity could be enhanced. Our results critically highlight the challenges in improving quantum sensors using entangled states of electronic spins, while providing an important benchmark in the quest for entanglement-assisted metrology.
    BibTeX:
    @article{Cooper19,
      author = {Cooper, Alexandre and Sun, Won Kyu Calvin and Jaskula, Jean-Christophe and Cappellaro, Paola},
      title = {Environment-assisted Quantum-enhanced Sensing with Electronic Spins in Diamond},
      journal = {Phys. Rev. Applied},
      publisher = {American Physical Society},
      year = {2019},
      volume = {12},
      pages = {044047},
      doi = {10.1103/PhysRevApplied.12.044047}
    }
    
  55. J.-C. Jaskula, K. Saha, A. Ajoy, D. J. Twitchen, M. Markham and P. Cappellaro
  56. "Cross-sensor feedback stabilization of an emulated quantum spin gyroscope"
    Phys. Rev. App. 11, 054010 (2019)
    Abstract: Quantum sensors, such as the Nitrogen Vacancy (NV) color center in diamond, are known for their exquisite sensitivity, but their performance is degraded by noise. To improve the long-term robustness of a quantum sensor, here we realize an integrated combinatorial spin sensor in the same micrometer-scale footprint, which exploits two different spin sensitivity to distinct physical quantities to stabilize one spin sensor with local information collected in realtime via the second sensor. We show that we can use the electronic spins of a large ensemble of NV centers as sensor of the local magnetic field fluctuations, affecting both spin sensors, in order to stabilize the output signal of interleaved Ramsey sequences performed on the 14N nuclear spin. An envisioned application of such a device is to sense rotation rates with a stability of several days, allowing navigation with limited or no requirement of geo-localization. Our results would enable stable rotation sensing for over several hours, which already reflects better performance than MEMS gyroscopes of comparable sensitivity and size.
    BibTeX:
    @article{Jaskula19,
      author = {Jean-Christophe Jaskula and Kasturi Saha and Ashok Ajoy and Daniel J. Twitchen and Matthew Markham and Paola Cappellaro},
      title = {Cross-sensor feedback stabilization of an emulated quantum spin gyroscope},
      journal = {Phys. Rev. App.},
      publisher = {American Physical Society},
      year = {2019},
      volume = {11},
      pages = {054010},
      doi = {10.1103/PhysRevApplied.11.054010}
    }
    
  57. S. Zhou, D. Layden, M. Zhang, J. Preskill, P. Cappellaro and L. Jiang
  58. "Error-corrected quantum sensing"
    In Proceedings of SPIE Optical, Opto-Atomic, and Entanglement-Enhanced Precision Metrology, Volume 10934, 1J (2019)
    Abstract: Quantum metrology has many important applications in science and technology, ranging from frequency spectroscopy to gravitational wave detection. Quantum mechanics imposes a fundamental limit on measurement precision, called the Heisenberg limit, which can be achieved for noiseless quantum systems, but is not achievable in general for systems subject to noise. Here we study how measurement precision can be enhanced through quantum error correction, a general method for protecting a quantum system from the damaging effects of noise. We find a necessary and sufficient condition for achieving the Heisenberg limit using quantum probes subject to Markovian noise, assuming that noiseless ancilla systems are available, and that fast, accurate quantum processing can be performed. When the sufficient condition is satisfied, the quantum error-correcting code achieving the best possible precision can be found by solving a semidefinite program. We also show that noiseless ancilla are not needed when the signal Hamiltonian and the error operators commute. Finally we provide two explicit, archetypal examples of quantum sensors: qubits undergoing dephasing and a lossy bosonic mode.
    BibTeX:
    @inproceedings{Zhou19,
      author = {Sisi Zhou, David Layden, Mengzhen Zhang, John Preskill, Paola Cappellaro, Liang Jiang},
      title = {Error-corrected quantum sensing},
      booktitle = {SPIE Optical, Opto-Atomic, and Entanglement-Enhanced Precision Metrology},
      publisher = {SPIE},
      year = {2019},
      volume = {10934},
      pages = {109341J},
      doi = {10.1117/12.2511587}
    }
    
  59. A. Sone, Q. Zhuang, C. Li, Y.-X. Liu and P. Cappellaro
  60. "Nonclassical correlations for quantum metrology in thermal equilibrium"
    Phys. Rev. A 99, 052318 (2019)
    Abstract: Nonclassical correlation beyond entanglement might provide a resource in quantum information tasks, such as quantum computation or quantum metrology. Quantum discord is a measure of nonclassical correlations beyond entanglement. Exploring the operational meaning of quantum discord as a resource in quantum information processing tasks, such as quantum metrology, is of essential importance to our understanding of nonclassical correlation. In our recent work [Phys. Rev. A, 98, 012115 (2018)], we have demonstrated that the diagonal discord plays a role in enhancing the high-temperature sensitivity of the greedy local thermometry scheme, where one measures the subsystems sequentially with the local optimal measurement. In this paper, we extend our results to a general greedy local parameter estimation scenario. In particular, we introduce a quantum discord, which we call discord for local metrology, to quantify the nonclassical correlations induced by the local optimal measurement on the subsystem. We demonstrate explicitly that discord for local metrology plays a role in sensitivity enhancement in the high-temperature limit by showing its relation to loss in quantum Fisher information. In particular, it coincides with diagonal discord for estimating a linear coupling parameter.
    BibTeX:
    @article{Sone19,
      author = {Sone, Akira and Zhuang, Quntao and Li, Changhao and Liu, Yi-Xiang and Cappellaro, Paola},
      title = {Nonclassical correlations for quantum metrology in thermal equilibrium},
      journal = {Phys. Rev. A},
      year = {2019},
      volume = {99},
      pages = {052318},
      doi = {10.1103/PhysRevA.99.052318}
    }
    
  61. F. Poggiali, S. Hernández-Gómez, P. Cappellaro and N. Fabbri
  62. "Optimal control of diamond spin qubits for quantum sensing in noisy environments"
    Quantum Information and Measurement (QIM) V: Quantum Technologies In Quantum Information and Measurement (QIM) V: Quantum Technologies , S3C.2 (2019)
    Abstract: We devise a robust quantum sensing scheme based on optimal control. We experimentally demonstrate sensitivity enhancement of diamond spin-qubits sensors to measure ultraweak time-varying magnetic fields in noisy environments.
    BibTeX:
    @inproceedings{Poggiali19,
      author = {Francesco Poggiali and Santiago Hern\'{a}ndez-G\'{o}mez and Paola Cappellaro and Nicole Fabbri},
      title = {Optimal control of diamond spin qubits for quantum sensing in noisy environments},
      booktitle = {Quantum Information and Measurement (QIM) V: Quantum Technologies},
      journal = {Quantum Information and Measurement (QIM) V: Quantum Technologies},
      publisher = {Optical Society of America},
      year = {2019},
      pages = {S3C.2},
      doi = {10.1364/QIM.2019.S3C.2}
    }
    
  63. Y.-X. Liu, A. Ajoy and P. Cappellaro
  64. "Nanoscale Vector dc Magnetometry via Ancilla-Assisted Frequency Up-Conversion"
    Phys. Rev. Lett. 122, 100501 (2019)
    Abstract: Sensing static magnetic fields with high sensitivity and spatial resolution is critical to many applications in fundamental physics, bioimaging, and materials science. Even more beneficial would be full vector magnetometry with nanoscale spatial resolution. Several versatile magnetometry platforms have emerged over the past decade, such as electronic spins associated with nitrogen vacancy (NV) centers in diamond. Achieving vector magnetometry has, however, often required using an ensemble of sensors or degrading the sensitivity. Here we introduce a hybrid magnetometry platform, consisting of a sensor and an ancillary qubit, that allows vector magnetometry of static fields. While more generally applicable, we demonstrate the method for an electronic NV sensor and a nuclear spin qubit. In particular, sensing transverse fields relies on frequency up-conversion of the dc fields through the ancillary qubit, allowing quantum lock-in detection with low-frequency noise rejection. In combination with the Ramsey detection of longitudinal fields, our frequency up-conversion scheme delivers a sensitive technique for vector dc magnetometry at the nanoscale.
    BibTeX:
    @article{Liu19,
      author = {Liu, Yi-Xiang and Ajoy, Ashok and Cappellaro, Paola},
      title = {Nanoscale Vector dc Magnetometry via Ancilla-Assisted Frequency Up-Conversion},
      journal = {Phys. Rev. Lett.},
      publisher = {American Physical Society},
      year = {2019},
      volume = {122},
      pages = {100501},
      doi = {10.1103/PhysRevLett.122.100501}
    }
    
  65. A. Ajoy, U. Bissbort, D. Poletti and P. Cappellaro
  66. "Selective Decoupling and Hamiltonian Engineering in Dipolar Spin Networks"
    Phys. Rev. Lett. 122, 013205 (2019)
    Abstract: We present a protocol to selectively decouple, recouple, and engineer effective interactions in mesoscopic dipolar spin networks. In particular, we develop a versatile protocol that relies upon magic angle spinning to perform Hamiltonian engineering. By using global control fields in conjunction with a local actuator, such as a diamond nitrogen vacancy center located in the vicinity of a nuclear spin network, both global and local control over the effective couplings can be achieved. We show that the resulting effective Hamiltonian can be well understood within a simple, intuitive geometric picture, and corroborate its validity by performing exact numerical simulations in few-body systems. Applications of our method are in the emerging fields of two-dimensional room temperature quantum simulators in diamond platforms, as well as in molecular magnet systems.
    BibTeX:
    @article{Ajoy19,
      author = {Ajoy, A. and Bissbort, U. and Poletti, D. and Cappellaro, P.},
      title = {Selective Decoupling and Hamiltonian Engineering in Dipolar Spin Networks},
      journal = {Phys. Rev. Lett.},
      publisher = {American Physical Society},
      year = {2019},
      volume = {122},
      pages = {013205},
      doi = {10.1103/PhysRevLett.122.013205}
    }
    
  67. D. Layden, S. Zhou, P. Cappellaro and L. Jiang
  68. "Ancilla-Free Quantum Error Correction Codes for Quantum Metrology"
    Phys. Rev. Lett. 122, 040502 (2019)
    Abstract: Quantum error correction has recently emerged as a tool to enhance quantum sensing under Markovian noise. It works by correcting errors in a sensor while letting a signal imprint on the logical state. This approach typically requires a specialized error-correcting code, as most existing codes correct away both the dominant errors and the signal. To date, however, few such specialized codes are known, among which most require noiseless, controllable ancillas. We show here that such ancillas are not needed when the signal Hamiltonian and the error operators commute, a common limiting type of decoherence in quantum sensors. We give a semidefinite program for finding optimal ancilla-free sensing codes in general, as well as closed-form codes for two common sensing scenarios: qubits undergoing dephasing, and a lossy bosonic mode. Finally, we analyze the sensitivity enhancement offered by the qubit code under arbitrary spatial noise correlations, beyond the ideal limit of orthogonal signal and noise operators.
    BibTeX:
    @article{Layden19,
      author = {Layden, David and Zhou, Sisi and Cappellaro, Paola and Jiang, Liang},
      title = {Ancilla-Free Quantum Error Correction Codes for Quantum Metrology},
      journal = {Phys. Rev. Lett.},
      publisher = {American Physical Society},
      year = {2019},
      volume = {122},
      pages = {040502},
      doi = {10.1103/PhysRevLett.122.040502}
    }
    
  69. K. X. Wei, P. Peng, O. Shtanko, I. Marvian, S. Lloyd, C. Ramanathan and P. Cappellaro
  70. "Emergent Prethermalization Signatures in Out-of-Time Ordered Correlations"
    Phys. Rev. Lett. 123, 090605 (2019)
    Abstract: How a many-body quantum system thermalizes—or fails to do so—under its own interaction is a fundamental yet elusive concept. Here we demonstrate nuclear magnetic resonance observation of the emergence of prethermalization by measuring out-of-time ordered correlations. We exploit Hamiltonian engineering techniques to tune the strength of spin-spin interactions and of a transverse magnetic field in a spin chain system, as well as to invert the Hamiltonian sign to reveal out-of-time ordered correlations. At large fields, we observe an emergent conserved quantity due to prethermalization, which can be revealed by an early saturation of correlations. Our experiment not only demonstrates a new protocol to measure out-of-time ordered correlations, but also provides new insights in the study of quantum thermodynamics.
    BibTeX:
    @article{Wei19,
      author = {Wei, K.~X. and Peng, P. and Shtanko, O. and Marvian, I. and Lloyd, S. and Ramanathan, C. and Cappellaro, P.},
      title = { Emergent Prethermalization Signatures in Out-of-Time Ordered Correlations },
      journal = {Phys. Rev. Lett.},
      publisher = {American Physical Society},
      year = {2019},
      volume = {123},
      pages = {090605},
      doi = {10.1103/PhysRevLett.123.090605}
    }
    

    2018

    UP ↑

  71. M. Chen, W. K. C. Sun, K. Saha, J.-C. Jaskula and P. Cappellaro
  72. "Protecting solid-state spins from a strongly coupled environment"
    New J. Phys. 20, 063011 (2018)
    Abstract: Quantum memories are critical for solid-state quantum computing devices and a good quantum memory requires both long storage time and fast read/write operations. A promising system is the nitrogen-vacancy (NV) center in diamond, where the NV electronic spin serves as the computing qubit and a nearby nuclear spin as the memory qubit. Previous works used remote, weakly coupled 13C nuclear spins, trading read/write speed for long storage time. Here we focus instead on the intrinsic strongly coupled 14N nuclear spin. We first quantitatively understand its decoherence mechanism, identifying as its source the electronic spin that acts as a quantum fluctuator. We then propose a scheme to protect the quantum memory from the fluctuating noise by applying dynamical decoupling on the environment itself. We demonstrate a factor of 3 enhancement of the storage time in a proof-of-principle experiment, showing the potential for a quantum memory that combines fast operation with long coherence time.
    BibTeX:
    @article{Chen18,
      author = {Chen, M. and Sun, W.~K.~C. and Saha, K. and Jaskula, J.-C. and Cappellaro, P.},
      title = {Protecting solid-state spins from a strongly coupled environment},
      journal = {New J. Phys.},
      year = {2018},
      volume = {20},
      number = {6},
      pages = {063011},
      doi = {10.1088/1367-2630/aac542}
    }
    
  73. S. Hernández-Gómez, F. Poggiali, P. Cappellaro and N. Fabbri
  74. "Noise spectroscopy of a quantum-classical environment with a diamond qubit"
    Phys. Rev. B 98, 214307 (2018)
    Abstract: Knowing a quantum system's environment is critical for its practical use as a quantum device. Qubit sensors can reconstruct the noise spectral density of a classical bath, provided long enough coherence time. Here, we present a protocol that can unravel the characteristics of a more complex environment, comprising both unknown coherently coupled quantum systems, and a larger quantum bath that can be modeled as a classical stochastic field. We exploit the rich environment of a nitrogen-vacancy center in diamond, tuning the environment behavior with a bias magnetic field, to experimentally demonstrate our method. We show how to reconstruct the noise spectral density even when limited by relatively short coherence times, and identify the local spin environment. Importantly, we demonstrate that the reconstructed model can have predictive power, describing the spin qubit dynamics under control sequences not used for noise spectroscopy, a feature critical for building robust quantum devices. At lower bias fields, where the effects of the quantum nature of the bath are more pronounced, we find that more than a single classical noise model are needed to properly describe the spin coherence under different controls, due to the back action of the qubit onto the bath.
    BibTeX:
    @article{Hernandez18,
      author = {Hernández-Gómez, S. and Poggiali, F. and Cappellaro, P. and Fabbri, N.},
      title = {Noise spectroscopy of a quantum-classical environment with a diamond qubit},
      journal = {Phys. Rev. B},
      publisher = {American Physical Society},
      year = {2018},
      volume = {98},
      pages = {214307},
      doi = {10.1103/PhysRevB.98.214307}
    }
    
