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K. Agarwal, E. G. D. Torre, B. Rauer, T. Langen, J. Schmiedmayer and E. Demler “Chiral Prethermalization in supersonically split condensates” arxiv (2014) [Abstract] [BibTeX] [URL]

Abstract: We study the dynamics of phase relaxation between a pair of one-dimensional condensates created by a supersonic unzipping of a single condensate. We use the Lorentz invariance of the low energy sector of such systems to show that dephasing results in an unusual prethermal state, in which right- and left-moving excitations have different, Doppler-shifted temperatures. The chirality of these modes can be probed experimentally by measuring the interference fringe contrasts with the release point of the split condensates moving at another supersonic velocity. Further, an accelerated motion of the release point can be used to observe a space-like analogue of the Unruh effect. A concrete experimental realization of the quantum zipper for a BEC of trapped atoms on a atom chip is outlined. We interpret these results in the context of the general question of the Lorentz transformation of temperature, and the close analogy with the dipolar anisotropy of the Cosmic Microwave Background.

BibTeX: @article{Agarwal2014, author = {Kartiek Agarwal and Emanuele G. Dalla Torre and Bernhard Rauer and Tim Langen and Joerg Schmiedmayer and Eugene Demler}, title = {Chiral Prethermalization in supersonically split condensates}, journal = {arxiv}, year = {2014} }

A. Ajoy, U. Bissbort, M. D. Lukin, R. L. Walsworth and P. Cappellaro “Atomic-scale nuclear spin imaging using quantum-assisted sensors in diamond” arxiv (2014) [Abstract] [BibTeX] [URL]

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 nano-scale sensors has given hope of achieving the long-standing goal of single-protein, high spatial-resolution structure determination in their natural environment and 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{Ajoy2014, author = {Ashok Ajoy and Ulf Bissbort and Mikhail D. Lukin and Ronald L. Walsworth and Paola Cappellaro}, title = {Atomic-scale nuclear spin imaging using quantum-assisted sensors in diamond}, journal = {arxiv}, year = {2014} }

K. Arai, C. Belthangady, H. Zhang, S. J. DeVience, D. Le Sage, D. R. Glenn, L. M. Pham, L. Rahn-Lee, M. D. Lukin, A. Yacoby, A. Komeili and R. L. Walsworth “Optical Magnetic Imaging with Nitrogen-Vacancy Centers in Diamond” Biophysical Journal In Biophysical Journal 106, 191a- (2014) [BibTeX] [URL]

BibTeX: @article{Arai14, author = {Arai, Keigo and Belthangady, Chinmay and Zhang, Huiliang and DeVience, Stephen J. and Le Sage, David and Glenn, David R. and Pham, Linh M. and Rahn-Lee, Lilah and Lukin, Mikhail D. and Yacoby, Amir and Komeili, Arash and Walsworth, Ronald L.}, title = {Optical Magnetic Imaging with Nitrogen-Vacancy Centers in Diamond}, booktitle = {Biophysical Journal}, journal = {Biophysical Journal}, publisher = {Elsevier}, year = {2014}, volume = {106}, number = {2}, pages = {191a--} }

D. Benjamin, I. Klich and E. Demler “Single-Band Model of Resonant Inelastic X-Ray Scattering by Quasiparticles in High-T CuPhysical Review Ate Superconductors” Physical Review Letters 112(2014) [BibTeX] [URL]

BibTeX: @article{Benjamin2014, author = {Benjamin, David and Klich, Israel and Demler, Eugene}, title = {Single-Band Model of Resonant Inelastic X-Ray Scattering by Quasiparticles in High-T CuPhysical Review Ate Superconductors}, journal = {Physical Review Letters}, publisher = {American Physical Society (APS)}, year = {2014}, volume = {112}, number = {24} }

D. Benjamin and E. Demler “Variational polaron method for Bose-Bose mixtures” Phys. Rev. A 89(2014) [BibTeX] [URL]

BibTeX: @article{Benjamin2014a, author = {Benjamin, David and Demler, Eugene}, title = {Variational polaron method for Bose-Bose mixtures}, journal = {Phys. Rev. A}, publisher = {American Physical Society (APS)}, year = {2014}, volume = {89}, number = {3} }

A. Cooper, E. Magesan, H. Yum and P. Cappellaro “Time-resolved magnetic sensing with electronic spins in diamond” Nature Comms. 5, 3141 (2014) [BibTeX] [URL]

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} }

S. J. DeVience, L. M. Pham, I. Lovchinsky, A. O. Sushkov, N. Bar-Gill, C. Belthangady, F. Casola, M. Corbett, H. Zhang, M. Lukin, H. Park, A. Yacoby and R. L. Walsworth “Nanoscale NMR Spectroscopy and Imaging of Multiple Nuclear Species” arxiv (2014) [Abstract] [BibTeX] [URL]

Abstract: Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are well-established techniques that provide valuable information in a diverse set of disciplines but are currently limited to macroscopic sample volumes. Here we demonstrate nanoscale NMR spectroscopy and imaging under ambient conditions of samples containing multiple nuclear species, using nitrogen-vacancy (NV) colour centres in diamond as sensors. With single, shallow NV centres in a diamond chip and samples placed on the diamond surface, we perform NMR spectroscopy and one-dimensional MRI on few-nanometre-sized samples containing $^1$H and $^19$F nuclei. Alternatively, we employ a high-density NV layer near the surface of a diamond chip to demonstrate wide-field optical NMR spectroscopy of nanoscale samples containing $^1$H, $^19$F, and $^31$P nuclei, as well as multi-species two-dimensional optical MRI with sub-micron resolution. For all diamond samples exposed to air, we identify a ubiquitous $^1$H NMR signal, consistent with a $sim 1$ nm layer of adsorbed hydrocarbons or water on the diamond surface and below any sample placed on the diamond. This work lays the foundation for nanoscale NMR and MRI applications such as studies of single proteins and functional biological imaging with subcellular resolution, as well as characterization of thin films with sub-nanometre resolution.

BibTeX: @article{DeVience2014, author = {Stephen J. DeVience and Linh M. Pham and Igor Lovchinsky and Alexander O. Sushkov and Nir Bar-Gill and Chinmay Belthangady and Francesco Casola and Madeleine Corbett and Huiliang Zhang and Mikhail Lukin and Hongkun Park and Amir Yacoby and Ronald L. Walsworth}, title = {Nanoscale NMR Spectroscopy and Imaging of Multiple Nuclear Species}, journal = {arxiv}, year = {2014} }

M. S. Grinolds, M. Warner, K. De Greve, Y. Dovzhenko, L. Thiel, R. L. Walsworth, S. Hong, P. Maletinsky and A. Yacoby “Subnanometre resolution in three-dimensional magnetic resonance imaging of individual dark spins” Nature Nanotechnology 9, 279-284 (2014) [BibTeX] [URL]

BibTeX: @article{Grinolds2014, author = {Grinolds, M. S. and Warner, M. and De Greve, K. and Dovzhenko, Y. and Thiel, L. and Walsworth, R. L. and Hong, S. and Maletinsky, P. and Yacoby, A.}, title = {Subnanometre resolution in three-dimensional magnetic resonance imaging of individual dark spins}, journal = {Nature Nanotechnology}, publisher = {Nature Publishing Group}, year = {2014}, volume = {9}, number = {4}, pages = {279--284} }

F. Grusdt, D. Abanin and E. Demler “Measuring Z-topological invariants in optical lattices using interferometry” Phys. Rev. A 89(2014) [BibTeX] [URL]

BibTeX: @article{Grusdt2014, author = {Grusdt, F. and Abanin, D. and Demler, E.}, title = {Measuring Z-topological invariants in optical lattices using interferometry}, journal = {Phys. Rev. A}, publisher = {American Physical Society (APS)}, year = {2014}, volume = {89}, number = {4} }

S. Hild, T. Fukuhara, P. Schau�, J. Zeiher, M. Knap, E. Demler, I. Bloch and C. Gross “Far-from-equilibrium spin transport in Heisenberg quantum magnets” arxiv (2014) [Abstract] [BibTeX] [URL]

Abstract: We study experimentally the far-from-equilibrium dynamics in ferromagnetic Heisenberg quantum magnets realized with ultracold atoms in an optical lattice. After controlled imprinting of a spin spiral pattern with adjustable wave vector, we measure the decay of the initial spin correlations through single-site resolved detection. On the experimentally accessible timescale of several exchange times we find a profound dependence of the decay rate on the wave vector. In one-dimensional systems we observe diffusion-like spin transport with a dimensionless diffusion coefficient of 0.22(1). We show how this behavior emerges from the microscopic properties of the closed quantum system. In contrast to the one-dimensional case, our transport measurements for two-dimensional Heisenberg systems indicate anomalous super-diffusion.

BibTeX: @article{Hild2014, author = {Sebastian Hild and Takeshi Fukuhara and Peter Schau� and Johannes Zeiher and Michael Knap and Eugene Demler and Immanuel Bloch and Christian Gross}, title = {Far-from-equilibrium spin transport in Heisenberg quantum magnets}, journal = {arxiv}, year = {2014} }

E. M. Kessler, I. Lovchinsky, A. O. Sushkov and M. D. Lukin “Quantum Error Correction for Metrology” Physical Review Letters 112, 150802 (2014) [BibTeX] [URL]

BibTeX: @article{Kessler2014, author = {Kessler, E. M. and Lovchinsky, I. and Sushkov, A. O. and Lukin, M. D.}, title = {Quantum Error Correction for Metrology}, journal = {Physical Review Letters}, publisher = {American Physical Society (APS)}, year = {2014}, volume = {112}, number = {15}, pages = {150802} }

E. M. Kessler, P. Komer, M. Bishof, L. Jiang, A. S. Sorensen, J. Ye and M. D. Lukin “Heisenberg-Limited Atom Clocks Based on Entangled Qubits” Physical Review Letters 112(2014) [BibTeX] [URL]

BibTeX: @article{Kessler2014a, author = {Kessler, E. M. and Komer, P. and Bishof, M. and Jiang, L. and Sorensen, A. S. and Ye, J. and Lukin, M. D.}, title = {Heisenberg-Limited Atom Clocks Based on Entangled Qubits}, journal = {Physical Review Letters}, publisher = {American Physical Society (APS)}, year = {2014}, volume = {112}, number = {19} }

P. Komer, E. M. Kessler, M. Bishof, L. Jiang, A. S. S??rensen, J. Ye and M. D. Lukin “A quantum network of clocks” Nat Phys 10, 582-587 (2014) [BibTeX] [URL]

BibTeX: @article{Komer2014, author = {Komer, P. and Kessler, E. M. and Bishof, M. and Jiang, L. and S??rensen, A. S. and Ye, J. and Lukin, M. D.}, title = {A quantum network of clocks}, journal = {Nat Phys}, publisher = {Nature Publishing Group}, year = {2014}, volume = {10}, number = {8}, pages = {582--587} }

S. Lloyd “Quantum optics: Cool computation, hot bits” Nature Photon 8, 90-91 (2014) [BibTeX] [URL]

BibTeX: @article{Lloyd2014, author = {Lloyd, Seth}, title = {Quantum optics: Cool computation, hot bits}, journal = {Nature Photon}, publisher = {Nature Publishing Group}, year = {2014}, volume = {8}, number = {2}, pages = {90--91} }

S. Lloyd, M. Mohseni and P. Rebentrost “Quantum principal component analysis” Nat Phys (2014) [BibTeX] [URL]

BibTeX: @article{Lloyd2014a, author = {Lloyd, Seth and Mohseni, Masoud and Rebentrost, Patrick}, title = {Quantum principal component analysis}, journal = {Nat Phys}, publisher = {Nature Publishing Group}, year = {2014} }

S. Lloyd and S. Montangero “Universal quantum computation in integrable systems” arxiv (2014) [Abstract] [BibTeX] [URL]

Abstract: Quantized integrable systems can be made to perform universal quantum computation by the application of a global time-varying control. The action-angle variables of the integrable system function as qubits or qudits, which can be coupled selectively by the global control to induce universal quantum logic gates. By contrast, chaotic quantum systems, even if controllable, do not generically allow quantum computation under global control.

BibTeX: @article{Lloyd2014b, author = {Seth Lloyd and Simone Montangero}, title = {Universal quantum computation in integrable systems}, journal = {arxiv}, year = {2014} }

S. Lloyd, S. Garnerone and P. Zanardi “Quantum algorithms for topological and geometric analysis of big data” arxiv (2014) [Abstract] [BibTeX] [URL]

Abstract: Extracting useful information from large data sets can be a daunting task. Topological methods for analyzing data sets provide a powerful technique for extracting such information. Persistent homology is a sophisticated tool for identifying such topological features — connected components, holes, or voids — and for determining how such features persist as the data is viewed at different scales. This paper provides quantum algorithms for calculating Betti numbers in persistent homology, and for finding eigenvectors and eigenvalues of the combinatorial Laplacian. The algorithms provide an exponential speedup over classical algorithms for topological data analysis.