  75. M. Hirose and P. Cappellaro
  76. "Time-optimal control with finite bandwidth"
    Quantum Inf. Process. 17, 88 (2018)
    Abstract: Time-optimal control theory provides recipes to achieve quantum operations with high fidelity and speed, as required in quantum technologies such as quantum sensing and computation. While technical advances have achieved the ultrastrong driving regime in many physical systems, these capabilities have yet to be fully exploited for the precise control of quantum systems, as other limitations, such as the generation of higher harmonics or the finite response time of the control apparatus, prevent the implementation of theoretical time-optimal control. Here we present a method to achieve time-optimal control of qubit systems that can take advantage of fast driving beyond the rotating wave approximation. We exploit results from time-optimal control theory to design driving protocols that can be implemented with realistic, finite-bandwidth control fields, and we find a relationship between bandwidth limitations and achievable control fidelity.
    BibTeX:
    @article{Hirose18,
      author = {Hirose, M. and Cappellaro, P.},
      title = {Time-optimal control with finite bandwidth},
      journal = {Quantum Inf. Process.},
      year = {2018},
      volume = {17},
      pages = {88},
      doi = {10.1007/s11128-018-1845-6}
    }
    
  77. D. Layden and P. Cappellaro
  78. "Spatial noise filtering through error correction for quantum sensing"
    npj Quantum Information 4, 30 (2018)
    Abstract: Quantum systems can be used to measure various quantities in their environment with high precision. Often, however, their sensitivity is limited by the decohering effects of this same environment. Dynamical decoupling schemes are widely used to filter environmental noise from signals, but their performance is limited by the spectral properties of the signal and noise at hand. Quantum error correction schemes have therefore emerged as a complementary technique without the same limitations. To date, however, they have failed to correct the dominant noise type in many quantum sensors, which couples to each qubit in a sensor in the same way as the signal. Here we show how quantum error correction can correct for such noise, which dynamical decoupling can only partially address. Whereas dynamical decoupling exploits temporal noise correlations in signal and noise, our scheme exploits spatial correlations. We give explicit examples in small quantum devices and demonstrate a method by which error-correcting codes can be tailored to their noise.
    BibTeX:
    @article{Layden18,
      author = {Layden, David and Cappellaro, Paola},
      title = {Spatial noise filtering through error correction for quantum sensing},
      journal = {npj Quantum Information},
      year = {2018},
      volume = {4},
      number = {1},
      pages = {30},
      doi = {10.1038/s41534-018-0082-2}
    }
    
  79. L. Marseglia, K. Saha, A. Ajoy, T. Schroeder, D. Englund, F. Jelezko, R. Walsworth, J. L. Pacheco, D. L. Perry, E. S. Bielejec and P. Cappellaro
  80. "Bright nanowire single photon source based on SiV centers in diamond"
    Opt. Express 26, 80-89 (2018)
    Abstract: The practical implementation of many quantum technologies relies on the development of robust and bright single photon sources that operate at room temperature. The negatively charged silicon-vacancy (SiV-) color center in diamond is a possible candidate for such a single photon source. However, due to the high refraction index mismatch to air, color centers in diamond typically exhibit low photon out-coupling. An additional shortcoming is due to the random localization of native defects in the diamond sample. Here we demonstrate deterministic implantation of Si ions with high conversion efficiency to single SiV− centers, targeted to fabricated nanowires. The co-localization of single SiV- centers with the nanostructures yields a ten times higher light coupling efficiency than for single SiV- centers in bulk diamond. This enhanced photon out-coupling, together with the intrinsic scalability of the SiV− creation method, enables a new class of devices for integrated photonics and quantum science.
    BibTeX:
    @article{Marseglia18,
      author = {Marseglia, L. and Saha, K. and Ajoy, A. and Schroeder, T. and Englund, D. and Jelezko, F. and Walsworth, R. and Pacheco, J. L. and Perry, D. L. and Bielejec, E. S. and Cappellaro, P.},
      title = {Bright nanowire single photon source based on SiV centers in diamond},
      journal = {Opt. Express},
      publisher = {OSA},
      year = {2018},
      volume = {26},
      number = {1},
      pages = {80--89}
    }
    
  81. F. Poggiali, P. Cappellaro and N. Fabbri
  82. "Optimal Control for One-Qubit Quantum Sensing"
    Phys. Rev. X 8, 021059 (2018)
    Abstract: Quantum systems can be exquisite sensors thanks to their sensitivity to external perturbations. This same characteristic also makes them fragile to external noise. Quantum control can tackle the challenge of protecting a quantum sensor from environmental noise, while strongly coupling the sensor with the field to be measured. As the compromise between these two conflicting requirements does not always have an intuitive solution, optimal control based on a numerical search could prove very effective. Here, we adapt optimal control theory to the quantum-sensing scenario by introducing a cost function that, unlike the usual fidelity of operation, correctly takes into account both the field to be measured and the environmental noise. We experimentally implement this novel control paradigm using a nitrogen vacancy center in diamond, finding improved sensitivity to a broad set of time-varying fields. The demonstrated robustness and efficiency of the numerical optimization, as well as the sensitivity advantage it bestows, will prove beneficial to many quantum-sensing applications.
    BibTeX:
    @article{Poggiali18,
      author = {Poggiali, F. and Cappellaro, P. and Fabbri, N.},
      title = {Optimal Control for One-Qubit Quantum Sensing},
      journal = {Phys. Rev. X},
      publisher = {American Physical Society},
      year = {2018},
      volume = {8},
      pages = {021059},
      doi = {10.1103/PhysRevX.8.021059}
    }
    
  83. A. Sone, Q. Zhuang and P. Cappellaro
  84. "Quantifying precision loss in local quantum thermometry via diagonal discord"
    Phys. Rev. A 98, 012115 (2018)
    Abstract: When quantum information is spread over a system through nonclassical correlation, it makes retrieving information by local measurements difficult—making global measurement necessary for optimal parameter estimation. In this paper, we consider temperature estimation of a system in a Gibbs state and quantify the separation between the estimation performance of the global optimal measurement scheme and a greedy local measurement scheme by diagonal quantum discord. In a greedy local scheme, instead of global measurements, one performs sequential local measurement on subsystems, which is potentially enhanced by feed-forward communication. We show that, for finite-dimensional systems, diagonal discord quantifies the difference in the quantum Fisher information quantifying the precision limits for temperature estimation of these two schemes, and we analytically obtain the relation in the high-temperature limit. We further verify this result by employing the examples of spins with Heisenberg's interaction.
    BibTeX:
    @article{Sone18b,
      author = {Sone, Akira and Zhuang, Quntao and Cappellaro, Paola},
      title = {Quantifying precision loss in local quantum thermometry via diagonal discord},
      journal = {Phys. Rev. A},
      publisher = {American Physical Society},
      year = {2018},
      volume = {98},
      pages = {012115},
      doi = {10.1103/PhysRevA.98.012115}
    }
    
  85. K. X. Wei, C. Ramanathan and P. Cappellaro
  86. "Exploring Localization in Nuclear Spin Chains"
    Phys. Rev. Lett. 120, 070501 (2018)
    Abstract: Characterizing out-of-equilibrium many-body dynamics is a complex but crucial task for quantum applications and understanding fundamental phenomena. A central question is the role of localization in quenching thermalization in many-body systems and whether such localization survives in the presence of interactions. Probing this question in real systems necessitates the development of an experimentally measurable metric that can distinguish between different types of localization. While it is known that the localized phase of interacting systems [many-body localization (MBL)] exhibits a long-time logarithmic growth in entanglement entropy that distinguishes it from the noninteracting case of Anderson localization (AL), entanglement entropy is difficult to measure experimentally. Here, we present a novel correlation metric, capable of distinguishing MBL from AL in high-temperature spin systems. We demonstrate the use of this metric to detect localization in a natural solid-state spin system using nuclear magnetic resonance (NMR). We engineer the natural Hamiltonian to controllably introduce disorder and interactions, and observe the emergence of localization. In particular, while our correlation metric saturates for AL, it slowly keeps increasing for MBL, demonstrating analogous features to entanglement entropy, as we show in simulations. Our results show that our NMR techniques, akin to measuring out-of-time correlations, are well suited for studying localization in spin systems.
    BibTeX:
    @article{Wei18,
      author = {Wei, Ken Xuan and Ramanathan, Chandrasekhar and Cappellaro, Paola},
      title = {Exploring Localization in Nuclear Spin Chains},
      journal = {Phys. Rev. Lett.},
      publisher = {American Physical Society},
      year = {2018},
      volume = {120},
      pages = {070501},
      doi = {10.1103/PhysRevLett.120.070501}
    }
    

    2017

    UP ↑

  87. A. Ajoy, Y.-X. Liu, K. Saha, L. Marseglia, J.-C. Jaskula, U. Bissbort and P. Cappellaro
  88. "Quantum interpolation for high-resolution sensing"
    Proc. Nat. Acad. Sc. 114, 2149–2153 (2017)
    Abstract: Recent advances in engineering and control of nanoscale quantum sensors have opened new paradigms in precision metrology. Unfortunately, hardware restrictions often limit the sensor performance. In nanoscale magnetic resonance probes, for instance, finite sampling times greatly limit the achievable sensitivity and spectral resolution. Here we introduce a technique for coherent quantum interpolation that can overcome these problems. Using a quantum sensor associated with the nitrogen vacancy center in diamond, we experimentally demonstrate that quantum interpolation can achieve spectroscopy of classical magnetic fields and individual quantum spins with orders of magnitude finer frequency resolution than conventionally possible. Not only is quantum interpolation an enabling technique to extract structural and chemical information from single biomolecules, but it can be directly applied to other quantum systems for superresolution quantum spectroscopy.
    BibTeX:
    @article{Ajoy17,
      author = {Ajoy, Ashok and Liu, Yi-Xiang and Saha, Kasturi and Marseglia, Luca and Jaskula, Jean-Christophe and Bissbort, Ulf and Cappellaro, Paola},
      title = {Quantum interpolation for high-resolution sensing},
      journal = {Proc. Nat. Acad. Sc.},
      year = {2017},
      volume = {114},
      number = {9},
      pages = {2149–2153},
      doi = {10.1073/pnas.1610835114}
    }
    
  89. S. T. Alsid
  90. "Optimizing chemical-vapor-deposition diamond for nitrogen-vacancy center ensemble magnetometry"
    Thesis at: Massachusetts Institute of Technology, (2017)
    BibTeX:
    @mastersthesis{Alsid17t,
      author = {Alsid, Scott T},
      title = {Optimizing chemical-vapor-deposition diamond for nitrogen-vacancy center ensemble magnetometry},
      school = {Massachusetts Institute of Technology},
      year = {2017}
    }
    
  91. A. Sone and P. Cappellaro
  92. "Hamiltonian identifiability assisted by a single-probe measurement"
    Phys. Rev. A 95, 022335 (2017)
    Abstract: We study the Hamiltonian identifiability of a many-body spin-1/2 system assisted by the measurement on a single quantum probe based on the eigensystem realization algorithm approach employed in Zhang and Sarovar, Phys. Rev. Lett. 113, 080401 (2014). We demonstrate a potential application of Gröbner basis to the identifiability test of the Hamiltonian, and provide the necessary experimental resources, such as the lower bound in the number of the required sampling points, the upper bound in total required evolution time, and thus the total measurement time. Focusing on the examples of the identifiability in the spin-chain model with nearest-neighbor interaction, we classify the spin-chain Hamiltonian based on its identifiability, and provide the control protocols to engineer the nonidentifiable Hamiltonian to be an identifiable Hamiltonian.
    BibTeX:
    @article{Sone17,
      author = {Sone, Akira and Cappellaro, Paola},
      title = {Hamiltonian identifiability assisted by a single-probe measurement},
      journal = {Phys. Rev. A},
      year = {2017},
      volume = {95},
      pages = {022335},
      doi = {10.1103/PhysRevA.95.022335}
    }
    
  93. A. Sone and P. Cappellaro
  94. "Exact dimension estimation of interacting qubit systems assisted by a single quantum probe"
    Phys. Rev. A 96, 062334 (2017)
    Abstract: Estimating the dimension of an Hilbert space is an important component of quantum system identification. In quantum technologies, the dimension of a quantum system (or its corresponding accessible Hilbert space) is an important resource, as larger dimensions determine, e.g., the performance of quantum computation protocols or the sensitivity of quantum sensors. Despite being a critical task in quantum system identification, estimating the Hilbert space dimension is experimentally challenging. While there have been proposals for various dimension witnesses capable of putting a lower bound on the dimension from measuring collective observables that encode correlations, in many practical scenarios, especially for multiqubit systems, the experimental control might not be able to engineer the required initialization, dynamics, and observables. Here we propose a more practical strategy that relies not on directly measuring an unknown multiqubit target system, but on the indirect interaction with a local quantum probe under the experimenter's control. Assuming only that the interaction model is given and the evolution correlates all the qubits with the probe, we combine a graph-theoretical approach and realization theory to demonstrate that the system dimension can be exactly estimated from the model order of the system. We further analyze the robustness in the presence of background noise of the proposed estimation method based on realization theory, finding that despite stringent constrains on the allowed noise level, exact dimension estimation can still be achieved.
    BibTeX:
    @article{Sone17a,
      author = {Sone, Akira and Cappellaro, Paola},
      title = {Exact dimension estimation of interacting qubit systems assisted by a single quantum probe},
      journal = {Phys. Rev. A},
      publisher = {American Physical Society},
      year = {2017},
      volume = {96},
      pages = {062334},
      doi = {10.1103/PhysRevA.96.062334}
    }
    
  95. C. L. Degen, F. Reinhard and P. Cappellaro
  96. "Quantum sensing"
    Rev. Mod. Phys. 89, 035002 (2017)
    Abstract: “Quantum sensing” describes the use of a quantum system, quantum properties, or quantum phenomena to perform a measurement of a physical quantity. Historical examples of quantum sensors include magnetometers based on superconducting quantum interference devices and atomic vapors or atomic clocks. More recently, quantum sensing has become a distinct and rapidly growing branch of research within the area of quantum science and technology, with the most common platforms being spin qubits, trapped ions, and flux qubits. The field is expected to provide new opportunities—especially with regard to high sensitivity and precision—in applied physics and other areas of science. This review provides an introduction to the basic principles, methods, and concepts of quantum sensing from the viewpoint of the interested experimentalist.
    BibTeX:
    @article{Degen17,
      author = {Degen, C. L. and Reinhard, F. and Cappellaro, P.},
      title = {Quantum sensing},
      journal = {Rev. Mod. Phys.},
      publisher = {American Physical Society},
      year = {2017},
      volume = {89},
      pages = {035002},
      doi = {10.1103/RevModPhys.89.035002}
    }
    
  97. F. Poggiali, P. Cappellaro and N. Fabbri
  98. "Measurement of the excited-state transverse hyperfine coupling in NV centers via dynamic nuclear polarization"
    Phys. Rev. B 95, 195308 (2017)
    Abstract: Precise knowledge of a quantum system's Hamiltonian is a critical pre-requisite for its use in many quantum information technologies. Here, we report a method for the precise characterization of the nonsecular part of the excited-state Hamiltonian of an electronic-nuclear spin system in diamond. The method relies on the investigation of the dynamic nuclear polarization mediated by the electronic spin, which is currently exploited as a primary tool for initializing nuclear qubits and performing enhanced nuclear magnetic resonance. By measuring the temporal evolution of the population of the ground-state hyperfine levels of a nitrogen-vacancy center, we obtain the first direct estimation of the excited-state transverse hyperfine coupling between its electronic and nitrogen nuclear spin. Our method could also be applied to other electron-nuclear spin systems, such as those related to defects in silicon carbide.
    BibTeX:
    @article{Poggiali17,
      author = {Poggiali, F. and Cappellaro, P. and Fabbri, N.},
      title = {Measurement of the excited-state transverse hyperfine coupling in NV centers via dynamic nuclear polarization},
      journal = {Phys. Rev. B},
      year = {2017},
      volume = {95},
      number = {19},
      pages = {195308},
      doi = {10.1103/PhysRevB.95.195308}
    }
    