BibTeX: @article{Lloyd2014c, author = {Seth Lloyd and Silvano Garnerone and Paolo Zanardi}, title = {Quantum algorithms for topological and geometric analysis of big data}, journal = {arxiv}, year = {2014} }

D. Lu, J. D. Biamonte, J. Li, H. Li, T. H. Johnson, V. Bergholm, M. Faccin, Z. Zimbor�s, R. Laflamme, J. Baugh and S. Lloyd “Chiral Quantum Walks” arxiv (2014) [Abstract] [BibTeX] [URL]

Abstract: Wigner separated the possible types of symmetries in quantum theory into those symmetries that are unitary and those that are antiunitary. Unitary symmetries have been well studied whereas antiunitary symmetries and the physical implications associated with time-reversal symmetry breaking have had little influence on quantum information science. Here we develop a quantum circuits version of time-reversal symmetry theory, classifying time-symmetric and time-asymmetric Hamiltonians and circuits in terms of their underlying network elements and geometric structures. These results reveal that many of the typical quantum circuit networks found across the field of quantum information science exhibit time-asymmetry. We then experimentally implement the most fundamental time-reversal asymmetric process, applying local gates in an otherwise time-symmetric circuit to induce time-reversal asymmetry and thereby achieve (i) directional biasing in the transition probability between basis states, (ii) the enhancement of and (iii) the suppression of these transport probabilities. Our results imply that the physical effect of time-symmetry breaking plays an essential role in coherent transport and its control represents an omnipresent yet essentially untapped resource in quantum transport science.

BibTeX: @article{Lu2014, author = {DaWei Lu and Jacob D. Biamonte and Jun Li and Hang Li and Tomi H. Johnson and Ville Bergholm and Mauro Faccin and Zolt�n Zimbor�s and Raymond Laflamme and Jonathan Baugh and Seth Lloyd}, title = {Chiral Quantum Walks}, journal = {arxiv}, year = {2014} }

B. C. Sawyer, J. W. Britton and J. J. Bollinger “Spin dephasing as a probe of mode temperature, motional state distributions, and heating rates in a two-dimensional ion crystal” Phys. Rev. A 89(2014) [BibTeX] [URL]

BibTeX: @article{Sawyer2014, author = {Sawyer, Brian C. and Britton, Joseph W. and Bollinger, John J.}, title = {Spin dephasing as a probe of mode temperature, motional state distributions, and heating rates in a two-dimensional ion crystal}, journal = {Phys. Rev. A}, publisher = {American Physical Society (APS)}, year = {2014}, volume = {89}, number = {3} }

M. Serbyn, M. Knap, S. Gopalakrishnan, Z. Papic, N. Y. Yao, C. R. Laumann, D. A. Abanin, M. D. Lukin and E. A. Demler “Interferometric probes of many-body localization” arxiv (2014) [Abstract] [BibTeX] [URL]

Abstract: We propose a method for detecting many-body localization (MBL) in disordered spin systems. The method involves pulsed, coherent spin manipulations that probe the dephasing of a given spin due to its entanglement with a set of distant spins. It allows one to distinguish the MBL phase from a non-interacting localized phase and a delocalized phase. In particular, we show that for a properly chosen pulse sequence the MBL phase exhibits a characteristic power-law decay reflecting its slow growth of entanglement. We find that this power-law decay is robust with respect to thermal and disorder averaging, provide numerical simulations supporting our results, and discuss possible experimental realizations in solid-state and cold atom systems.

BibTeX: @article{Serbyn2014, author = {M. Serbyn and M. Knap and S. Gopalakrishnan and Z. Papic and N. Y. Yao and C. R. Laumann and D. A. Abanin and M. D. Lukin and E. A. Demler}, title = {Interferometric probes of many-body localization}, journal = {arxiv}, year = {2014} }

A. Shashi, F. Grusdt, D. A. Abanin and E. Demler “Radio-frequency spectroscopy of polarons in ultracold Bose gases” Phys. Rev. A 89(2014) [BibTeX] [URL]

BibTeX: @article{Shashi2014, author = {Shashi, Aditya and Grusdt, Fabian and Abanin, Dmitry A. and Demler, Eugene}, title = {Radio-frequency spectroscopy of polarons in ultracold Bose gases}, journal = {Phys. Rev. A}, publisher = {American Physical Society (APS)}, year = {2014}, volume = {89}, number = {5} }

V. M. Stojanovic, M. Vanevic, E. Demler and L. Tian “Transmon-based simulator of nonlocal electron-phonon coupling: A platform for observing sharp small-polaron transitions” Phys. Rev. B 89(2014) [BibTeX] [URL]

BibTeX: @article{Stojanovic2014, author = {Stojanovic, Vladimir M. and Vanevic, Mihajlo and Demler, Eugene and Tian, Lin}, title = {Transmon-based simulator of nonlocal electron-phonon coupling: A platform for observing sharp small-polaron transitions}, journal = {Phys. Rev. B}, publisher = {American Physical Society (APS)}, year = {2014}, volume = {89}, number = {14} }

X. Wang, M. Allegra, K. Jacobs, S. Lloyd, C. Lupo and M. Mohseni “Quantum brachistochrone curves as geodesics: obtaining accurate control protocols for time-optimal quantum gates” arxiv (2014) [Abstract] [BibTeX] [URL]

Abstract: Most methods of optimal control cannot obtain accurate time-optimal protocols. The quantum brachistochrone equation is an exception, and has the potential to provide accurate time-optimal protocols for essentially any quantum control problem. So far this potential has not been realized, however, due to the inadequacy of conventional numerical methods to solve it. Here, using differential geometry, we reformulate the quantum brachistochrone curves as geodesics on the unitary group. With this identification we are able to obtain a numerical method that efficiently solves the brachistochrone problem. We apply it to two examples demonstrating its power.

BibTeX: @article{Wang2014, author = {Xiaoting Wang and Michele Allegra and Kurt Jacobs and Seth Lloyd and Cosmo Lupo and Masoud Mohseni}, title = {Quantum brachistochrone curves as geodesics: obtaining accurate control protocols for time-optimal quantum gates}, journal = {arxiv}, year = {2014} }

N. Y. Yao, C. P. Moca, I. Weymann, J. D. Sau, M. D. Lukin, E. A. Demler and G. Zarand “Phase diagram and excitations of a Shiba molecule” arxiv (2014) [Abstract] [BibTeX] [URL]

Abstract: We analyze the phase diagram associated with a pair of magnetic impurities trapped in a superconducting host. The natural interplay between Kondo screening, superconductivity and exchange interactions leads to a rich array of competing phases, whose transitions are characterized by discontinuous changes of the total spin. Our analysis is based on a combination of numerical renormalization group techniques as well as semi-classical analytics. In addition to the expected screened and unscreened phases, we observe a new molecular doublet phase where the impurity spins are only partially screened by a single extended quasiparticle. Direct signatures of the various Shiba molecule states can be observed via RF spectroscopy.