    2016

    UP ↑

  99. A. Ajoy, Y. X. Liu and P. Cappellaro
  100. "DC Magnetometry at the T2 Limit"
    ArXiv:1611.04691 (2016)
    Abstract: Sensing static or slowly varying magnetic fields with high sensitivity and spatial resolution is critical to many applications in fundamental physics, bioimaging and materials science. Several versatile magnetometry platforms have emerged over the past decade, such as electronic spins associated with Nitrogen Vacancy (NV) centers in diamond. However, their high sensitivity to external fields also makes them poor sensors of DC fields. Indeed, the usual method of Ramsey magnetometry leaves them prone to environmental noise, limiting the allowable interrogation time to the short dephasing time T2*. Here we introduce a hybridized magnetometery platform, consisting of a sensor and ancilla, that allows sensing static magnetic fields with interrogation times up to the much longer T2 coherence time, allowing significant potential gains in field sensitivity. While more generally applicable, we demonstrate the method for an electronic NV sensor and a nuclear ancilla. It relies on frequency upconversion of transverse DC fields through the ancilla, allowing quantum lock-in detection with low-frequency noise rejection. In our experiments, we demonstrate sensitivities better than 6uT/vHz, comparable to the Ramsey method, and narrow-band signal noise filtering better than 64kHz. With technical optimization, we expect more than an one order of magnitude improvement in each of these parameters. Since our method measures transverse fields, in combination with the Ramsey detection of longitudinal fields, it ushers in a compelling technique for sensitive vector DC magnetometry at the nanoscale.
    BibTeX:
    @article{Ajoy16xx,
      author = {Ajoy, A. and Liu, Y.~X. and Cappellaro, P.},
      title = {DC Magnetometry at the T2 Limit},
      journal = {ArXiv:1611.04691 },
      year = {2016}
    }
    
  101. L. Marseglia, K. Saha, A. Ajoy, T. Schroder, D. R. Englund, T. TERAJI, junichi isoya, F. Jelezko, R. Walsworth, J. L. Pacheco, D. Perry, E. Bielejec and P. Cappellaro
  102. "A bright nanowire single photon source"
    Conference on Lasers and Electro-Optics In Conference on Lasers and Electro-Optics , FTu3D.1 (2016)
    Abstract: Silicon-vacancy (SiV) centers in diamond are bright sources of indistinguishable single photons. We report fabrication of nanowires coupled to single SiV by deterministic ion implantation, yielding greatly enhanced light coupling compared to SiV in bulk.
    BibTeX:
    @inproceedings{Marseglia16,
      author = {Luca Marseglia and Kasturi Saha and Ashok Ajoy and Tim Schroder and Dirk R. Englund and Tokuyuki TERAJI and junichi isoya and Fedor Jelezko and Ronald Walsworth and Jose L. Pacheco and Daniel Perry and Edward Bielejec and Paola Cappellaro},
      title = {A bright nanowire single photon source},
      booktitle = {Conference on Lasers and Electro-Optics},
      journal = {Conference on Lasers and Electro-Optics},
      publisher = {Optical Society of America},
      year = {2016},
      pages = {FTu3D.1},
      doi = {10.1364/CLEO_QELS.2016.FTu3D.1}
    }
    
  103. A. Ajoy
  104. "Quantum Assisted Sensing, Simulation and Control"
    Thesis at: Massachusetts Institute of Technology, (2016)
    Abstract: This thesis describes experimental and theoretical work making contributions in the general areas of quantum
    metrology, simulation and control. Specifically, we describe new approaches for high resolution magnetic
    resonance imaging using quantum sensors, that could potentially achieve the determination of single molecular
    structure. We show experiments that boost the sensing resolution of these sensors by over a factor of 100. We
    also develop separate methods for the sensitive detection of DC magnetic fields, and rotations. Also developed
    are techniques for the engineering of many-body Hamiltonians with restricted just global control fields, a task of
    wide interest for quantum simulation. Finally the toolbox available for quantum control is expanded for various
    applications in quantum information processing and simulation.
    BibTeX:
    @phdthesis{Ajoy16t,
      author = {Ajoy, Ashok},
      title = {Quantum Assisted Sensing, Simulation and Control},
      school = {Massachusetts Institute of Technology},
      year = {2016}
    }
    
  105. A. Cooper-Roy
  106. "Coherent control of electron spins in diamond for quantum information science and quantum sensing"
    Thesis at: Massachusetts Institute of Technology, (2016)
    Abstract: This thesis introduces and experimentally demonstrates coherent control techniques
    to exploit electron spins in diamond for applications in quantum information processing
    and quantum sensing. Specifically, optically-detected magnetic double resonance
    measurements are performed on quantum states of single and multiple electron spins
    associated with nitrogen-vacancy centers and paramagnetic centers in synthetic diamond
    crystals. The Walsh reconstruction method is introduced as a general framework
    to estimate the parameters of deterministic and stochastic time-varying fields
    with quantum sensors. The Walsh method generalizes sampling techniques based
    on dynamical decoupling sequences and enables measuring the temporal profile of
    time-varying magnetic fields with increased precision in the presence of noise and
    environmental fluctuations. Coherent control techniques are further introduced to
    identify, integrate, and exploit unknown quantum systems located in the environment
    of a quantum probe. In particular, entangled states of two and three electron
    spins in diamond are created to estimate the amplitude of time-varying magnetic
    fields. These results demonstrate a scalable approach to measure time-varying fields
    with quantum probes in solid-state materials for applications in environment-assisted
    quantum metrology, such as real-time functional imaging of neural activity at the
    level of a single neuron, magnetic resonance spectroscopy and imaging of biological
    complexes in living cells; and characterization of the structure and dynamics of new
    magnetic materials.
    BibTeX:
    @phdthesis{Cooper16t,
      author = {Cooper-Roy, Alexandre},
      title = {Coherent control of electron spins in diamond for quantum information science and quantum sensing},
      school = {Massachusetts Institute of Technology},
      year = {2016}
    }
    
  107. M. Hirose and P. Cappellaro
  108. "Coherent feedback control of a single qubit in diamond"
    Nature 532, 77-80 (2016)
    Abstract: Engineering desired operations on qubits subjected to the deleterious effects of their environment is a critical task in quantum information processing, quantum simulation and sensing. The most common approach relies on open-loop quantum control techniques, including optimal-control algorithms based on analytical or numerical solutions, Lyapunov design3 and Hamiltonian engineering. An alternative strategy, inspired by the success of classical control, is feedback contro. Because of the complications introduced by quantum measurement, closed-loop control is less pervasive in the quantum setting and, with exceptions, its experimental implementations have been mainly limited to quantum optics experiments. Here we implement a feedback-control algorithm using a solid-state spin qubit system associated with the nitrogen vacancy centre in diamond, using coherent feedback to overcome the limitations of measurement-based feedback, and show that it can protect the qubit against intrinsic dephasing noise for milliseconds. In coherent feedback, the quantum system is connected to an auxiliary quantum controller (ancilla) that acquires information about the output state of the system (by an entangling operation) and performs an appropriate feedback action (by a conditional gate). In contrast to open-loop dynamical decoupling techniques, feedback control can protect the qubit even against Markovian noise and for an arbitrary period of time (limited only by the coherence time of the ancilla), while allowing gate operations. It is thus more closely related to quantum error-correction schemes, although these require larger and increasing qubit overheads. Increasing the number of fresh ancillas enables protection beyond their coherence time. We further evaluate the robustness of the feedback protocol, which could be applied to quantum computation and sensing, by exploring a trade-off between information gain and decoherence protection, as measurement of the ancilla-qubit correlation after the feedback algorithm voids the protection, even if the rest of the dynamics is unchanged.
    BibTeX:
    @article{Hirose16,
      author = {Hirose, Masashi and Cappellaro, Paola},
      title = {Coherent feedback control of a single qubit in diamond},
      journal = {Nature},
      year = {2016},
      volume = {532},
      number = {7597},
      pages = {77--80},
      doi = {10.1038/nature17404}
    }
    
  109. L. M. Pham, S. J. DeVience, F. Casola, I. Lovchinsky, A. O. Sushkov, E. Bersin, J. Lee, E. Urbach, P. Cappellaro, H. Park, A. Yacoby, M. Lukin and R. L. Walsworth
  110. "NMR technique for determining the depth of shallow nitrogen-vacancy centers in diamond"
    Phys. Rev. B 93, 045425 (2016)
    Abstract: We demonstrate a robust experimental method for determining the depth of individual shallow nitrogen-vacancy (NV) centers in diamond with ~1nm uncertainty. We use a confocal microscope to observe single NV centers and detect the proton nuclear magnetic resonance (NMR) signal produced by objective immersion oil, which has well understood nuclear spin properties, on the diamond surface. We determine the NV center depth by analyzing the NV NMR data using a model that describes the interaction of a single NV center with the statistically polarized proton spin bath. We repeat this procedure for a large number of individual, shallow NV centers and compare the resulting NV depths to the mean value expected from simulations of the ion implantation process used to create the NV centers, with reasonable agreement.
    BibTeX:
    @article{Pham16,
      author = {Pham, Linh M. and DeVience, Stephen J. and Casola, Francesco and Lovchinsky, Igor and Sushkov, Alexander O. and Bersin, Eric and Lee, Junghyun and Urbach, Elana and Cappellaro, Paola and Park, Hongkun and Yacoby, Amir and Lukin, Mikhail and Walsworth, Ronald L.},
      title = {NMR technique for determining the depth of shallow nitrogen-vacancy centers in diamond},
      journal = {Phys. Rev. B},
      publisher = {American Physical Society},
      year = {2016},
      volume = {93},
      pages = {045425},
      doi = {10.1103/PhysRevB.93.045425}
    }
    

    2015

    UP ↑

  111. M. Chen, M. Hirose and P. Cappellaro
  112. "Measurement of transverse hyperfine interaction by forbidden transitions"
    Phys. Rev. B 92, 020101 (2015)
    Abstract: Precise characterization of a system's Hamiltonian is crucial to its high-fidelity control that would enable many quantum technologies, ranging from quantum computation to communication and sensing. In particular, non-secular parts of the Hamiltonian are usually more difficult to characterize, even if they can give rise to subtle but non-negligible effects. Here we present a strategy for the precise estimation of the transverse hyperfine coupling between an electronic and a nuclear spin, exploiting effects due to forbidden transitions during the Rabi driving of the nuclear spin. We applied the method to precisely determine the transverse coupling between a Nitrogen-Vacancy center electronic spin and its Nitrogen nuclear spin. In addition, we show how this transverse hyperfine, that has been often neglected in experiments, is crucial to achieving large enhancements of the nuclear Rabi driving.
    BibTeX:
    @article{Chen15,
      author = {Chen, Mo and Hirose, Masashi and Cappellaro, Paola},
      title = {Measurement of transverse hyperfine interaction by forbidden transitions},
      journal = {Phys. Rev. B},
      publisher = {American Physical Society},
      year = {2015},
      volume = {92},
      pages = {020101},
      doi = {10.1103/PhysRevB.92.020101}
    }
    
  113. K. Arai, C. Belthangady, H. Zhang, N. Bar-Gill, S. DeVience, P. Cappellaro, A. Yacoby and R. Walsworth
  114. "Fourier magnetic imaging with nanoscale resolution and compressed sensing speed-up using electronic spins in diamond"
    Nat Nano 10, 859-864 (2015)
    Abstract: Optically detected magnetic resonance using nitrogen?vacancy (NV) colour centres in diamond is a leading modality for nanoscale magnetic field imaging, as it provides single electron spin sensitivity, three-dimensional resolution better than 1?nm and applicability to a wide range of physical and biological, samples under ambient conditions. To date, however, NV-diamond magnetic imaging has been performed using 'real-space' techniques, which are either limited by optical diffraction to ~250?nm resolution or require slow, point-by-point scanning for nanoscale resolution, for example, using an atomic force microscope, magnetic tip, or super-resolution optical imaging. Here, we introduce an alternative technique of Fourier magnetic imaging using NV-diamond. In analogy with conventional magnetic resonance imaging (MRI), we employ pulsed magnetic field gradients to phase-encode spatial information on NV electronic spins in wavenumber or 'k-space' followed by a fast Fourier transform to yield real-space images with nanoscale resolution, wide field of view and compressed sensing speed-up.
    BibTeX:
    @article{Arai15,
      author = {Arai, K. and Belthangady, C. and Zhang, H. and Bar-Gill, N. and DeVience, S.J. and Cappellaro, P. and Yacoby, A. and Walsworth, R.L.},
      title = {Fourier magnetic imaging with nanoscale resolution and compressed sensing speed-up using electronic spins in diamond},
      journal = {Nat Nano},
      publisher = {Nature Publishing Group},
      year = {2015},
      volume = {10},
      pages = {859--864}
    }
    
  115. C. Aiello, M. Allegra, B. Hemmerling, X. Wan and P. Cappellaro
  116. "Algebraic synthesis of time-optimal unitaries in SU(2) with alternating controls"
    Quantum Information Processing 14, 3233 (2015)
    Abstract: We present an algebraic framework to study the time-optimal synthesis of arbitrary unitaries in SU(2), when the control set is restricted to rotations around two non-parallel axes in the Bloch sphere. Our method bypasses commonly used control-theoretical techniques and easily imposes necessary conditions on time-optimal sequences. In a straightforward fashion, we prove that time-optimal sequences are solely parametrized by three rotation angles and derive general bounds on those angles as a function of the relative rotation speed of each control and the angle between the axes. Results are substantially different whether both clockwise and counterclockwise rotations about the given axes are allowed, or only clockwise rotations. In the first case, we prove that any finite time-optimal sequence is composed at most of five control concatenations, while for the more restrictive case, we present scaling laws on the maximum length of any finite time-optimal sequence. The bounds we find for both cases are stricter than previously published ones and severely constrain the structure of time-optimal sequences, allowing for an efficient numerical search of the time-optimal solution. Our results can be used to find the time-optimal evolution of qubit systems under the action of the considered control set and thus potentially increase the number of realizable unitaries before decoherence.
    BibTeX:
    @article{Aiello15q,
      author = {Aiello, ClariceD. and Allegra, Michele and Hemmerling, Boerge and Wan, Xiaoting and Cappellaro, Paola},
      title = {Algebraic synthesis of time-optimal unitaries in SU(2) with alternating controls},
      journal = {Quantum Information Processing},
      publisher = {Springer US},
      year = {2015},
      volume = {14}
      pages = {3233},
      doi = {10.1007/s11128-015-1045-6}
    }
    
  117. P. Cappellaro
  118. "Polarizing Nuclear Spins in Silicon Carbide"
    Physics , 56 (2015)
    Abstract: An optical technique polarizes the spin of nuclei in silicon carbide, offering a potential new route to
    nuclear spin-based quantum memory.
    A Viewpoint on:
    Optical Polarization of Nuclear Spins in Silicon Carbide
    Abram L. Falk, Paul V. Klimov, Viktor Ivady, Krisztian Szasz, David J. Christle, William F. Koehl, Adam Gali, and, and
    David D. Awschalom
    Physical Review Letters 114, 247603 2015
    BibTeX:
    @article{Cappellaro15,
      author = {Cappellaro, P.},
      title = {Polarizing Nuclear Spins in Silicon Carbide},
      journal = {Physics},
      year = {2015},
      number = {8},
      pages = {56},
      doi = {10.1103/Physics.8.56}
    }
    
  119. A. Ajoy, U. Bissbort, M. D. Lukin, R. L. Walsworth and P. Cappellaro
  120. "Atomic-Scale Nuclear Spin Imaging Using Quantum-Assisted Sensors in Diamond"
    Phys. Rev. X 5, 011001 (2015)
    Abstract: Nuclear spin imaging at the atomic level is essential for the understanding of fundamental biological phenomena and for applications such as drug discovery. The advent of novel nanoscale sensors promises to achieve the long-standing goal of single-protein, high spatial-resolution structure determination under ambient conditions. In particular, quantum sensors based on the spin-dependent photoluminescence of nitrogen-vacancy (NV) centers in diamond have recently been used to detect nanoscale ensembles of external nuclear spins. While NV sensitivity is approaching single-spin levels, extracting relevant information from a very complex structure is a further challenge since it requires not only the ability to sense the magnetic field of an isolated nuclear spin but also to achieve atomic-scale spatial resolution. Here, we propose a method that, by exploiting the coupling of the NV center to an intrinsic quantum memory associated with the nitrogen nuclear spin, can reach a tenfold improvement in spatial resolution, down to atomic scales. The spatial resolution enhancement is achieved through coherent control of the sensor spin, which creates a dynamic frequency filter selecting only a few nuclear spins at a time. We propose and analyze a protocol that would allow not only sensing individual spins in a complex biomolecule, but also unraveling couplings among them, thus elucidating local characteristics of the molecule structure.
    BibTeX:
    @article{Ajoy15,
      author = {Ajoy, A. and Bissbort, U. and Lukin, M.D. and Walsworth, R.L. and Cappellaro, P.},
      title = {Atomic-Scale Nuclear Spin Imaging Using Quantum-Assisted Sensors in Diamond},
      journal = {Phys. Rev. X},
      publisher = {American Physical Society},
      year = {2015},
      volume = {5},
      pages = {011001},
      doi = {10.1103/PhysRevX.5.011001}
    }
    
  121. C. D. Aiello and P. Cappellaro
  122. "Time-optimal control by a quantum actuator"
    Phys. Rev. A 91, 042340 (2015)
    Abstract: Indirect control of qubits by a quantum actuator has been proposed as an appealing strategy to manipulate qubits that couple only weakly to external fields. While universal quantum control can be easily achieved when the actuator-qubit coupling is anisotropic, the efficiency of this approach is less clear. Here we analyze the time efficiency of quantum actuator control. We describe a strategy to find time-optimal control sequences by the quantum actuator and compare their gate times with direct driving, identifying regimes where the actuator control performs faster. As a paradigmatic example, we focus on a specific implementation based on the nitrogen-vacancy center electronic spin in diamond (the actuator) and nearby C13 nuclear spins (the qubits).
    BibTeX:
    @article{Aiello15,
      author = {Aiello, Clarice D. and Cappellaro, Paola},
      title = {Time-optimal control by a quantum actuator},
      journal = {Phys. Rev. A},
      publisher = {American Physical Society},
      year = {2015},
      volume = {91},
      pages = {042340},
      doi = {10.1103/PhysRevA.91.042340}
    }
    
  123. M. Hirose
  124. "Quantum Control of Spin Systems in diamond"
    Thesis at: Massachusetts Institute of Technology,
    Department of Nuclear Science and Engineering (2015)
    Abstract: The precise control of a system which behaves according to the principles of quantum mechanics is an indispensable task in order to fully harness unique properties of quantum mechanics, such as superposition and entanglement, for practical applications. Leveraging the quantum nature of the system would enable for example the implementation of quantum computation and quantum metrology. However, any realistic quantum system is inevitably coupled to its environment. The interaction with its surroundings irrevocably destroys the quantum nature of the system: mitigating decoherence is thus one of the central problems in quantum control. In this thesis, we develop novel control methods to protect a qubit from decoherence by two distinct approaches and demonstrate them experimentally using the nitrogen-vacancy (NV) center in diamond.