BibTeX: @article{Yao2014, author = {N. Y. Yao and C. P. Moca and I. Weymann and J. D. Sau and M. D. Lukin and E. A. Demler and G. Zarand}, title = {Phase diagram and excitations of a Shiba molecule}, journal = {arxiv}, year = {2014} }

C. D. Aiello, M. Hirose and P. Cappellaro “Composite-pulse magnetometry with a solid-state quantum sensor” Nature Communications 4(2013) [Abstract] [BibTeX] [URL]

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 ?THz-1/2 range, comparable with those obtained with Ramsey spectroscopy, and coherence times approaching T1.

BibTeX: @article{Aiello2013, author = {Aiello, C.~D. and Hirose, M. and Cappellaro, P.}, title = {Composite-pulse magnetometry with a solid-state quantum sensor}, journal = {Nature Communications}, year = {2013}, volume = {4} }

K. Arai, D. Le Sage, S. J. DeVience, D. R. Glenn, L. M. Pham, L. Rahn-Lee, M. D. Lukin, A. Yacoby, A. Komeili and R. L. Walsworth “Wide-Field Magnetic Imaging using Nitrogen-Vacancy Color Centers in Diamond” Biophysical Journal 104, 193a (2013) [BibTeX] [URL]

BibTeX: @article{Arai2013, author = {Arai, Keigo and Le Sage, David and DeVience, Stephen J. and Glenn, David R. and Pham, Linh M. and Rahn-Lee, Lilah and Lukin, Mikhail D. and Yacoby, Amir and Komeili, Arash and Walsworth, Ronald L.}, title = {Wide-Field Magnetic Imaging using Nitrogen-Vacancy Color Centers in Diamond}, journal = {Biophysical Journal}, publisher = {Elsevier BV}, year = {2013}, volume = {104}, number = {2}, pages = {193a} }

N. Bar-Gill, L. M. Pham, A. Jarmola, D. Budker and R. L. Walsworth “Solid-state electronic spin coherence time approaching one second” Nature Communications 4(2013) [Abstract] [BibTeX] [URL]

Abstract: Solid-state spin systems such as nitrogen-vacancy colour centres in diamond are promising for applications of quantum information, sensing and metrology. However, a key challenge for such solid-state systems is to realize a spin coherence time that is much longer than the time for quantum spin manipulation protocols. Here we demonstrate an improvement of more than two orders of magnitude in the spin coherence time (T2) of nitrogen-vacancy centres compared with previous measurements: T2?0.6?s at 77?K. We employed dynamical decoupling pulse sequences to suppress nitrogen-vacancy spin decoherence, and found that T2 is limited to approximately half of the longitudinal spin relaxation time over a wide range of temperatures, which we attribute to phonon-induced decoherence. Our results apply to ensembles of nitrogen-vacancy spins, and thus could advance quantum sensing, enable squeezing and many-body entanglement, and open a path to simulating driven, interaction-dominated quantum many-body Hamiltonians.

BibTeX: @article{Bar-Gill2013, author = {Bar-Gill, N. and Pham, L.~M. and Jarmola, A. and Budker, D. and Walsworth, R.~L.}, title = {Solid-state electronic spin coherence time approaching one second}, journal = {Nature Communications}, year = {2013}, volume = {4} }

C. Belthangady, N. Bar-Gill, L. M. Pham, K. Arai, D. Le Sage, P. Cappellaro and R. L. Walsworth “Dressed-State Resonant Coupling between Bright and Dark Spins in Diamond” Phys. Rev. Lett. 110, 157601 (2013) [Abstract] [BibTeX] [URL]

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} }

S. Bennett, N. Yao, J. Otterbach, P. Zoller, P. Rabl and M. Lukin “Phonon-Induced Spin-Spin Interactions in Diamond Nanostructures: Application to Spin Squeezing” Physical Review Letters 110(2013) [BibTeX] [URL]

BibTeX: @article{Bennett2013, author = {Bennett, S. and Yao, N. and Otterbach, J. and Zoller, P. and Rabl, P. and Lukin, M.}, title = {Phonon-Induced Spin-Spin Interactions in Diamond Nanostructures: Application to Spin Squeezing}, journal = {Physical Review Letters}, publisher = {American Physical Society (APS)}, year = {2013}, volume = {110}, number = {15} }

J. J. Bollinger, J. W. Britton and B. C. Sawyer “Simulating quantum magnetism with correlated non-neutral ion plasmas” In American Institute of Physics Conference Series 1521, 200-209 (2013) [Abstract] [BibTeX] [URL]

Abstract: By employing forces that depend on the internal electronic state (or spin) of an atomic ion, the Coulomb potential energy of a strongly coupled array of ions can be modified in a spin-dependent way to mimic effective quantum spin Hamiltonians. Both ferromagnetic and anti-ferromagnetic interactions can be implemented. We use simple models to explain how the effective spin interactions are engineered with trapped-ion crystals. We summarize the type of effective spin interactions that can be readily generated, and discuss an experimental implementation using single-plane ion crystals in a Penning trap.

BibTeX: @inproceedings{Bollinger2013, author = {Bollinger, J.~J. and Britton, J.~W. and Sawyer, B.~C.}, editor = {Sarasola, X. and Schweikhard, L. and Pedersen, T.~S.}, title = {Simulating quantum magnetism with correlated non-neutral ion plasmas}, booktitle = {American Institute of Physics Conference Series}, year = {2013}, volume = {1521}, pages = {200-209} }

A. Cooper, E. Magesan, H. Yum and P. Cappellaro “Time-resolved magnetic sensing with electronic spins in diamond” ArXiv:1305.6082 (2013) [Abstract] [BibTeX] [URL]

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{Cooper2013, author = {Cooper, A. and Magesan, E. and Yum, H.N. and Cappellaro, P.}, title = {Time-resolved magnetic sensing with electronic spins in diamond}, journal = {ArXiv:1305.6082}, year = {2013} }

E. Dalla Torre, J. Otterbach, E. Demler, V. Vuletic and M. Lukin “Dissipative Preparation of Spin Squeezed Atomic Ensembles in a Steady State” Physical Review Letters 110(2013) [BibTeX] [URL]

BibTeX: @article{DallaTorre2013, author = {Dalla Torre, Emanuele and Otterbach, Johannes and Demler, Eugene and Vuletic, Vladan and Lukin, Mikhail}, title = {Dissipative Preparation of Spin Squeezed Atomic Ensembles in a Steady State}, journal = {Physical Review Letters}, publisher = {American Physical Society (APS)}, year = {2013}, volume = {110}, number = {12} }

M. Foss-Feig, K. R. A. Hazzard, J. J. Bollinger, A. M. Rey and C. W. Clark “Dynamical quantum correlations of Ising models on an arbitrary lattice and their resilience to decoherence” New Journal of Physics 15, 113008 (2013) [BibTeX] [URL]