    The first method rests on an open-loop control scheme and it is tailored to improve quantum sensing tasks. We develop a continuous dynamical decoupling (CoDD) method that allows us to tune the degree of protection from a dephasing environment. Exploiting this flexibility, we show that the CoDD can be used to measure magnetic fields with sensitivity comparable to existing methods, while providing superior versatility in practical experimental settings. This protocol can adapt to various sensing conditions, such as measurement time and sensitive frequency, that might occur in biological and material science.

    The second method exploits a coherent feedback protocol. We take advantage of a long-lived nuclear spin as an ancillary spin to protect the qubit of interest from decoherence. We show that the protocol protects the qubit as long as open-loop dynamical decoupling control schemes and it can be used against more general types of noise than the open-loop protocol. This method thus offers an alternative protocol to protect the qubit from decoherence in quantum computation and quantum metrology.

    BibTeX:
    @phdthesis{Hirose15t,
      author = {Hirose, Masashi},
      title = {Quantum Control of Spin Systems in diamond},
      school = {Massachusetts Institute of Technology},
      year = {2015}
    }
    
  125. N. Lopez
  126. "All-Optical Method of Nanoscale Magnetometry for Ensembles of Nitrogen-Vacancy Defects in Diamond"
    B.S. Thesis at: Massachusetts Institute of Technology,
    Department of Nuclear Science & Engineering (2015)
    Abstract In this thesis, we discuss two problems of quantum dynamics in the presence of alternating controls. Alternating controls arise in many protocols designed to extend the duration over which a qubit is a useful computational resource. This is accomplished by control sequences that either retard decoherence, or that accomplish a quantum operation in as short a time as possible. The first problem tackles the use of a composite-pulse control sequence known as 'rotary-echo' for quantum magnetometry purposes. The sequence consists in the continuous drive of a qubit, with field phases that alternate at specific intervals. We implement such a magnetometry protocol using an electronic qubit in diamond, and experimentally confirm the flexibility yielded by the tuning of sequence parameters that achieves a good compromise between decoherence resilience and sensitivity. The second problem theoretically investigates the time-optimal evolution of a qubit in the case of a restricted control set composed of alternating rotations around two non-parallel axes on the Bloch sphere. Using accessible algebraic methods, we show that experimental parameters, such as the angle between the two rotation axes, restrict the necessary structure of time-optimal sequences. We propose to implement such an evolution through alternate driving as an advantageous alternative to the slow, noisy direct addressing of a nuclear qubit anisotropically hyperfine-coupled to an electronic spin in diamond.
    BibTeX:
    @phdthesis{Lopez15t,
      author = {Nicolas Lopez},
      title = {All-Optical Method of Nanoscale Magnetometry
    for Ensembles of Nitrogen-Vacancy Defects in
    Diamond},
      school = {Massachusetts Institute of Technology},
      year = {2014}
    }
    

    2014

    UP ↑

  127. A. Cooper, E. Magesan, H. Yum and P. Cappellaro
  128. "Time-resolved magnetic sensing with electronic spins in diamond"
    Nature Comms. 5, 3141 (2014)
    Abstract: Quantum probes enable the sensitive detection of time-varying fields with high spatial resolution, opening the possibility to explore biological functions as well as materials and physical phenomena at the nanometer scale. In particular, nitrogen-vacancy (NV) centers in diamond have recently emerged as promising sensors of magnetic and electric fields. Although coherent control techniques have measured the amplitude of constant or oscillating fields, these techniques are unable to measure time-varying fields with unknown dynamics. Here we introduce a coherent acquisition method to accurately reconstruct the arbitrary profile of time-varying fields using coherent control sequences associated with the Walsh functions. These sequences act as digital filters that efficiently extract information about the dynamics of the field while suppressing decoherence. We experimentally demonstrate the Walsh reconstruction method by performing proof-of-principle reconstruction of the magnetic field radiated by a physical model of a neuron using a single electronic spin in diamond. These results will be useful for performing time-resolved magnetic sensing with quantum probes in a broad array of physical and biological systems at the nanometer scale.
    BibTeX:
    @article{Cooper14,
      author = {Cooper, A. and Magesan, E. and Yum, H.N. and Cappellaro, P.},
      title = {Time-resolved magnetic sensing with electronic spins in diamond},
      journal = {Nature Comms.},  
      year = {2014},  
      volume = {5},
      pages = {3141},
      doi = {10.1038/ncomms4141}
    }
    
  129. P. Cappellaro
  130. "Implementation of State Transfer Hamiltonians in Spin Chains with Magnetic Resonance Techniques"
    In Quantum State Transfer and Network Engineering , 183-222 (2014)
    Abstract: Nuclear spin systems and magnetic resonance techniques have provided a fertile platform for experimental investigation of quantum state transfer in spin chains. From the first observation of polarization transfer, predating the formal definition of quantum state transfer, to the realization of state transfer simulations in small molecules and in larger solid-state spin systems, the experiments have drawn on the strengths of nuclear magnetic resonance (NMR), in particular on its long history of well-developed control techniques. NMR implementations have been invaluable both as proof-of-principle demonstrations of quantum state transfer protocols and to explore dynamics occurring in real systems that go beyond what can be analytically solved or numerically simulated. In addition, control techniques developed in these systems to engineer the Hamiltonians required for transport can be adopted in potentially scalable quantum information processing architectures. In this contribution we describe recent results and outline future directions of research in magnetic-resonance based implementation of quantum state transfer in spin chains.
    BibTeX:
    @incollection{Cappellaro14,
      author = {Cappellaro, Paola},
      editor = {Nikolopoulos, Georgios M. and Jex, Igor},
      title = {Implementation of State Transfer Hamiltonians in Spin Chains with Magnetic Resonance Techniques},
      booktitle = {Quantum State Transfer and Network Engineering},
      publisher = {Springer Berlin Heidelberg},
      year = {2014},
      pages = {183-222},
      doi = {10.1007/978-3-642-39937-4_6}
    }
    
  131. C. D. Aiello
  132. "Qubit Dynamics under Alternating Controls"
    Thesis at: Massachusetts Institute of Technology,
    Department of Electrical Engineering & Computer Science and Engineering (2014)
    Abstract In this thesis, we discuss two problems of quantum dynamics in the presence of alternating controls. Alternating controls arise in many protocols designed to extend the duration over which a qubit is a useful computational resource. This is accomplished by control sequences that either retard decoherence, or that accomplish a quantum operation in as short a time as possible. The first problem tackles the use of a composite-pulse control sequence known as 'rotary-echo' for quantum magnetometry purposes. The sequence consists in the continuous drive of a qubit, with field phases that alternate at specific intervals. We implement such a magnetometry protocol using an electronic qubit in diamond, and experimentally confirm the flexibility yielded by the tuning of sequence parameters that achieves a good compromise between decoherence resilience and sensitivity. The second problem theoretically investigates the time-optimal evolution of a qubit in the case of a restricted control set composed of alternating rotations around two non-parallel axes on the Bloch sphere. Using accessible algebraic methods, we show that experimental parameters, such as the angle between the two rotation axes, restrict the necessary structure of time-optimal sequences. We propose to implement such an evolution through alternate driving as an advantageous alternative to the slow, noisy direct addressing of a nuclear qubit anisotropically hyperfine-coupled to an electronic spin in diamond.
    BibTeX:
    @phdthesis{Aiello14t,
      author = {Clarice D. Aiello},
      title = {Qubit dynamics under alternating controls},
      school = {Massachusetts Institute of Technology},
      year = {2014}
    }
    

    2013

    UP ↑

  133. E. Magesan, A. Cooper and P. Cappellaro
  134. "Compressing measurements in quantum dynamic parameter estimation,"
    Phys. Rev A 88, 062109 (2013)
    Abstract: We present methods that can provide an exponential savings in the resources required to perform dynamic parameter estimation using quantum systems. The key idea is to merge classical compressive sensing techniques with quantum control methods to efficiently estimate time-dependent parameters in the system Hamiltonian. We show that incoherent measurement bases and, more generally, suitable random measurement matrices can be created by performing simple control sequences on the quantum system. Since random measurement matrices satisfying the restricted isometry property can be used to reconstruct any sparse signal in an efficient manner, and many physical processes are approximately sparse in some basis, these methods can potentially be useful in a variety of applications such as quantum sensing and magnetometry. We illustrate the theoretical results throughout the presentation with various practically relevant numerical examples.
    BibTeX: @article{Magesan13c, author = {Magesan, E. and Cooper, A and Yum, H.N. and Cappellaro, P.}, title = {Compressing measurements in quantum dynamic parameter estimation}, journal = {Phys. Rev. A}, abstract = {We present methods that can provide an exponential savings in the resources required to perform dynamic parameter estimation using quantum systems. The key idea is to merge classical compressive sensing techniques with quantum control methods to efficiently estimate time-dependent parameters in the system Hamiltonian. We show that incoherent measurement bases and, more generally, suitable random measurement matrices can be created by performing simple control sequences on the quantum system. Since random measurement matrices satisfying the restricted isometry property can be used to reconstruct any sparse signal in an efficient manner, and many physical processes are approximately sparse in some basis, these methods can potentially be useful in a variety of applications such as quantum sensing and magnetometry. We illustrate the theoretical results throughout the presentation with various practically relevant numerical examples.}, year = {2013}, volume = {88}, pages = {062109}, doi = {10.1103/PhysRevA.88.062109}, url={http://link.aps.org/doi/10.1103/PhysRevA.88.062109} }
  135. C. D. Aiello, M. Hirose and P. Cappellaro
  136. "Composite-pulse magnetometry with a solid-state quantum sensor"
    Nat. Commun. 4, 1419- (2013)
    Abstract: The sensitivity of quantum magnetometer is challenged by control errors and, especially in the solid state, by their short coherence times. Refocusing techniques can overcome these limitations and improve the sensitivity to periodic fields, but they come at the cost of reduced bandwidth and cannot be applied to sense static or aperiodic fields. Here we experimentally demonstrate that continuous driving of the sensor spin by a composite pulse known as rotary-echo yields a flexible magnetometry scheme, mitigating both driving power imperfections and decoherence. A suitable choice of rotary-echo parameters compensates for different scenarios of noise strength and origin. The method can be applied to nanoscale sensing in variable environments or to realize noise spectroscopy. In a room-temperature implementation, based on a single electronic spin in diamond, composite-pulse magnetometry provides a tunable trade-off between sensitivities in the mTHz^1/2 range, comparable with those obtained with Ramsey spectroscopy, and coherence times approaching T1.
    BibTeX:
    @article{Aiello13,
      author = {Aiello, Clarice D. and Hirose, Masashi and Cappellaro, Paola},
      title = {Composite-pulse magnetometry with a solid-state quantum sensor},
      journal = {Nat. Commun.},
      publisher = {Nat Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.},
      year = {2013},
      volume = {4},
      pages = {1419--},
      doi = {10.1038/ncomms2375}
    }
    
  137. A. Ajoy and P. Cappellaro
  138. "Perfect quantum transport in arbitrary spin networks"
    Phys. Rev. B 87, 064303 (2013)
    Abstract: Spin chains have been proposed as wires to transport information between distributed registers in a quantum information processor. Unfortunately, the challenges in manufacturing linear chains with engineered couplings has hindered experimental implementations. Here we present strategies to achieve perfect quantum information transport in arbitrary spin networks. Our proposal is based on the weak coupling limit for pure state transport, where information is transferred between two end spins that are only weakly coupled to the rest of the network. This regime allows ignoring the complex, internal dynamics of the bulk network and relying on virtual transitions or on the coupling to a single bulk eigenmode. We further introduce control methods capable of tuning the transport process and achieve perfect fidelity with limited resources, involving only manipulation of the end qubits. These strategies could be thus applied not only to engineered systems with relaxed fabrication precision, but also to naturally occurring networks; specifically, we discuss the practical implementation of quantum state transfer between two separated nitrogen vacancy (NV) centers through a network of nitrogen substitutional impurities.
    BibTeX:
    @article{Ajoy13,
      author = {Ajoy, Ashok and Cappellaro, Paola},
      title = {Perfect quantum transport in arbitrary spin networks},
      journal = {Phys. Rev. B},
      year = {2013},
      volume = {87},
      pages = {064303},
      doi = {10.1103/PhysRevB.87.064303}
    }
    
  139. A. Ajoy and P. Cappellaro
  140. "Quantum simulation via filtered Hamiltonian engineering: application to perfect quantum transport in spin networks"
    Phys. Rev. Lett. 110, 220503 (2013)
    Abstract: We propose a method for Hamiltonian engineering that requires no local control but only relies on collective qubit rotations and field gradients. The technique achieves a spatial modulation of the coupling strengths via a dynamical construction of a weighting function combined with a Bragg grating. As an example, we demonstrate how to generate the ideal Hamiltonian for perfect quantum information transport between two separated nodes of a large spin network. We engineer a spin chain with optimal couplings starting from a large spin network, such as one naturally occurring in crystals, while decoupling all unwanted interactions. For realistic experimental parameters, our method can be used to drive almost perfect quantum information transport at room temperature. The Hamiltonian engineering method can be made more robust under decoherence and coupling disorder by a novel apodization scheme. Thus, the method is quite general and can be used to engineer the Hamiltonian of many complex spin lattices with different topologies and interactions.
    BibTeX:
    @article{Ajoy13l,
      author = {Ajoy, A. and Cappellaro, P.},
      title = {Quantum simulation via filtered Hamiltonian engineering: application to perfect quantum transport in spin networks},
      journal = {Phys. Rev. Lett.},
      year = {2013},
      volume = {110},
      pages = {220503},
      doi = {10.1103/PhysRevLett.110.220503}
    }
    
  141. C. Belthangady, N. Bar-Gill, L. M. Pham, K. Arai, D. Le Sage, P. Cappellaro and R. L. Walsworth
  142. "Dressed-State Resonant Coupling between Bright and Dark Spins in Diamond"
    Phys. Rev. Lett. 110, 157601 (2013)
    Abstract: Under ambient conditions, spin impurities in solid-state systems are found in thermally mixed states and are optically "dark"; i.e., the spin states cannot be optically controlled. Nitrogen-vacancy (NV) centers in diamond are an exception in that the electronic spin states are "bright"; i.e., they can be polarized by optical pumping, coherently manipulated with spin-resonance techniques, and read out optically, all at room temperature. Here we demonstrate a scheme to resonantly couple bright NV electronic spins to dark substitutional-nitrogen (P1) electronic spins by dressing their spin states with oscillating magnetic fields. This resonant coupling mechanism can be used to transfer spin polarization from NV spins to nearby dark spins and could be used to cool a mesoscopic bath of dark spins to near-zero temperature, thus providing a resource for quantum information and sensing, and aiding studies of quantum effects in many-body spin systems.
    BibTeX:
    @article{Belthangady13,
      author = {Belthangady, C. and Bar-Gill, N. and Pham, L. M. and Arai, K. and Le Sage, D. and Cappellaro, P. and Walsworth, R. L.},
      title = {Dressed-State Resonant Coupling between Bright and Dark Spins in Diamond},
      journal = {Phys. Rev. Lett.},
      year = {2013},
      volume = {110},
      pages = {157601},
      doi = {10.1103/PhysRevLett.110.157601}
    }
    
  143. G. Kaur, A. Ajoy and P. Cappellaro
  144. "Decay of spin coherences in one-dimensional spin systems"
    New J. Phys. 15, 093035 (2013)
    Abstract: Strategies to protect multi-qubit states against decoherence are difficult to formulate because of their complex many-body dynamics. A better knowledge of the decay dynamics would help in the construction of dynamical decoupling control schemes. Here we use solid-state nuclear magnetic resonance techniques to experimentally investigate decay of coherent multi-spin states in linear spin chains. Leveraging on the quasi-one-dimension geometry of fluorapatite crystal spin systems, we can gain a deeper insight on the multi-spin states created by the coherent evolution, and their subsequent decay, than it is possible in three-dimensional (3D) systems. We are then able to formulate an analytical model that captures the key features of the decay. We can thus compare the decoherence behavior for different initial states of the spin chain and link their decay rate to the state characteristics, in particular their coherence and long-range correlation among spins. Our experimental and theoretical study shows that the spin chains undergo a rich dynamics, with a slower decay rate than for the 3D case, and thus might be more amenable to decoupling techniques.
    BibTeX:
    @article{Kaur13,
      author = {G Kaur and A Ajoy and P Cappellaro},
      title = {Decay of spin coherences in one-dimensional spin systems},
      journal = {New J. Phys.},
      year = {2013},
      volume = {15},
      number = {9},
      pages = {093035},
      doi = {10.1088/1367-2630/15/9/093035}
    }
    
  145. E. Magesan, A. Cooper, H. Yum and P. Cappellaro
  146. "Reconstructing the profile of time-varying magnetic fields with quantum sensors"
    Phys. Rev. A 88, 032107 (2013)
    Abstract: Quantum systems have shown great promise for precision metrology thanks to advances in their control. This has allowed not only the sensitive estimation of external parameters but also the reconstruction of their temporal profile. In particular, quantum control techniques and orthogonal function theory have been applied to the reconstruction of the complete profiles of time-varying magnetic fields. Here, we provide a detailed theoretical analysis of the reconstruction method based on the Walsh functions, highlighting the relationship between the orthonormal Walsh basis, sensitivity of field reconstructions, data compression techniques, and dynamical decoupling theory. Specifically, we show how properties of the Walsh basis and a detailed sensitivity analysis of the reconstruction protocol provide a method to characterize the error between the reconstructed and true fields. In addition, we prove various results about the negligibility function on binary sequences which lead to data compression techniques in the Walsh basis and a more resource-efficient reconstruction protocol. The negligibility proves a fruitful concept to unify the information content of Walsh functions and their dynamical decoupling power, which makes the reconstruction method robust against noise.
    BibTeX:
    @article{Magesan13,
      author = {Magesan, Easwar and Cooper, Alexandre and Yum, Honam and Cappellaro, Paola},
      title = {Reconstructing the profile of time-varying magnetic fields with quantum sensors},
      journal = {Phys. Rev. A},
      year = {2013},
      volume = {88},
      pages = {032107},
      doi = {10.1103/PhysRevA.88.032107}
    }
    
  147. E. Magesan and P. Cappellaro
  148. "Experimentally efficient methods for estimating the performance of quantum measurements"
    Phys. Rev. A 88, 022127 (2013)
    Abstract: Efficient methods for characterizing the performance of quantum measurements are important in the experimental quantum sciences. Ideally, one requires both a physically relevant distinguishability measure between measurement operations and a well-defined experimental procedure for estimating the distinguishability measure. Here, we propose the average measurement fidelity and error between quantum measurements as distinguishability measures. We present protocols for obtaining bounds on these quantities that are both estimable using experimentally accessible quantities and scalable in the size of the quantum system. We also explain why the bounds should be valid in large generality and illustrate the method via numerical examples.
    BibTeX:
    @article{Magesan13a,
      author = {Magesan, Easwar and Cappellaro, Paola},
      title = {Experimentally efficient methods for estimating the performance of quantum measurements},
      journal = {Phys. Rev. A},
      year = {2013},
      volume = {88},
      pages = {022127},
      doi = {10.1103/PhysRevA.88.022127}
    }
    

    2012

    UP ↑

  149. A. Ajoy and P. Cappellaro
  150. "Mixed-state quantum transport in correlated spin networks"
    Phys. Rev. A 85, 042305 (2012)
    Abstract: Quantum spin networks can be used to transport information between separated registers in a quantum-information processor. To find a practical implementation, the strict requirements of ideal models for perfect state transfer need to be relaxed, allowing for complex coupling topologies and general initial states. Here we analyze transport in complex quantum spin networks in the maximally mixed state and derive explicit conditions that should be satisfied by propagators for perfect state transport. Using a description of the transport process as a quantum walk over the network, we show that it is necessary to phase-correlate the transport processes occurring along all the possible paths in the network. We provide a Hamiltonian that achieves this correlation and use it in a constructive method to derive engineered couplings for perfect transport in complicated network topologies.
    BibTeX:
    @article{Ajoy12,
      author = {Ajoy, Ashok and Cappellaro, Paola},
      title = {Mixed-state quantum transport in correlated spin networks},
      journal = {Phys. Rev. A},
      year = {2012},
      volume = {85},
      pages = {042305},
      doi = {10.1103/PhysRevA.85.042305}
    }
    
  151. A. Ajoy and P. Cappellaro
  152. "Stable three-axis nuclear-spin gyroscope in diamond"
    Phys. Rev. A 86, 062104 (2012)
    Abstract: Gyroscopes find wide applications in everyday life from navigation and inertial sensing to rotation sensors in hand-held devices and automobiles. Current devices, based on either atomic or solid-state systems, impose a choice between long-time stability and high sensitivity in a miniaturized system. Here, we introduce a quantum sensor that overcomes these limitations by providing a sensitive and stable three-axis gyroscope in the solid state. We achieve high sensitivity by exploiting the long coherence time of the 14N nuclear spin associated with the nitrogen-vacancy center in diamond, combined with the efficient polarization and measurement of its electronic spin. Although the gyroscope is based on a simple Ramsey interferometry scheme, we use coherent control of the quantum sensor to improve its coherence time and robustness against long-time drifts. Such a sensor can achieve a sensitivity of eta~0.5 (mdeg s-1)/Root Hz mm3 while offering enhanced stability in a small footprint. In addition, we exploit the four axes of delocalization of the nitrogen-vacancy center to measure not only the rate of rotation, but also its direction, thus obtaining a compact three-axis gyroscope.
    BibTeX:
    @article{Ajoy12g,
      author = {Ajoy, Ashok and Cappellaro, Paola},
      title = {Stable three-axis nuclear-spin gyroscope in diamond},
      journal = {Phys. Rev. A},
      year = {2012},
      volume = {86},
      pages = {062104},
      doi = {10.1103/PhysRevA.86.062104}
    }
    
  153. N. Bar-Gill, L. Pham, C. Belthangady, D. Le Sage, P. Cappellaro, J. Maze, M. Lukin, A. Yacoby and R. Walsworth
  154. "Suppression of spin-bath dynamics for improved coherence of multi-spin-qubit systems"
    Nat Commun. 3, 858 (2012)
    Abstract: Multi-qubit systems are crucial for the advancement and application of quantum science. Such systems require maintaining long coherence times while increasing the number of qubits available for coherent manipulation. For solid-state spin systems, qubit coherence is closely related to fundamental questions of many-body spin dynamics. Here we apply a coherent spectroscopic technique to characterize the dynamics of the composite solid-state spin environment of nitrogen-vacancy colour centres in room temperature diamond. We identify a possible new mechanism in diamond for suppression of electronic spin-bath dynamics in the presence of a nuclear spin bath of sufficient concentration. This suppression enhances the efficacy of dynamical decoupling techniques, resulting in increased coherence times for multi-spin-qubit systems, thus paving the way for applications in quantum information, sensing and metrology.
    BibTeX:
    @article{Bar-Gill12,
      author = {Bar-Gill, N. and Pham, L.M. and Belthangady, C. and Le Sage, D. and Cappellaro, P. and Maze, J.R. and Lukin, M.D. and Yacoby, A. and Walsworth, R.},
      title = {Suppression of spin-bath dynamics for improved coherence of multi-spin-qubit systems},
      journal = {Nat Commun.},
      year = {2012},
      volume = {3},
      pages = {858},
      doi = {10.1038/ncomms1856}
    }
    
  155. P. Cappellaro
  156. "Spin-bath narrowing with adaptive parameter estimation"
    Phys. Rev. A 85, 030301(R) (2012)
    Abstract: We present a measurement scheme capable of achieving the quantum limit of parameter estimation using an adaptive strategy that minimizes the parameter's variance at each step. The adaptive rule we propose makes the scheme robust against errors, in particular imperfect readouts, a critical requirement to extend adaptive schemes from quantum optics to solid-state sensors. Thanks to recent advances in single-shot readout capabilities for electronic spins in the solid state (such as nitrogen vacancy centers in diamond), this scheme can also be applied to estimate the polarization of a spin bath coupled to the sensor spin. In turns, the measurement process decreases the entropy of the spin bath resulting in longer coherence times of the sensor spin.
    BibTeX:
    @article{Cappellaro12,
      author = {Cappellaro, Paola},
      title = {Spin-bath narrowing with adaptive parameter estimation},
      journal = {Phys. Rev. A},
      year = {2012},
      volume = {85},
      pages = {030301(R)},
      doi = {10.1103/PhysRevA.85.030301}
    }
    
  157. P. Cappellaro, G. Goldstein, J. S. Hodges, L. Jiang, J. R. Maze, A. S. Sørensen and M. D. Lukin
  158. "Environment-assisted metrology with spin qubits"
    Phys. Rev. A 85, 032336 (2012)
    Abstract: We investigate the sensitivity of a recently proposed method for precision measurement [ Phys. Rev. Lett. 106 140502 (2011)], focusing on an implementation based on solid-state spin systems. The scheme amplifies a quantum sensor response to weak external fields by exploiting its coupling to spin impurities in the environment. We analyze the limits to the sensitivity due to decoherence and propose dynamical decoupling schemes to increase the spin coherence time. The sensitivity is also limited by the environment spin polarization; therefore, we discuss strategies to polarize the environment spins and present a method to extend the scheme to the case of zero polarization. The coherence time and polarization determine a figure of merit for the environment's ability to enhance the sensitivity compared to echo-based sensing schemes. This figure of merit can be used to engineer optimized samples for high-sensitivity nanoscale magnetic sensing, such as diamond nanocrystals with controlled impurity density.
    BibTeX:
    @article{Cappellaro12a,
      author = {Cappellaro, P. and Goldstein, G. and Hodges, J. S. and Jiang, L. and Maze, J. R. and Sørensen, A. S. and Lukin, M. D.},
      title = {Environment-assisted metrology with spin qubits},
      journal = {Phys. Rev. A},
      year = {2012},
      volume = {85},
      pages = {032336},
      doi = {10.1103/PhysRevA.85.032336}
    }
    
  159. M. Hirose, C. D. Aiello and P. Cappellaro
  160. "Continuous dynamical decoupling magnetometry"
    Phys. Rev. A 86, 062320 (2012)
    Abstract: Solid-state qubits hold the promise to achieve an unmatched combination of sensitivity and spatial resolution. To achieve their potential, the qubits need, however, to be shielded from the deleterious effects of the environment. While dynamical decoupling techniques can improve the coherence time, they impose a compromise between sensitivity and the frequency range of the field to be measured. Moreover, the performance of pulse sequences is ultimately limited by control bounds and errors. Here we analyze a versatile alternative based on continuous driving. We find that continuous dynamical decoupling schemes can be used for ac magnetometry, providing similar frequency constraints on the ac field and improved sensitivity for some noise regimes. In addition, the flexibility of phase and amplitude modulation could yield superior robustness to driving errors and a better adaptability to external experimental scenarios.
    BibTeX:
    @article{Hirose12,
      author = {Hirose, Masashi and Aiello, Clarice D. and Cappellaro, Paola},
      title = {Continuous dynamical decoupling magnetometry},
      journal = {Phys. Rev. A},
      year = {2012},
      volume = {86},
      pages = {062320},
      doi = {10.1103/PhysRevA.86.062320}
    }
    
  161. G. Kaur and P. Cappellaro
  162. "Initialization and readout of spin chains for quantum information transport"
    New J. Phys. 14, 083005 (2012)
    Abstract: Linear chains of spins acting as quantum wires are a promising approach for achieving scalable quantum information processors. Nuclear spins in apatite crystals provide an ideal test bed for the experimental study of quantum information transport, as they closely emulate a one-dimensional spin chain, while magnetic resonance techniques can be used to drive the spin chain dynamics and probe the accompanying transport mechanisms. Here we demonstrate initialization and readout capabilities in these spin chains, even in the absence of single-spin addressability. These control schemes enable preparing desired states for quantum information transport and probing their evolution under the transport Hamiltonian. We further optimize the control schemes by a detailed analysis of 19 F NMR lineshape.
    BibTeX:
    @article{Kaur12,
      author = {Gurneet Kaur and Paola Cappellaro},
      title = {Initialization and readout of spin chains for quantum information transport},
      journal = {New J. Phys.},
      year = {2012},
      volume = {14},
      number = {8},
      pages = {083005},
      doi = {10.1088/1367-2630/14/8/083005}
    }
    
  163. L. M. Pham, N. Bar-Gill, C. Belthangady, D. Le Sage, P. Cappellaro, M. D. Lukin, A. Yacoby and R. L. Walsworth
  164. "Enhanced solid-state multispin metrology using dynamical decoupling"
    Phys. Rev. B 86, 045214 (2012)
    Abstract: We use multipulse dynamical decoupling to increase the coherence lifetime (T2) of large numbers of nitrogen-vacancy (NV) electronic spins in room temperature diamond, thus enabling scalable applications of multispin quantum information processing and metrology. We realize an order-of-magnitude extension of the NV multispin T2 in three diamond samples with widely differing spin impurity environments. In particular, for samples with nitrogen impurity concentration ?1 ppm, we extend T2 to >2 ms, comparable to the longest coherence time reported for single NV centers, and demonstrate a tenfold enhancement in NV multispin sensing of ac magnetic fields.
    BibTeX:
    @article{Pham12,
      author = {Pham, L. M. and Bar-Gill, N. and Belthangady, C. and Le Sage, D. and Cappellaro, P. and Lukin, M. D. and Yacoby, A. and Walsworth, R. L.},
      title = {Enhanced solid-state multispin metrology using dynamical decoupling},
      journal = {Phys. Rev. B},
      year = {2012},
      volume = {86},
      pages = {045214},
      doi = {10.1103/PhysRevB.86.045214}
    }
    
  165. F. Ticozzi, R. Lucchese, P. Cappellaro and L. Viola
  166. "Hamiltonian Control of Quantum Dynamical Semigroups: Stabilization and Convergence Speed"
    IEEE TAC 57, 1931 -1944 (2012)
    Abstract: We consider finite-dimensional Markovian open quantum systems, and characterize the extent to which time-independent Hamiltonian control may allow to stabilize a target quantum state or subspace and optimize the resulting convergence speed. For a generic Lindblad master equation, we introduce a dissipation-induced decomposition of the associated Hilbert space, and show how it serves both as a tool to analyze global stability properties for given control resources and as the starting point to synthesize controls that ensure rapid convergence. The resulting design principles are illustrated in realistic Markovian control settings motivated by quantum information processing, including quantum-optical systems and nitrogen-vacancy centers in diamond.
    BibTeX:
    @article{Ticozzi12,
      author = {Ticozzi, F. and Lucchese, R. and Cappellaro, P. and Viola, L.},
      title = {Hamiltonian Control of Quantum Dynamical Semigroups: Stabilization and Convergence Speed},
      journal = {IEEE TAC},
      year = {2012},
      volume = {57},
      number = {8},
      pages = {1931 -1944},
      doi = {10.1109/TAC.2012.2195858}
    }
    