BibTeX: @article{Foss-Feig2013, author = {Foss-Feig, M and Hazzard, K R A and Bollinger, J J and Rey, A M and Clark, C W}, title = {Dynamical quantum correlations of Ising models on an arbitrary lattice and their resilience to decoherence}, journal = {New Journal of Physics}, publisher = {IOP Publishing}, year = {2013}, volume = {15}, number = {11}, pages = {113008} }

M. Foss-Feig, K. R. A. Hazzard, J. J. Bollinger, A. M. Rey and C. W. Clark “Dynamical quantum correlations of Ising models on an arbitrary lattice and their resilience to decoherence” ArXiv:1306.0172 (2013) [Abstract] [BibTeX] [URL]

Abstract: Ising models, and the physical systems described by them, play a central role in generating entangled states for use in quantum metrology and quantum information. In particular, ultracold atomic gases, trapped ion systems, and Rydberg atoms realize long-ranged Ising models, which even in the absence of a transverse field can give rise to highly non-classical dynamics and long-range quantum correlations. In the first part of this paper, we present a detailed theoretical framework for studying the dynamics of such systems driven (at time t=0) into arbitrary unentangled non-equilibrium states, thus greatly extending and unifying the work of Ref. [1]. Specifically, we derive exact expressions for closed-time-path ordered correlation functions, and use these to study experimentally relevant observables, e.g. Bloch vector and spin-squeezing dynamics. In the second part, these correlation functions are then used to derive closed-form expressions for the dynamics of arbitrary spin-spin correlation functions in the presence of both T_1 (spontaneous spin relaxation/excitation) and T_2 (dephasing) type decoherence processes. Even though the decoherence is local, our solution reveals that the competition between Ising dynamics and T_1 decoherence gives rise to an emergent non-local dephasing effect, thereby drastically amplifying the degradation of quantum correlations. In addition to identifying the mechanism of this deleterious effect, our solution points toward a scheme to eliminate it via measurement-based coherent feedback.

BibTeX: @article{Foss-Feig2013a, author = {Michael Foss-Feig and Kaden R A Hazzard and John J Bollinger and Ana Maria Rey and Charles W Clark}, title = {Dynamical quantum correlations of Ising models on an arbitrary lattice and their resilience to decoherence}, journal = {ArXiv:1306.0172}, year = {2013} }

M. Foss-Feig, K. R. A. Hazzard, J. J. Bollinger and A. M. Rey “Nonequilibrium dynamics of arbitrary-range Ising models with decoherence: An exact analytic solution” Phys. Rev. A 87, 042101 (2013) [Abstract] [BibTeX] [URL]

Abstract: The interplay between interactions and decoherence in many-body systems is of fundamental importance in quantum physics. In a step toward understanding this interplay, we obtain an exact analytic solution for the nonequilibrium dynamics of Ising models with arbitrary couplings (and therefore in arbitrary dimension) and subject to local Markovian decoherence. Our solution shows that decoherence significantly degrades the nonclassical correlations developed during coherent Ising spin dynamics, which relax much faster than predicted by treating decoherence and interactions separately. We also show that the competition of decoherence and interactions induces a transition from oscillatory to overdamped dynamics that is absent at the single-particle or mean-field level. These calculations are applicable to ongoing quantum information and emulation efforts using a variety of atomic, molecular, optical, and solid-state systems. In particular, we apply our results to the NIST Penning trapped-ion experiment and show that the current experiment is capable of producing entanglement amongst hundreds of quantum spins.

BibTeX: @article{Foss-Feig2013b, author = {Foss-Feig, M. and Hazzard, K.~R.~A. and Bollinger, J.~J. and Rey, A.~M.}, title = {Nonequilibrium dynamics of arbitrary-range Ising models with decoherence: An exact analytic solution}, journal = {Phys. Rev. A}, year = {2013}, volume = {87}, number = {4}, pages = {042101} }

M. S. Grinolds, S. Hong, P. Maletinsky, L. Luan, M. D. Lukin, R. L. Walsworth and A. Yacoby “Nanoscale magnetic imaging of a single electron spin under ambient conditions” Nat Phys 9, 215-219 (2013) [BibTeX] [URL]

BibTeX: @article{Grinolds2013, author = {Grinolds, M. S. and Hong, S. and Maletinsky, P. and Luan, L. and Lukin, M. D. and Walsworth, R. L. and Yacoby, A.}, title = {Nanoscale magnetic imaging of a single electron spin under ambient conditions}, journal = {Nat Phys}, publisher = {Nature Publishing Group}, year = {2013}, volume = {9}, number = {4}, pages = {215--219} }

J. Hodges, N. Yao, D. Maclaurin, C. Rastogi, M. Lukin and D. Englund “Timekeeping with electron spin states in diamond” Phys. Rev. A 87(2013) [BibTeX] [URL]

BibTeX: @article{Hodges2013, author = {Hodges, J. and Yao, N. and Maclaurin, D. and Rastogi, C. and Lukin, M. and Englund, D.}, title = {Timekeeping with electron spin states in diamond}, journal = {Phys. Rev. A}, publisher = {American Physical Society (APS)}, year = {2013}, volume = {87}, number = {3} }

G. Kucsko, P. C. Maurer, N. Y. Yao, M. Kubo, H. J. Noh, P. K. Lo, H. Park and M. D. Lukin “Nanometre-scale thermometry in a living cell” Nature 500, 54-58 (2013) [BibTeX] [URL]

BibTeX: @article{Kucsko2013, author = {Kucsko, G. and Maurer, P. C. and Yao, N. Y. and Kubo, M. and Noh, H. J. and Lo, P. K. and Park, H. and Lukin, M. D.}, title = {Nanometre-scale thermometry in a living cell}, journal = {Nature}, publisher = {Nature Publishing Group}, year = {2013}, volume = {500}, number = {7460}, pages = {54--58} }

G. Kucsko, P. C. Maurer, N. Y. Yao, M. Kubo, H. J. Noh, P. K. Lo, H. Park and M. D. Lukin “Nanometer scale quantum thermometry in a living cell” Nature 500,, 54-58 (2013) [Abstract] [BibTeX] [URL]

Abstract: Sensitive probing of temperature variations on nanometer scales represents an outstanding challenge in many areas of modern science and technology. In particular, a thermometer capable of sub-degree temperature resolution as well as integration within a living system could provide a powerful new tool for many areas of biological research, including temperature-induced control of gene expression and cell-selective treatment of disease. Here, we demonstrate a new approach to nanoscale thermometry that utilizes coherent manipulation of the electronic spin associated with nitrogen-vacancy (NV) color centers in diamond. We show the ability to detect temperature variations down to 1.8 mK (sensitivity of 9 mK/sqrt(Hz)) in an ultra-pure bulk diamond sample. Using NV centers in diamond nanocrystals (nanodiamonds), we directly measure the local thermal environment at length scales down to 200 nm. Finally, by introducing both nanodiamonds and gold nanoparticles into a single human embryonic fibroblast, we demonstrate temperature-gradient control and mapping at the sub-cellular level, enabling unique potential applications in life sciences.