    2011

    UP ↑

  167. P. Cappellaro, L. Viola and C. Ramanathan
  168. "Coherent-state transfer via highly mixed quantum spin chains"
    Phys. Rev. A 83, 032304 (2011)
    Abstract: Spin chains have been proposed as quantum wires in many quantum-information processing architectures. Coherent transmission of quantum information in spin chains over short distances is enabled by their internal dynamics, which drives the transport of single-spin excitations in perfectly polarized chains. Given the practical challenge of preparing the chain in a pure state, we propose to use a chain that is initially in the maximally mixed state. We compare the transport properties of pure and mixed-state chains and find similarities that enable the experimental study of pure-state transfer via mixed-state chains. We also demonstrate protocols for the perfect transfer of quantum information in these chains. Remarkably, mixed-state chains allow the use of Hamiltonians that do not preserve the total number of single-spin excitations and are more readily obtainable from the naturally occurring magnetic dipolar interaction. We discuss experimental implementations using solid-state nuclear magnetic resonance and defect centers in diamond.
    BibTeX:
    @article{Cappellaro11,
      author = {Cappellaro, Paola and Viola, Lorenza and Ramanathan, Chandrasekhar },
      title = {Coherent-state transfer via highly mixed quantum spin chains},
      journal = {Phys. Rev. A},
      year = {2011},
      volume = {83},
      number = {3},
      pages = {032304},
      doi = {10.1103/PhysRevA.83.032304}
    }
    
  169. G. Goldstein, P. Cappellaro, J. R. Maze, J. S. Hodges, L. Jiang, A. S. Sørensen and M. D. Lukin
  170. "Environment Assisted Precision Measurement"
    Phys. Rev. Lett. 106, 140502 (2011)
    Abstract: We describe a method to enhance the sensitivity of precision measurements that takes advantage of the environment of a quantum sensor to amplify the response of the sensor to weak external perturbations. An individual qubit is used to sense the dynamics of surrounding ancillary qubits, which are in turn affected by the external field to be measured. The resulting sensitivity enhancement is determined by the number of ancillas that are coupled strongly to the sensor qubit; it does not depend on the exact values of the coupling strengths and is resilient to many forms of decoherence. The method achieves nearly Heisenberg-limited precision measurement, using a novel class of entangled states. We discuss specific applications to improve clock sensitivity using trapped ions and magnetic sensing based on electronic spins in diamond.
    BibTeX:
    @article{Goldstein11,
      author = {Goldstein, G. and Cappellaro, P. and Maze, J.~R. and Hodges, J.~S. and Jiang, L. and Sørensen, A.~S. and Lukin, M.~D.},
      title = {Environment Assisted Precision Measurement},
      journal = {Phys. Rev. Lett.},
      year = {2011},
      volume = {106},
      number = {14},
      pages = {140502},
      doi = {10.1103/PhysRevLett.106.140502}
    }
    
  171. L. M. Pham, D. Le Sage, P. L. Stanwix, T. K. Yeung, D. Glenn, A. Trifonov, P. Cappellaro, P. R. Hemmer, M. D. Lukin, H. Park, A. Yacoby and R. L. Walsworth
  172. "Magnetic field imaging with nitrogen-vacancy ensembles"
    New J. Phys. 13, 045021 (2011)
    Abstract: We demonstrate a method of imaging spatially varying magnetic fields using a thin layer of nitrogen-vacancy (NV) centers at the surface of a diamond chip. Fluorescence emitted by the two-dimensional NV ensemble is detected by a CCD array, from which a vector magnetic field pattern is reconstructed. As a demonstration, ac current is passed through wires placed on the diamond chip surface, and the resulting ac magnetic field patterns are imaged using an echo-based technique with sub-micron resolution over a 140� um 140 um field of view, giving single-pixel sensitivity . We discuss ongoing efforts to further improve the sensitivity, as well as potential bioimaging applications such as real-time imaging of activity in functional, cultured networks of neurons.
    BibTeX:
    @article{Pham11,
      author = {Pham, L M and Le Sage, D and Stanwix, P L and Yeung, T K and Glenn, D and Trifonov, A and Cappellaro, P and Hemmer, P R and Lukin, M D and Park, H and Yacoby, A and Walsworth, R L},
      title = {Magnetic field imaging with nitrogen-vacancy ensembles},
      journal = {New J. Phys.},
      publisher = {IOP Publishing},
      year = {2011},
      volume = {13},
      number = {4},
      pages = {045021},
      doi = {10.1088/1367-2630/13/4/045021}
    }
    
  173. C. Ramanathan, P. Cappellaro, L. Viola and D. G. Cory
  174. "Experimental characterization of coherent magnetization transport in a one-dimensional spin system"
    New J. Phys. 13, 103015 (2011)
    Abstract: We experimentally characterize the non-equilibrium, room-temperature magnetization dynamics of a spin chain evolving under an effective double-quantum (DQ) Hamiltonian. We show that the Liouville space operators corresponding to the magnetization and the two-spin correlations evolve 90 degrees out of phase with each other, and drive the transport dynamics. For a nearest-neighbor-coupled N -spin chain, the dynamics are found to be restricted to a Liouville operator space whose dimension scales only as N 2 , leading to a slow growth of multi-spin correlations. Even though long-range couplings are present in the real system, we find excellent agreement between the analytical predictions and our experimental results, confirming that leakage out of the restricted Liouville space is slow on the timescales investigated. Our results indicate that the group velocity of the magnetization is6.04 ± 0.38 μ m s −1 , corresponding to a coherent transport over N ≈ 26 spins on the experimental timescale. As the DQ Hamiltonian is related to the standard one-dimensional XX Hamiltonian by a similarity transform, our results can be directly extended to XX quantum spin chains, which have been extensively studied in the context of both quantum magnetism and quantum information processing.
    BibTeX:
    @article{Ramanathan11,
      author = {Chandrasekhar Ramanathan and Paola Cappellaro and Lorenza Viola and David G Cory},
      title = {Experimental characterization of coherent magnetization transport in a one-dimensional spin system},
      journal = {New J. Phys.},
      year = {2011},
      volume = {13},
      number = {10},
      pages = {103015},
      doi = {10.1088/1367-2630/13/10/103015}
    }
    

    2010

    UP ↑

  175. C. Altafini, P. Cappellaro and D. Cory
  176. "Feedback schemes for radiation damping suppression in NMR: A control-theoretical perspective"
    Systems & Control Letters 59, 782 - 786 (2010)
    Abstract: In Nuclear Magnetic Resonance (NMR) spectroscopy, the measurement of the collective spin magnetization is weakly invasive and its back-action is called radiation damping. The aim of this paper is to provide a control-theoretical analysis of the problem of suppressing radiation damping effects. We show that the various real-time feedback schemes commonly used in NMR can be cast in terms of high gain feedback, of exact cancellation based on knowledge of the radiation damping field, and of 2-degree of freedom control designs, with the exact cancellation as prefeedback. We further show that the formulation in control-theoretical terms naturally leads to devising other possible closed-loop schemes, such as a general high gain feedback stabilization design not requiring the knowledge of the radiation damping field.
    BibTeX:
    @article{Altafini10,
      author = {C. Altafini and P. Cappellaro and D. Cory},
      title = {Feedback schemes for radiation damping suppression in NMR: A control-theoretical perspective},
      journal = {Systems & Control Letters},
      year = {2010},
      volume = {59},
      number = {12},
      pages = {782 - 786},
      doi = {10.1016/j.sysconle.2010.09.004}
    }
    
  177. G. Goldstein, M. D. Lukin and P. Cappellaro
  178. "Quantum Limits on Parameter Estimation"
    ArXiv:1001.4804 (2010)
    Abstract: We present a new proof of the quantum Cramer-Rao bound for precision parameter estimation [1-3] and extend it to a more general class of measurement procedures. We analyze a generalized framework for parameter estimation that covers most experimentally accessible situations, where multiple rounds of measurements, auxiliary systems or external control of the evolution are available. The proof presented demonstrates the equivalence of these more general metrology procedures to the simplest optimal strategy for which the bound is proven: a single measurement of a two-level system interacting with a time-independent Hamiltonian.
    BibTeX:
    @article{Goldstein10x,
      author = {Goldstein, G. and Lukin, M.~D. and Cappellaro, P.},
      title = {Quantum Limits on Parameter Estimation},
      journal = {ArXiv:1001.4804},
      year = {2010}
    }
    
  179. C. A. Meriles, L. Jiang, G. Goldstein, J. S. Hodges, J. Maze, M. D. Lukin and P. Cappellaro
  180. "Imaging mesoscopic nuclear spin noise with a diamond magnetometer"
    J. Chem. Phys. 133, 124105 (2010)
    Abstract: Magnetic resonance imaging can characterize and discriminate among tissues using their diverse physical and biochemical properties. Unfortunately, submicrometer screening of biological specimens is presently not possible, mainly due to lack of detection sensitivity. Here we analyze the use of a nitrogen-vacancy center in diamond as a magnetic sensor for nanoscale nuclear spin imaging and spectroscopy. We examine the ability of such a sensor to probe the fluctuations of the "classical" dipolar field due to a large number of neighboring nuclear spins in a densely protonated sample. We identify detection protocols that appropriately take into account the quantum character of the sensor and find a signal-to-noise ratio compatible with realistic experimental parameters. Through various example calculations we illustrate different kinds of image contrast. In particular, we show how to exploit the comparatively long nuclear spin correlation times to reconstruct a local, high-resolution sample spectrum.
    BibTeX:
    @article{Meriles10,
      author = {Carlos A. Meriles and Liang Jiang and Garry Goldstein and Jonathan S. Hodges and Jeronimo Maze and Mikhail D. Lukin and Paola Cappellaro},
      title = {Imaging mesoscopic nuclear spin noise with a diamond magnetometer},
      journal = {J. Chem. Phys.},
      publisher = {AIP},
      year = {2010},
      volume = {133},
      number = {12},
      pages = {124105},
      doi = {10.1063/1.3483676}
    }
    
  181. P. L. Stanwix, L. M. Pham, J. R. Maze, D. Le Sage, T. K. Yeung, P. Cappellaro, P. R. Hemmer, A. Yacoby, M. D. Lukin and R. L. Walsworth
  182. "Coherence of nitrogen-vacancy electronic spin ensembles in diamond"
    Phys. Rev. B 82, 201201 (2010)
    Abstract: We present an experimental and theoretical study of electronic spin decoherence in ensembles of nitrogen-vacancy (NV) color centers in bulk high-purity diamond at room temperature. Under appropriate conditions, we find ensemble NV spin coherence times (T2) comparable to that of single NV with T2>600??s for a sample with natural abundance of 13C and paramagnetic impurity density ?1015?cm?3. We also observe a sharp decrease in the coherence time with misalignment of the static magnetic field relative to the NV electronic spin axis, consistent with theoretical modeling of NV coupling to a 13C nuclear-spin bath. The long coherence times and increased signal-to-noise provided by room-temperature NV ensembles will aid many applications of NV centers in precision magnetometry and quantum information.
    BibTeX:
    @article{Stanwix10,
      author = {Stanwix, P. L. and Pham, L. M. and Maze, J. R. and Le Sage, D. and Yeung, T. K. and Cappellaro, P. and Hemmer, P. R. and Yacoby, A. and Lukin, M. D. and Walsworth, R. L.},
      title = {Coherence of nitrogen-vacancy electronic spin ensembles in diamond},
      journal = {Phys. Rev. B},
      year = {2010},
      volume = {82},
      pages = {201201},
      doi = {10.1103/PhysRevB.82.201201}
    }
    

    2009

    UP ↑

  183. C. Altafini, P. Cappellaro and D. Cory
  184. "Feedback schemes for radiation damping suppression in NMR: a control-theoretical perspective"
    In Decision and Control, 2009 held jointly with the 2009 28th Chinese Control Conference. CDC/CCC 2009. Proceedings of the 48th IEEE Conference on , 1445 -1450 (2009)
    Abstract: In NMR spectroscopy, the collective measurement is weakly invasive and its back-action is called radiation damping. The aim of this paper is to provide a control-theoretical analysis of the problem of suppressing this radiation damping. It is shown that the two feedback schemes commonly used in the NMR practice correspond one to a high gain output feedback for the simple case of maintaining the spin 1/2 in its inverted state, and the second to a 2-degree of freedom control design with a prefeedback that exactly cancels the radiation damping field. A general high gain feedback stabilization design not requiring the knowledge of the radiation damping time constant is also investigated.
    BibTeX:
    @inproceedings{Altafini09,
      author = {Altafini, C. and Cappellaro, P. and Cory, D.},
      title = {Feedback schemes for radiation damping suppression in NMR: a control-theoretical perspective},
      booktitle = {Decision and Control, 2009 held jointly with the 2009 28th Chinese Control Conference. CDC/CCC 2009. Proceedings of the 48th IEEE Conference on},
      year = {2009},
      pages = {1445 -1450},
      doi = {10.1109/CDC.2009.5400761}
    }
    
  185. P. Cappellaro, L. Jiang, J. S. Hodges and M. D. Lukin
  186. "Coherence and Control of Quantum Registers Based on Electronic Spin in a Nuclear Spin Bath"
    Phys. Rev. Lett. 102, 210502 (2009)
    Abstract: We consider a protocol for the control of few-qubit registers comprising one electronic spin embedded in a nuclear spin bath. We show how to isolate a few proximal nuclear spins from the rest of the bath and use them as building blocks for a potentially scalable quantum information processor. We describe how coherent control techniques based on magnetic resonance methods can be adapted to these solid-state spin systems, to provide not only efficient, high fidelity manipulation but also decoupling from the spin bath. As an example, we analyze feasible performances and practical limitations in the realistic setting of nitrogen-vacancy centers in diamond.
    BibTeX:
    @article{Cappellaro09,
      author = {P. Cappellaro and L. Jiang and J. S. Hodges and M. D. Lukin},
      title = {Coherence and Control of Quantum Registers Based on Electronic Spin in a Nuclear Spin Bath},
      journal = {Phys. Rev. Lett.},
      year = {2009},
      volume = {102},
      number = {21},
      pages = {210502},
      doi = {10.1103/PhysRevLett.102.210502}
    }
    
  187. P. Cappellaro and M. D. Lukin
  188. "Quantum correlation in disordered spin systems: Applications to magnetic sensing"
    Phys. Rev. A 80, 032311 (2009)
    Abstract: We propose a strategy to generate a many-body entangled state in a collection of randomly placed, dipolarly coupled electronic spins in the solid state. By using coherent control to restrict the evolution into a suitable collective subspace, this method enables the preparation of GHZ-like and spin-squeezed states even for randomly positioned spins, while in addition protecting the entangled states against decoherence. We consider the application of this squeezing method to improve the sensitivity of nanoscale magnetometer based on nitrogen-vacancy spin qubits in diamond.
    BibTeX:
    @article{Cappellaro09b,
      author = {P. Cappellaro and M. D. Lukin},
      title = {Quantum correlation in disordered spin systems: Applications to magnetic sensing},
      journal = {Phys. Rev. A},
      year = {2009},
      volume = {80},
      number = {3},
      pages = {032311},
      doi = {10.1103/PhysRevA.80.032311}
    }
    
  189. P. Cappellaro, J. Maze, L. Childress, M. Dutt, J. Hodges, S. Hong, L. Jiang, P. Stanwix, J. Taylor, E. Togan and others
  190. "Quantum Control of Spins and Photons at Nanoscales"
    21st International Conference on Atomic Physics 21, 78 (2009)
    Abstract: The detection of weak magnetic fields with high spatial resolution is an outstanding problem in diverse areas ranging from fundamental physics and material science to data storage and bio-imaging. Here we describe an innovative approach to magnetometry that takes advantage of recently developed techniques for coherent control of solid-state spin qubits. We experimentally demonstrate this novel magnetometer employing an individual electronic spin associated with a Nitrogen-Vacancy (NV) center in diamond. Using an ultra-pure diamond sample, we achieve shot-noise-limited detection of nanotesla magnetic fields at kHz frequencies after 100 seconds of averaging. In addition, we demonstrate 0.5 microtesla/$Hz$ sensitivity for a diamond nanocrystal with volume of (30 nm)$^3$. This magnetic sensor provides an unprecedented combination of high sensitivity and spatial resolution --potentially allowing for the detection of a single nuclear spin's precession within one second.
    BibTeX:
    @inproceedings{Cappellaro09p,
      author = {Cappellaro, P. and Maze, JM and Childress, L. and Dutt, MVG and Hodges, JS and Hong, S. and Jiang, L. and Stanwix, PL and Taylor, JM and Togan, E. and others},
      title = {Quantum Control of Spins and Photons at Nanoscales},
      journal = {21st International Conference on Atomic Physics},
      publisher = {World Scientific},
      year = {2009},
      volume = {21},
      pages = {78},
      doi = {10.1142/9789814273008_0009}
    }
    
  191. J. R. Maze, P. Cappellaro, L. Childress, M. V. G. Dutt, J. S. Hodges, S. Hong, L. Jiang, P. L. Stanwix, J. M. Taylor, E. Togan, A. S. Zibrov, P. Hemmer, A. Yacoby, R. L. Walsworth and M. D. Lukin
  192. "Nanoscale magnetic sensing using spin qubits in diamond"
    Advanced Optical Concepts in Quantum Computing, Memory, and Communication II In Advanced Optical Concepts in Quantum Computing, Memory, and Communication II 7225, 722509 (2009)
    Abstract: The ability to sense nanotelsa magnetic fields with nanoscale spatial resolution is an outstanding technical challenge relevant to the physical and biological sciences. For example, detection of such weak localized fields will enable sensing of magnetic resonance signals from individual electron or nuclear spins in complex biological molecules and the readout of classical or quantum bits of information encoded in an electron or nuclear spin memory. Here we present a novel approach to nanoscale magnetic sensing based on coherent control of an individual electronic spin contained in the Nitrogen-Vacancy (NV) center in diamond. At room temperature, using an ultra-pure diamond sample, we achieve shot-noise-limited detection of 3 nanotesla magnetic fields oscillating at kHz frequencies after 100 seconds of signal averaging. Furthermore, we experimentally demonstrate nanoscale resolution using a diamond nanocrystal of 30 nm diameter for which we achieve a sensitivity of 0.5 microtesla / Hz1/2.
    BibTeX:
    @inproceedings{Maze09p,
      author = {J. R. Maze and P. Cappellaro and L. Childress and M. V. G. Dutt and J. S. Hodges and S. Hong and L. Jiang and P. L. Stanwix and J. M. Taylor and E. Togan and A. S. Zibrov and P. Hemmer and A. Yacoby and R. L. Walsworth and M. D. Lukin},
      editor = {Zameer U. Hasan and Alan E. Craig and Philip R. Hemmer},
      title = {Nanoscale magnetic sensing using spin qubits in diamond},
      booktitle = {Advanced Optical Concepts in Quantum Computing, Memory, and Communication II},
      journal = {Advanced Optical Concepts in Quantum Computing, Memory, and Communication II},
      publisher = {SPIE},
      year = {2009},
      volume = {7225},
      number = {1},
      pages = {722509},
      doi = {10.1117/12.813802}
    }
    
  193. P. Rabl, P. Cappellaro, M. V. G. Dutt, L. Jiang, J. R. Maze and M. D. Lukin
  194. "Strong magnetic coupling between an electronic spin qubit and a mechanical resonator"
    Phys. Rev. B 79, 041302 (2009)
    Abstract: We describe a technique that enables a strong coherent coupling between a single electronic spin qubit associated with a nitrogen-vacancy impurity in diamond and the quantized motion of a magnetized nanomechanical resonator tip. This coupling is achieved via careful preparation of dressed spin states which are highly sensitive to the motion of the resonator but insensitive to perturbations from the nuclear-spin bath. In combination with optical pumping techniques, the coherent exchange between spin and motional excitations enables ground-state cooling and controlled generation of arbitrary quantum superpositions of resonator states. Optical spin readout techniques provide a general measurement toolbox for the resonator with quantum limited precision.
    BibTeX:
    @article{Rabl09,
      author = {P. Rabl and P. Cappellaro and M. V. Gurudev Dutt and L. Jiang and J. R. Maze and M. D. Lukin},
      title = {Strong magnetic coupling between an electronic spin qubit and a mechanical resonator},
      journal = {Phys. Rev. B},
      year = {2009},
      volume = {79},
      number = {4},
      pages = {041302},
      doi = {10.1103/PhysRevB.79.041302}
    }
    
  195. W. Zhang, P. Cappellaro, N. Antler, B. Pepper, D. G. Cory, V. V. Dobrovitski, C. Ramanathan and L. Viola
  196. "NMR multiple quantum coherences in quasi-one-dimensional spin systems: Comparison with ideal spin-chain dynamics"
    Phys. Rev. A 80, 052323 (2009)
    Abstract: The 19F spins in a crystal of fluorapatite have often been used to experimentally approximate a one-dimensional spin system. Under suitable multipulse control, the nuclear-spin dynamics may be modeled to first approximation by a double-quantum one-dimensional Hamiltonian, which is analytically solvable for nearest-neighbor couplings. Here, we use solid-state nuclear magnetic resonance techniques to investigate the multiple quantum coherence dynamics of fluorapatite, with an emphasis on understanding the region of validity for such a simplified picture. Using experimental, numerical, and analytical methods, we explore the effects of long-range intrachain couplings, cross-chain couplings, as well as couplings to a spin environment, all of which tend to damp the oscillations of the multiple quantum coherence signal at sufficiently long times. Our analysis characterizes the extent to which fluorapatite can faithfully simulate a one-dimensional quantum wire.
    BibTeX:
    @article{Zhang09,
      author = {Zhang, Wenxian and Cappellaro, Paola and Antler, Natania and Pepper, Brian and Cory, David G. and Dobrovitski, Viatcheslav V. and Ramanathan, Chandrasekhar and Viola, Lorenza},
      title = {NMR multiple quantum coherences in quasi-one-dimensional spin systems: Comparison with ideal spin-chain dynamics},
      journal = {Phys. Rev. A},
      year = {2009},
      volume = {80},
      number = {5},
      pages = {052323},
      doi = {10.1103/PhysRevA.80.052323}
    }
    

    2008

    UP ↑

  197. L. Jiang, M. V. G. Dutt, E. Togan, L. Childress, P. Cappellaro, J. M. Taylor and M. D. Lukin
  198. "Coherence of an Optically Illuminated Single Nuclear Spin Qubit"
    Phys. Rev. Lett. 100, 073001 (2008)
    Abstract: We investigate the coherence properties of individual nuclear spin quantum bits in diamond [Dutt et al., Science 316, 1312 (2007)] when a proximal electronic spin associated with a nitrogen-vacancy (N-V) center is being interrogated by optical radiation. The resulting nuclear spin dynamics are governed by time-dependent hyperfine interaction associated with rapid electronic transitions, which can be described by a spin-fluctuator model. We show that due to a process analogous to motional averaging in nuclear magnetic resonance, the nuclear spin coherence can be preserved after a large number of optical excitation cycles. Our theoretical analysis is in good agreement with experimental results. It indicates a novel approach that could potentially isolate the nuclear spin system completely from the electronic environment.
    BibTeX:
    @article{Jiang08,
      author = {L. Jiang and M. V. Gurudev Dutt and E. Togan and L. Childress and P. Cappellaro and J. M. Taylor and M. D. Lukin},
      title = {Coherence of an Optically Illuminated Single Nuclear Spin Qubit},
      journal = {Phys. Rev. Lett.},
      year = {2008},
      volume = {100},
      number = {7},
      pages = {073001},
      doi = {10.1103/PhysRevLett.100.073001}
    }
    
  199. J. R. Maze, P. L. Stanwix, J. S. Hodges, S. Hong, J. M. Taylor, P. Cappellaro, L. Jiang, A. Zibrov, A. Yacoby, R. Walsworth and M. D. Lukin
  200. "Nanoscale magnetic sensing with an individual electronic spin qubit in diamond"
    Nature 455, 644-647 (2008)
    Abstract: Detection of weak magnetic fields with nanoscale spatial resolution is an outstanding problem in the biological and physical sciences. For example, at a distance of 10 nm, the spin of a single electron produces a magnetic field of about 1 muT, and the corresponding field from a single proton is a few nanoteslas. A sensor able to detect such magnetic fields with nanometre spatial resolution would enable powerful applications, ranging from the detection of magnetic resonance signals from individual electron or nuclear spins in complex biological molecules to readout of classical or quantum bits of information encoded in an electron or nuclear spin memory. Here we experimentally demonstrate an approach to such nanoscale magnetic sensing, using coherent manipulation of an individual electronic spin qubit associated with a nitrogen-vacancy impurity in diamond at room temperature8. Using an ultra-pure diamond sample, we achieve detection of 3 nT magnetic fields at kilohertz frequencies after 100 s of averaging. In addition, we demonstrate a sensitivity of 0.5 muT Hz-1/2 for a diamond nanocrystal with a diameter of 30 nm.
    BibTeX:
    @article{Maze08,
      author = {J. R. Maze and P. L. Stanwix and J. S. Hodges and S. Hong and J. M. Taylor and P. Cappellaro and L. Jiang and A.S. Zibrov and A. Yacoby and R. Walsworth and M. D. Lukin},
      title = {Nanoscale magnetic sensing with an individual electronic spin qubit in diamond},
      journal = {Nature},
      year = {2008},
      volume = {455},
      pages = {644--647},
      doi = {10.1038/nature07279}
    }
    
  201. J. M. Taylor, P. Cappellaro, L. Childress, L. Jiang, D. Budker, P. R. Hemmer, A. Yacoby, R. Walsworth and M. D. Lukin
  202. "High-sensitivity diamond magnetometer with nanoscale resolution"
    Nat Phys. 4, 810-816 (2008)
    Abstract: The detection of weak magnetic fields with high spatial resolution is an important problem in diverse areas ranging from fundamental physics and material science to data storage and biomedical science. Here, we explore a novel approach to the detection of weak magnetic fields that takes advantage of recently developed techniques for the coherent control of solid-state electron spin quantum bits. Specifically, we investigate a magnetic sensor based on nitrogen-vacancy centres in room-temperature diamond. We discuss two important applications of this technique: a nanoscale magnetometer that could potentially detect precession of single nuclear spins and an optical magnetic-field imager combining spatial resolution ranging from micrometres to millimetres with a sensitivity approaching a few fT Hz-1/2.
    BibTeX:
    @article{Taylor08,
      author = {Taylor, J. M. and Cappellaro, P. and Childress, L. and Jiang, L. and Budker, D. and Hemmer, P. R. and Yacoby, A. and Walsworth, R. and Lukin, M. D.},
      title = {High-sensitivity diamond magnetometer with nanoscale resolution},
      journal = {Nat Phys.},
      publisher = {Nature Publishing Group},
      year = {2008},
      volume = {4},
      number = {10},
      pages = {810--816},
      doi = {10.1038/nphys1075}
    }
    

    2007

    UP ↑

  203. P. Cappellaro, J. S. Hodges, T. F. Havel and D. G. Cory
  204. "Subsystem pseudopure states"
    Phys. Rev. A 75, 042321 (2007)
    Abstract: A critical step in experimental quantum information processing (QIP) is to implement control of quantum systems protected against decoherence via informational encodings, such as quantum error-correcting codes, noiseless subsystems, and decoherence-free subspaces. These encodings lead to the promise of fault-tolerant QIP, but they come at the expense of resource overheads. Part of the challenge in studying control over multiple logical qubits is that QIP testbeds have not had sufficient resources to analyze encodings beyond the simplest ones. The most relevant resources are the number of available qubits and the cost to initialize and control them. Here we demonstrate an encoding of logical information that permits control over multiple logical qubits without full initialization, an issue that is particularly challenging in liquid-state NMR. The method of subsystem pseudopure states will allow the study of decoherence control schemes on up to six logical qubits using liquid-state NMR implementations.
    BibTeX:
    @article{Cappellaro07,
      author = {Cappellaro, P. and Hodges, J. S. and Havel, T. F. and Cory, D. G.},
      title = {Subsystem pseudopure states},
      journal = {Phys. Rev. A},
      year = {2007},
      volume = {75},
      pages = {042321},
      doi = {10.1103/PhysRevA.75.042321}
    }
    
  205. P. Cappellaro, C. Ramanathan and D. G. Cory
  206. "Dynamics and control of a quasi-one-dimensional spin system"
    Phys. Rev. A 76, 032317 (2007)
    Abstract: We study experimentally a system comprised of linear chains of spin-1/2 nuclei that provides a test bed for multibody dynamics and quantum-information processing. This system is a paradigm for a class of quantum-information processing devices that can perform particular tasks even without universal control of the whole quantum system. We investigate the extent of control achievable on the system with current experimental apparatus and methods to gain information on the system state, when full tomography is not possible and in any case highly inefficient.
    BibTeX:
    @article{Cappellaro07a,
      author = {P. Cappellaro and C. Ramanathan and D. G. Cory},
      title = {Dynamics and control of a quasi-one-dimensional spin system},
      journal = {Phys. Rev. A},
      year = {2007},
      volume = {76},
      number = {3},
      pages = {032317},
      doi = {10.1103/PhysRevA.76.032317}
    }
    
  207. P. Cappellaro, C. Ramanathan and D. G. Cory
  208. "Simulations of Information Transport in Spin Chains"
    Phys. Rev. Lett. 99, 250506 (2007)
    Abstract: Transport of quantum information in linear spin chains has been the subject of much theoretical work. Experimental studies by NMR in solid state spin systems (a natural implementation of such models) is complicated since the dipolar Hamiltonian is not solely comprised of nearest-neighbor XY-Heisenberg couplings. We present here a similarity transformation between the XY Hamiltonian and the double-quantum Hamiltonian, an interaction which is achievable with the collective control provided by radio-frequency pulses. Not only can this second Hamiltonian simulate the information transport in a spin chain, but it also creates coherent states, whose intensities give an experimental signature of the transport. This scheme makes it possible to study experimentally the transport of polarization beyond exactly solvable models and explore the appearance of quantum coherence and interference effects.
    BibTeX:
    @article{Cappellaro07l,
      author = {P. Cappellaro and C. Ramanathan and D. G. Cory},
      title = {Simulations of Information Transport in Spin Chains},
      journal = {Phys. Rev. Lett.},
      year = {2007},
      volume = {99},
      number = {25},
      pages = {250506},
      doi = {10.1103/PhysRevLett.99.250506}
    }
    
  209. P. Cappellaro, J. S. Hodges, T. F. Havel and D. G. Cory
  210. "Control of qubits encoded in decoherence-free subspaces"
    Las. Phys. 17, 545-551 (2007)
    Abstract: Decoherence-free subspaces protect quantum information from the effects of noise that is correlated across the physical qubits used to implement them. Given the ability to impose suitable Hamiltonians upon such a multi-qubit system, one can also implement a set of logical gates which enables universal computation on this information without compromising this protection. Real physical systems, however, seldom come with the correct Hamiltonians built-in, let alone the ability to turn them off and on at will. In the course of our development of quantum information processing devices based on liquid-state NMR, we have found the task of operating on quantum information encoded in decoherence-free subspaces rather more challenging than is commonly assumed. This contribution presents an overview of these challenges and the methods we have developed for overcoming them in practice. These methods promise to be broadly applicable to many of the physical systems proposed for the implementation of quantum information processing devices.
    BibTeX:
    @article{Cappellaro07p,
      author = {Cappellaro, P. and Hodges, J.~S. and Havel, T.~F. and Cory, D.~G.},
      title = {Control of qubits encoded in decoherence-free subspaces},
      journal = {Las. Phys.},
      year = {2007},
      volume = {17},
      pages = {545-551},
      doi = {10.1134/S1054660X0704038X}
    }
    
  211. J. S. Hodges, P. Cappellaro, T. F. Havel, R. Martinez and D. G. Cory
  212. "Experimental implementation of a logical Bell state encoding"
    Phys. Rev. A 75, 042320 (2007)
    Abstract: Liquid-phase NMR is a general-purpose testbed for developing methods of coherent control relevant to quantum information processing. Here we extend these studies to the coherent control of logical qubits and in particular to the unitary gates necessary to create entanglement between logical qubits. We report an experimental implementation of a conditional logical gate between two logical qubits that are each in decoherence-free subspaces that protect the quantum information from fully correlated dephasing.
    BibTeX:
    @article{Hodges07,
      author = {Hodges, J. S. and Cappellaro, P. and Havel, T. F. and Martinez, R. and Cory, D. G.},
      title = {Experimental implementation of a logical Bell state encoding},
      journal = {Phys. Rev. A},
      year = {2007},
      volume = {75},
      number = {4},
      pages = {042320},
      doi = {10.1103/PhysRevA.75.042320}
    }
    