BibTeX: @article{Kucsko2013a, author = {G. Kucsko and P. C. Maurer and N. Y. Yao and M. Kubo and H. J. Noh and P. K. Lo and H. Park and M. D. Lukin}, title = {Nanometer scale quantum thermometry in a living cell}, journal = {Nature}, year = {2013}, volume = {500,}, pages = {54-58} }

D. Leibrandt, M. Thorpe, C.-W. Chou, T. Fortier, S. Diddams and T. Rosenband “Absolute and Relative Stability of an Optical Frequency Reference Based on Spectral Hole Burning in Eu:25” Physical Review Letters 111(2013) [BibTeX] [URL]

BibTeX: @article{Leibrandt2013, author = {Leibrandt, David and Thorpe, Michael and Chou, Chin-Wen and Fortier, Tara and Diddams, Scott and Rosenband, Till}, title = {Absolute and Relative Stability of an Optical Frequency Reference Based on Spectral Hole Burning in Eu:25}, journal = {Physical Review Letters}, publisher = {American Physical Society (APS)}, year = {2013}, volume = {111}, number = {23} }

D. R. Leibrandt, J. C. Bergquist and T. Rosenband “Cavity-stabilized laser with acceleration sensitivity below 10$^-12$ g$^-1$” Phys. Rev. A 87, 023829 (2013) [Abstract] [BibTeX] [URL]

Abstract: We characterize the frequency sensitivity of a cavity-stabilized laser to inertial forces and temperature fluctuations, and perform real-time feedforward to correct for these sources of noise. We measure the sensitivity of the cavity to linear accelerations, rotational accelerations, and rotational velocities by rotating it about three axes with accelerometers and gyroscopes positioned around the cavity. The worst-direction linear acceleration sensitivity of the cavity is 2(1)�10-11 g-1 measured over 0-50 Hz, which is reduced by a factor of 50 to below 10-12 g-1 for low-frequency accelerations by real-time feedforward corrections of all of the aforementioned inertial forces. A similar idea is demonstrated in which laser frequency drift due to temperature fluctuations is reduced by a factor of 70 via real-time feedforward from a temperature sensor located on the outer wall of the cavity vacuum chamber.

BibTeX: @article{Leibrandt2013a, author = {Leibrandt, D.~R. and Bergquist, J.~C. and Rosenband, T. }, title = {Cavity-stabilized laser with acceleration sensitivity below 10$^-12$ g$^-1$}, journal = {Phys. Rev. A}, year = {2013}, volume = {87}, number = {2}, pages = {023829} }

D. Le Sage, K. Arai, D. R. Glenn, S. J. DeVience, L. M. Pham, L. Rahn-Lee, M. D. Lukin, A. Yacoby, A. Komeili and R. L. Walsworth “Optical magnetic imaging of living cells” Nature 496, 486-489 (2013) [BibTeX] [URL]

BibTeX: @article{LeSage2013, author = {Le Sage, D. and Arai, K. and Glenn, D. R. and DeVience, S. J. and Pham, L. M. and Rahn-Lee, L. and Lukin, M. D. and Yacoby, A. and Komeili, A. and Walsworth, R. L.}, title = {Optical magnetic imaging of living cells}, journal = {Nature}, publisher = {Nature Publishing Group}, year = {2013}, volume = {496}, number = {7446}, pages = {486--489} }

S. Lloyd “Core-halo instability in dynamical systems” arxiv (2013) [Abstract] [BibTeX] [URL]

Abstract: This paper proves an instability theorem for dynamical systems. As one adds interactions between subystems in a complex system, structured or random, a threshold of connectivity is reached beyond which the overall dynamics inevitably goes unstable. The threshold occurs at the point at which flows and interactions between subsystems (`surface’ effects) overwhelm internal stabilizing dynamics (`volume’ effects). The theorem is used to identify instability thresholds in systems that possess a core-halo or core-periphery structure, including the gravo-thermal catastrophe — i.e., star collapse and explosion — and the interbank payment network. In the core-halo model, the same dynamical instability underlies both gravitational and financial collapse.

BibTeX: @article{Lloyd2013, author = {Seth Lloyd}, title = {Core-halo instability in dynamical systems}, journal = {arxiv}, year = {2013} }

S. Lloyd, M. Mohseni and P. Rebentrost “Quantum algorithms for supervised and unsupervised machine learning” arxiv (2013) [Abstract] [BibTeX] [URL]

Abstract: Machine-learning tasks frequently involve problems of manipulating and classifying large numbers of vectors in high-dimensional spaces. Classical algorithms for solving such problems typically take time polynomial in the number of vectors and the dimension of the space. Quantum computers are good at manipulating high-dimensional vectors in large tensor product spaces. This paper provides supervised and unsupervised quantum machine learning algorithms for cluster assignment and cluster finding. Quantum machine learning can take time logarithmic in both the number of vectors and their dimension, an exponential speed-up over classical algorithms.

BibTeX: @article{Lloyd2013a, author = {Seth Lloyd and Masoud Mohseni and Patrick Rebentrost}, title = {Quantum algorithms for supervised and unsupervised machine learning}, journal = {arxiv}, year = {2013} }

S. Lloyd and J. Preskill “Unitarity of black hole evaporation in final-state projection models” arxiv (2013) [Abstract] [BibTeX] [URL]

Abstract: Almheiri et al. have emphasized that otherwise reasonable beliefs about black hole evaporation are incompatible with the monogamy of quantum entanglement, a general property of quantum mechanics. We investigate the final-state projection model of black hole evaporation proposed by Horowitz and Maldacena, pointing out that this model admits cloning of quantum states and polygamous entanglement, allowing unitarity of the evaporation process to be reconciled with smoothness of the black hole event horizon. Though the model seems to require carefully tuned dynamics to ensure exact unitarity of the black hole S-matrix, for a generic final-state boundary condition the deviations from unitarity are exponentially small in the black hole entropy; furthermore observers inside black holes need not detect any deviations from standard quantum mechanics. Though measurements performed inside old black holes could potentially produce causality-violating phenomena, the computational complexity of decoding the Hawking radiation may render the causality violation unobservable. Final-state projection models illustrate how inviolable principles of standard quantum mechanics might be circumvented in a theory of quantum gravity.

BibTeX: @article{Lloyd2013b, author = {Seth Lloyd and John Preskill}, title = {Unitarity of black hole evaporation in final-state projection models}, journal = {arxiv}, year = {2013} }

S. Lloyd, M. Mohseni and P. Rebentrost “Quantum self analysis” ArXiv:1307.0401 (2013) [Abstract] [BibTeX] [URL]

Abstract: The usual way to reveal properties of an unknown quantum state, given many copies of a system in that state, is to perform measurements of different observables and to analyze the measurement results statistically. Here we show that the unknown quantum state can play an active role in its own analysis. In particular, given multiple copies of a quantum system with density matrix R, then it is possible to perform the unitary transformation e^-i R t. As a result, one can create quantum coherence among different copies of the system to perform `quantum self analysis,’ revealing the eigenvectors and eigenvalues of the unknown state in time exponentially faster than any existing algorithm.

BibTeX: @article{Lloyd2013c, author = {Seth Lloyd and Masoud Mohseni and Patrick Rebentrost}, title = {Quantum self analysis}, journal = {ArXiv:1307.0401}, year = {2013} }

E. Magesan, A. Cooper, H. Yum and P. Cappellaro “Reconstructing the profile of time-varying magnetic fields with quantum sensors” Phys. Rev. A 88, 032107 (2013) [Abstract] [BibTeX] [URL]

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} }

E. Magesan, A. Cooper and P. Cappellaro “Compressing measurements in quantum dynamic parameter estimation” Phys. Rev. A 88, 062109 (2013) [Abstract] [BibTeX] [URL]

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 significantly reduce the number of signal coefficients that are required for reconstruction of time-varying parameters with high fidelity. 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. Random measurement matrices satisfying the restricted isometry property can be used efficiently to reconstruct signals that are sparse in any basis. Because many physical processes are approximately sparse in some basis, these methods can benefit a variety of applications such as quantum sensing and magnetometry with nitrogen-vacancy centers.

BibTeX: @article{Magesan13c, author = {Magesan, Easwar and Cooper, Alexandre and Cappellaro, Paola}, title = {Compressing measurements in quantum dynamic parameter estimation}, journal = {Phys. Rev. A}, year = {2013}, volume = {88}, pages = {062109} }

E. Magesan, A. Cooper, H. Yum and P. Cappellaro “Reconstructing the Profile of Time-Varying Magnetic Fields With Quantum Sensors” ArXiv:1305.6604 (2013) [Abstract] [BibTeX] [URL]

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{Magesan2013, author = {Easwar Magesan and Alexandre Cooper and Honam Yum and Paola Cappellaro}, title = {Reconstructing the Profile of Time-Varying Magnetic Fields With Quantum Sensors}, journal = {ArXiv:1305.6604}, year = {2013} }

E. Magesan, A. Cooper and P. Cappellaro “Compressing measurements in quantum dynamic parameter estimation” ArXiv:1308.0313 (2013) [Abstract] [BibTeX] [URL]

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 Physical Review Actically relevant numerical examples.

BibTeX: @article{Magesan2013a, author = {Easwar Magesan and Alexandre Cooper and Paola Cappellaro}, title = {Compressing measurements in quantum dynamic parameter estimation}, journal = {ArXiv:1308.0313}, year = {2013} }

E. Magesan and P. Cappellaro “Experimentally efficient methods for estimating the performance of quantum measurements” Physical Review A 88, 022127 (2013) [BibTeX] [URL]

BibTeX: @article{Magesan2013b, author = {Magesan, Easwar and Cappellaro, Paola}, title = {Experimentally efficient methods for estimating the performance of quantum measurements}, journal = {Physical Review A}, year = {2013}, volume = {88}, pages = {022127} }

R. McConnell, H. Zhang, S. Cuk, J. Hu, M. Schleier-Smith and V. Vuletic “Generating entangled spin states for quantum metrology by single-photon detection” Phys. Rev. A 88(2013) [BibTeX] [URL]

BibTeX: @article{McConnell2013, author = {McConnell, Robert and Zhang, Hao and Cuk, Senka and Hu, Jiazhong and Schleier-Smith, Monika and Vuletic, Vladan}, title = {Generating entangled spin states for quantum metrology by single-photon detection}, journal = {Phys. Rev. A}, publisher = {American Physical Society (APS)}, year = {2013}, volume = {88}, number = {6} }

L. Mourokh and S. Lloyd “Optimal rates for electron transfer in Marcus theory” Phys. Rev. E 88(2013) [BibTeX] [URL]

BibTeX: @article{Mourokh2013, author = {Mourokh, Lev and Lloyd, Seth}, title = {Optimal rates for electron transfer in Marcus theory}, journal = {Phys. Rev. E}, publisher = {American Physical Society (APS)}, year = {2013}, volume = {88}, number = {4} }

S. Pirandola, N. J. Cerf, S. L. Braunstein and S. Lloyd “Bosonic Minimum Output Entropy and Gaussian Discord” arxiv (2013) [Abstract] [BibTeX] [URL]

Abstract: We connect two fundamental conjectures in quantum information theory. One is the bosonic minimum output entropy conjecture, claiming that the output entropy of a Gaussian channel is minimized by a pure Gaussian state at the input. The other conjecture is the optimality of Gaussian discord, according to which the computation of quantum discord for Gaussian states can be restricted to Gaussian measurements. Here we prove that the first conjecture implies the second one for a large family of Gaussian states, with an equivalence holding when the Choi-Jamiolkowski isomorphism can be established. Furthermore, by exploiting the validity of the entropy conjecture for some canonical channels, we compute the unrestricted quantum discord for broad classes of Gaussian states.

BibTeX: @article{Pirandola2013, author = {Stefano Pirandola and Nicolas J. Cerf and Samuel L. Braunstein and Seth Lloyd}, title = {Bosonic Minimum Output Entropy and Gaussian Discord}, journal = {arxiv}, year = {2013} }

S. Pirandola, C. Ottaviani, G. Spedalieri, C. Weedbrook, S. L. Braunstein, S. Lloyd, T. Gehring, C. S. Jacobsen and U. L. Andersen “High-rate quantum cryptography in untrusted networks” arxiv (2013) [Abstract] [BibTeX] [URL]

Abstract: We extend the field of continuous-variable quantum cryptography to a network formulation where two honest parties connect to an untrusted relay by insecure quantum links. To generate secret correlations, they transmit coherent states to the relay where a continuous-variable Bell detection is performed and the outcome broadcast. Even though the detection could be fully corrupted and the links subject to optimal coherent attacks, the honest parties can still extract a secret key, achieving high rates when the relay is proximal to one party, as typical in public networks with access points or proxy servers. Our theory is confirmed by an experiment generating key-rates which are orders of magnitude higher than those achievable with discrete-variable protocols. Thus, using the cheapest possible quantum resources, we experimentally show the possibility of high-rate quantum key distribution in network topologies where direct links are missing between end-users and intermediate relays cannot be trusted.