    2006

    UP ↑

  213. P. Cappellaro, J. S. Hodges, T. F. Havel and D. G. Cory
  214. "Principles of Control for Decoherence-Free Subsystems"
    J. Chem. Phys. 125, 044514 (2006)
    Abstract: Decoherence-free subsystems (DFSs) are a powerful means of protecting quantum information against noise with known symmetry properties. Although Hamiltonians that can implement a universal set of logic gates on DFS encoded qubits without ever leaving the protected subsystem theoretically exist, the natural Hamiltonians that are available in specific implementations do not necessarily have this property. Here we describe some of the principles that can be used in such cases to operate on encoded qubits without losing the protection offered by the DFSs. In particular, we show how dynamical decoupling can be used to control decoherence during the unavoidable excursions outside of the DFS. By means of cumulant expansions, we show how the fidelity of quantum gates implemented by this method on a simple two physical qubit DFS depends on the correlation time of the noise responsible for decoherence. We further show by means of numerical simulations how our previously introduced ?strongly modulating pulses? for NMR quantum information processing can permit high-fidelity operations on multiple DFS encoded qubits in practice, provided that the rate at which the system can be modulated is fast compared to the correlation time of the noise. The principles thereby illustrated are expected to be broadly applicable to many implementations of quantum information processors based on DFS encoded qubits.
    BibTeX:
    @article{Cappellaro06,
      author = {Cappellaro, P. and Hodges, J. S. and Havel, T. F. and Cory, D. G},
      title = {Principles of Control for Decoherence-Free Subsystems},
      journal = {J. Chem. Phys.},
      year = {2006},
      volume = {125},
      pages = {044514},
      doi = {10.1063/1.2216702}
    }
    
  215. P. Cappellaro, J. Emerson, N. Boulant, C. Ramanathan, S. Lloyd and D. G. Cory
  216. "Spin amplifier for single spin measurement"
    In Quantum Computing in Solid State Systems , 306-312 (2006)
    Abstract: A new approach to the measurement of the state of a collapsed single spin is described by using many entangled spins as an amplifier. A single target spin is coupled via the natural dipolar Hamiltonian to a large collection of spins. Applying external radio frequency (r.f.) pulses, we can control the evolution of the system so that the ensemble spins reach one of two orthogonal states whose collective properties differ depending on the state of the target spin and are easily measured. The result of an experiment simulating this method on an ensemble liquid state NMR quantum processor is reported. That entanglement assisted metrology is compatible with the real control we have over physical spins is suggested, since the measurement process can actually be described in terms of the physical Hamiltonian of the spin system. By building on this work, and with the needed technical advances, it should be possible to detect a single nuclear spin.
    BibTeX:
    @incollection{Cappellaro06p,
      author = {Cappellaro, Paola and Emerson, Joseph and Boulant, Nicolas and Ramanathan, Chandrasekhar and Lloyd, Seth and Cory, David G.},
      editor = {Ruggiero, B. and Delsing, P. and Granata, C. and Pashkin, Y. and Silvestrini, P.},
      title = {Spin amplifier for single spin measurement},
      booktitle = {Quantum Computing in Solid State Systems},
      publisher = {Springer New York},
      year = {2006},
      pages = {306-312},
      doi = {10.1007/0-387-31143-2_37}
    }
    
  217. P. Cappellaro
  218. "Quantum Information Processing in Multi-Spin Systems"
    Thesis at: Massachusetts Institute of Technology, Department of Nuclear Science and Engineering (2006)
    Abstract: Coherence and entanglement in multi-spin systems are valuable resources for quantum information processing. In this thesis, I explore the manipulation of quantum information in complex multi-spin systems, with particular reference to Nuclear Magnetic Resonance implementations. In systems with a few spins, such as molecules in the liquid phase, the use of multi-spin coherent states provides a hedge against the noise, via the encoding of information in logical degrees of freedom distributed over several spins. Manipulating multi-spin coherent states also increases the complexity of quantum operations required in a quantum processor. Here I present schemes to mitigate this problem, both in the state initialization, with particular attention to bulk ensemble quantum information processing, and in the coherent control and gate implementations. In the many-body limit provided by nuclear spins in single crystals, the limitations in the available control increase the complexity of manipulating the system; also, the equations of motion are no longer exactly solvable even in the closed-system limit. Entanglement and multi-spin coherences are essential for extending the control and the accessible information on the system. I employ entanglement in a large ensemble of spins in order to obtain an amplification of the small perturbation created by a single spin on the spin ensemble, in a scheme for the measurement of a single nuclear spin state. I furthermore use multiple quantum coherences in mixed multi-spin states as a tool to explore many-body behavior of linear chain of spins, showing their ability to perform quantum information processing tasks such as simulations and transport of information. The theoretical and experimental results of this thesis suggest that although coherent multi-spin states are particularly fragile and complex to control they could make possible the execution of quantum information processing tasks that have no classical counterparts.
    BibTeX:
    @phdthesis{Cappellaro06t,
      author = {Paola Cappellaro},
      title = {Quantum Information Processing in Multi-Spin Systems},
      school = {Massachusetts Institute of Technology},
      year = {2006}
    }
    
  219. H. J. Cho, P. Cappellaro, D. G. Cory and C. Ramanathan
  220. "Decay of highly correlated spin states in a dipolar-coupled solid: NMR study of CaF2"
    Phys. Rev. B 74, 224434 (2006)
    Abstract: We have measured the decay of NMR multiple quantum coherence intensities both under the internal dipolar Hamiltonian as well as when this interaction is effectively averaged to zero, in the cubic calcium fluoride (CaF2) spin system and the pseudo-one-dimensional system of fluoroapatite. In calcium fluoride the decay rates depend both on the number of correlated spins in the cluster, as well as on the coherence number. For smaller clusters, the decays depend strongly on coherence number, but this dependence weakens as the size of the cluster increases. The same scaling was observed when the coherence distribution was measured in both the usual Zeeman or z basis and the x basis. The coherence decay in the one-dimensional fluoroapatite system did not change significantly as a function of the multiple quantum growth time, in contrast to the calcium fluoride case. While the growth of coherence orders is severely restricted in this case, the number of correlated spins should continue to grow, albeit more slowly. All coherence intensities were observed to decay as Gaussian functions in time. In all cases the standard deviation of the observed decay appeared to scale linearly with coherence number.
    BibTeX:
    @article{Cho06,
      author = {Hyung Joon Cho and Paola Cappellaro and David G. Cory and Chandrasekhar Ramanathan},
      title = {Decay of highly correlated spin states in a dipolar-coupled solid: NMR study of CaF2},
      journal = {Phys. Rev. B},
      year = {2006},
      volume = {74},
      number = {22},
      pages = {224434},
      doi = {10.1103/PhysRevB.74.224434}
    }
    
  221. J. Hodges, P. Cappellaro, T. Havel and D. Cory
  222. "Quantum Control of Nuclear Spins"
    In Decision and Control, 2006 45th IEEE Conference on , 2488 -2494 (2006)
    Abstract: Nuclear magnetic resonance (NMR) spectroscopy has proven to be a facile means of achieving small-scale demonstrations of quantum information processing. This was in large part made possible by the sophisticated methods of quantum control that have been developed by the NMR community over a span of more than 50 years. The traditional control methods, already perhaps the most complex examples of open-loop control available, were nevertheless designed primarily to assist in identifying the physical parameters of the underlying spin system, rather than to control the system with high precision. We have therefore extended the traditional methods in a variety of ways so as to achieve precise control, by taking advantage of prior knowledge of the physical parameters as determined by traditional methods. Due to the experimental challenges of real-time control, these developments relied upon our ability to simulate the evolution of the system under the action of radio-frequency control fields with essentially arbitrary precision, subject only to the available computing power. This talk presents an overview of our work in quantum control together with some of the on-going challenges still facing us
    BibTeX:
    @inproceedings{Hodges06,
      author = {Hodges, J. and Cappellaro, P. and Havel, T.F. and Cory, D.G.},
      title = {Quantum Control of Nuclear Spins},
      booktitle = {Decision and Control, 2006 45th IEEE Conference on},
      year = {2006},
      pages = {2488 -2494},
      doi = {10.1109/CDC.2006.377562}
    }
    
  223. C. A. Perez-Delgado, M. Mosca, P. Cappellaro and D. G. Cory
  224. "Single Spin Measurement Using Cellular Automata Techniques"
    Phys. Rev. Lett. 97, 100501 (2006)
    Abstract: We analyze a conceptual approach to single-spin measurement. The method uses techniques from the theory of quantum cellular automata to correlate a large number of ancillary spins to the one to be measured. It has the distinct advantage of being efficient: under ideal conditions, it requires the application of only O(3 N) steps (each requiring a constant number of rf pulses) to create a system of N correlated spins. Numerical simulations suggest that it is also, to a certain extent, robust against pulse errors, and imperfect initial polarization of the ancilla spin system.
    BibTeX:
    @article{Perez-Delgado06,
      author = {Carlos A. Perez-Delgado and Michele Mosca and Paola Cappellaro and David G. Cory},
      title = {Single Spin Measurement Using Cellular Automata Techniques},
      journal = {Phys. Rev. Lett.},
      year = {2006},
      volume = {97},
      number = {10},
      pages = {100501},
      doi = {10.1103/PhysRevLett.97.100501}
    }
    

    2005 & Earlier

    UP ↑

  225. P. Cappellaro, J. Emerson, N. Boulant, C. Ramanathan, S. Lloyd and D. G. Cory
  226. "Entanglement Assisted Metrology"
    Phys. Rev. Lett. 94, 020502 (2005)
    Abstract: We propose a new approach to the measurement of a single spin state, based on nuclear magnetic resonance (NMR) techniques and inspired by the coherent control over many-body systems envisaged by quantum information processing. A single target spin is coupled via the magnetic dipolar interaction to a large ensemble of spins. Applying radio frequency pulses, we can control the evolution so that the spin ensemble reaches one of two orthogonal states whose collective properties differ depending on the state of the target spin and are easily measured. We first describe this measurement process using quantum gates; then we show how equivalent schemes can be defined in terms of the Hamiltonian and thus implemented under conditions of real control, using well established NMR techniques. We demonstrate this method with a proof of principle experiment in ensemble liquid state NMR and simulations for small spin systems.
    BibTeX:
    @article{Cappellaro05,
      author = {Paola Cappellaro and Joseph Emerson and Nicolas Boulant and Chandrasekhar Ramanathan and Seth Lloyd and David G Cory},
      title = {Entanglement Assisted Metrology},
      journal = {Phys. Rev. Lett.},
      year = {2005},
      volume = {94},
      pages = {020502},
      doi = {10.1103/PhysRevLett.94.020502}
    }
    
  227. T. Havel, P. Cappellaro, C. Ramanathan and D. Cory
  228. "Quantum Information Processing with Nuclear Spin-Based Devices"
    In Technical Proceedings of the 2005 NSTI Nanotechnology Conference and Trade Show 3, 161 - 164 (2005)
    Abstract: This talk will illustrate the benefits offered by QIP with two devices which use quantum entanglement among nuclear spins to defeat the Heisenberg limits: (1) a spin gyroscope that operates by detecting the frequency shift in the spins? precession rate in the rotating frame, which is potentially more accurate than mechanical or optical devices; (2) a quantum amplifier which correlates the states of a macroscopic number of nuclear spins with the state of a single target spin, so that a collective measurement of the state of the amplifier?s spins reveals that of the target spin.
    BibTeX:
    @inproceedings{Havel05,
      author = {T.F. Havel and P. Cappellaro and C. Ramanathan and D.G. Cory},
      title = {Quantum Information Processing with Nuclear Spin-Based Devices},
      booktitle = {Technical Proceedings of the 2005 NSTI Nanotechnology Conference and Trade Show},
      year = {2005},
      volume = {3},
      pages = {161 - 164}
    }
    
  229. G. S. Boutis, P. Cappellaro, H. Cho, C. Ramanathan and D. G. Cory
  230. "Pulse error compensating symmetric magic-echo trains"
    J. Mag. Res. 161, 132-137 (2003)
    Abstract: We present improved line-narrowing sequences for dipolar coupled spin systems, based on a train of magic-echoes which are compensated for the effects of finite pulse widths and utilize symmetry properties of supercycles. Sequences are introduced for spectroscopy and imaging by proper choice of a phase alternating scheme. Using a 16 pulse time-suspension magic-echo cycle, the highest level of line-narrowing achieved was 2.7 Hz for the [100] direction of a single crystal of calcium fluoride, a reduction in linewidth by 4 orders of magnitude.
    BibTeX:
    @article{Boutis03,
      author = {Boutis, G. S. and Cappellaro, P. and Cho, H. and Ramanathan, C. and Cory, D. G.},
      title = {Pulse error compensating symmetric magic-echo trains},
      journal = {J. Mag. Res.},
      year = {2003},
      volume = {161},
      pages = {132-137},
      doi = {10.1016/S1090-7807(03)00010-7}
    }
    
  231. C. Ramanathan, H. Cho, P. Cappellaro, G. S. Boutis and D. G. Cory
  232. "Encoding multiple quantum coherences in non-commuting bases"
    Chem. Phys. Lett. 369, 311 (2003)
    Abstract: Multiple quantum (MQ) coherences are characterized by their coherence number and the number of spins that make up the state, though only the coherence number is normally measured. We present a simple set of measurements that extend our knowledge of the MQ state by recording the coherences in two non-commuting bases, the x and the z bases (related by a similarity transformation). The measurement of coherences in a basis other than the usual z basis also permits the study of spin dynamics under Hamiltonians that conserve z basis coherence number.
    BibTeX:
    @article{Ramanathan03,
      author = {Chandrasekhar Ramanathan and Hyungjoon Cho and Paola Cappellaro and Gregory S Boutis and David G Cory},
      title = {Encoding multiple quantum coherences in non-commuting bases},
      journal = {Chem. Phys. Lett.},
      year = {2003},
      volume = {369},
      pages = {311},
      doi = {10.1016/S0009-2614(02)02020-1}
    }
    
  233. C. Ramanathan, H. Cho, P.Cappellaro, G.S. Boutis and D. Cory
  234. "Exploring large nuclear spin systems in the solid state using NMR"
    In Proceedings of the 6th International Conference on Quantum Communication, Measurement and Computing. , 267-270 (2003)
    Abstract:Exploring large nuclear spin systems in the solid state using NMR.
    BibTeX:
    @inproceedings{Ramanathan03p,
      author = {C. Ramanathan, H. Cho, P.Cappellaro,G.S. Boutis and D.G. Cory},
      editor = {Jeffrey H. Shapiro and Osamu Hirota},
      title = {Exploring large nuclear spin systems in the solid state using NMR},
      booktitle = {Proceedings of the 6th International Conference on Quantum Communication, Measurement and Computing.},
      publisher = {Rinton Press},
      year = {2003},
      pages = {267-270}
    }
    
  235. C. Birattari, P. Cappellaro, A. Mitaroff and M. Silari
  236. "Development of an Extended Range Bonner Sphere Spectrometer"
    In Advanced Monte Carlo for Radiation Physics, Particle Transport Simulation and Applications Berlin, Heidelberg , 1157-1162 (2001)
    Abstract: To improve the response to high-energy neutrons of a conventional Bonner Sphere Spectrometer, the response functions of several detector configurations of different sizes and materials were calculated with the Monte Carlo programme FLUKA. The two most promising configurations were selected, built and afterwards exposed to neutrons of an Am-Be source and to a broad high-energy neutron spectrum at CERN. The comparison between the measured and calculated detector responses of the new spheres in these radiation fields confirms their simulated response functions and justifies their implementation into the conventional Bonner Sphere Spectrometer.
    BibTeX:
    @inproceedings{Birattari01,
      author = {Birattari, C. and Cappellaro, P. and Mitaroff, A. and Silari, M.},
      editor = {Kling, Andreas and Baro, Fernando J. C. and Nakagawa, Masayuki and Tavora, Luis and Vaz, Pedro},
      title = {Development of an Extended Range Bonner Sphere Spectrometer},
      booktitle = {Advanced Monte Carlo for Radiation Physics, Particle Transport Simulation and Applications},
      publisher = {Springer Berlin Heidelberg},
      year = {2001},
      pages = {1157--1162}
    }