BibTeX: @article{Pirandola2013a, author = {Stefano Pirandola and Carlo Ottaviani and Gaetana Spedalieri and Christian Weedbrook and Samuel L. Braunstein and Seth Lloyd and Tobias Gehring and Christian S. Jacobsen and Ulrik L. Andersen}, title = {High-rate quantum cryptography in untrusted networks}, journal = {arxiv}, year = {2013} }

P. Rebentrost, M. Mohseni and S. Lloyd “Quantum support vector machine for big data classification” arxiv (2013) [Abstract] [BibTeX] [URL]

Abstract: Supervised machine learning is the classification of new data based on already classified training examples. In this work, we show that the support vector machine, an optimized binary classifier, can be implemented on a quantum computer, with complexity logarithmic in the size of the vectors and the number of training examples. In cases when classical sampling algorithms require polynomial time, an exponential speed-up is obtained. At the core of this quantum big data algorithm is a non-sparse matrix exponentiation technique for efficiently performing a matrix inversion of the training data inner-product (kernel) matrix.

BibTeX: @article{Rebentrost2013, author = {Patrick Rebentrost and Masoud Mohseni and Seth Lloyd}, title = {Quantum support vector machine for big data classification}, journal = {arxiv}, year = {2013} }

T. Rosenband and D. R. Leibrandt “Exponential scaling of clock stability with atom number” ArXiv:1303.6357 (2013) [Abstract] [BibTeX] [URL]

Abstract: In trapped-atom clocks, the primary source of decoherence is often the phase noise of the oscillator. For this case, we derive theoretical performance gains by combining several atomic ensembles. For example, M ensembles of N atoms can be combined with a variety of probe periods, to reduce the frequency variance to M 2^-M times that of standard Ramsey clocks. A similar exponential improvement is possible if the atomic phases of some of the ensembles evolve at reduced frequencies. These ensembles may be constructed from atoms or molecules with lower-frequency transitions, or generated by dynamical decoupling. The ensembles with reduced frequency or probe period are responsible only for counting the integer number of 2 pi phase wraps, and do not affect the clock’s systematic errors. Quantum phase measurement with Gaussian initial states allows for smaller ensemble sizes than Ramsey spectroscopy.

BibTeX: @article{Rosenband2013, author = {T. Rosenband and D. R. Leibrandt}, title = {Exponential scaling of clock stability with atom number}, journal = {ArXiv:1303.6357}, year = {2013} }

A. O. Sushkov, N. Chisholm, I. Lovchinsky, M. Kubo, P. K. Lo, S. D. Bennett, D. Hunger, A. Akimov, R. L. Walsworth, H. Park and M. D. Lukin “All-optical sensing of a single-molecule electron spin” arxiv (2013) [Abstract] [BibTeX] [URL]

Abstract: We demonstrate an all-optical method for magnetic sensing of individual molecules in ambient conditions at room temperature. Our approach is based on shallow nitrogen-vacancy (NV) centers near the surface of a diamond crystal, which we use to detect single paramagnetic molecules covalently attached to the diamond surface. The manipulation and readout of the NV centers is all-optical and provides a sensitive probe of the magnetic field fluctuations stemming from the dynamics of the electronic spins of the attached molecules. As a specific example, we demonstrate detection of a single paramagnetic molecule containing a gadolinium (Gd$^3+$) ion. We confirm single-molecule resolution using optical fluorescence and atomic force microscopy to co-localize one NV center and one Gd$^3+$-containing molecule. Possible applications include nanoscale and in vivo magnetic spectroscopy and imaging of individual molecules.

BibTeX: @article{Sushkov2013, author = {A. O. Sushkov and N. Chisholm and I. Lovchinsky and M. Kubo and P. K. Lo and S. D. Bennett and D. Hunger and A. Akimov and R. L. Walsworth and H. Park and M. D. Lukin}, title = {All-optical sensing of a single-molecule electron spin}, journal = {arxiv}, year = {2013} }

U. Warring, C. Ospelkaus, Y. Colombe, R. Jördens, D. Leibfried and D. J. Wineland “Individual-Ion Addressing with Microwave Field Gradients” Physical Review Letters 110, 173002 (2013) [Abstract] [BibTeX] [URL]

Abstract: Individual-qubit addressing is a prerequisite for many instances of quantum information processing. We demonstrate this capability on trapped-ion qubits with microwave near fields delivered by electrode structures integrated into a microfabricated surface-electrode trap. We describe four approaches that may be used in quantum information experiments with hyperfine levels as qubits. We implement individual control on two Mg+25 ions separated by 4.3?m and find spin-flip crosstalk errors on the order of 10-3.

BibTeX: @article{Warring2013, author = {Warring, U. and Ospelkaus, C. and Colombe, Y. and Jördens, R. and Leibfried, D. and Wineland, D.~J.}, title = {Individual-Ion Addressing with Microwave Field Gradients}, journal = {Physical Review Letters}, year = {2013}, volume = {110}, number = {17}, pages = {173002} }

U. Warring, C. Ospelkaus, Y. Colombe, K. R. Brown, J. M. Amini, M. Carsjens, D. Leibfried and D. J. Wineland “Techniques for microwave near-field quantum control of trapped ions” Phys. Rev. A 87, 013437 (2013) [Abstract] [BibTeX] [URL]

Abstract: Microwave near-field quantum control of spin and motional degrees of freedom of 25Mg+ ions can be used to generate two-ion entanglement, as recently demonstrated in Ospelkaus [Nature0028-083610.1038/nature10290 476, 181 (2011)]. Here, we describe additional details of the setup and calibration procedures for these experiments. We discuss the design and characteristics of the surface-electrode trap and the microwave system and compare experimental measurements of the microwave near fields with numerical simulations. Additionally, we present a method that utilizes oscillating magnetic-field gradients to detect micromotion induced by the ponderomotive radio-frequency potential in linear traps. Finally, we discuss the present limitations of microwave-driven two-ion entangling gates in our system.

BibTeX: @article{Warring2013a, author = {Warring, U. and Ospelkaus, C. and Colombe, Y. and Brown, K.~R. and Amini, J.~M. and Carsjens, M. and Leibfried, D. and Wineland, D.~J. }, title = {Techniques for microwave near-field quantum control of trapped ions}, journal = {Phys. Rev. A}, year = {2013}, volume = {87}, number = {1}, pages = {013437} }

M. v. d. Worm, B. C. Sawyer, J. J. Bollinger and M. Kastner “Relaxation timescales and decay of correlations in a long-range interacting quantum simulator” New Journal of Physics 15, 083007 (2013) [BibTeX] [URL]

BibTeX: @article{Worm2013, author = {Worm, Mauritz van den and Sawyer, Brian C and Bollinger, John J and Kastner, Michael}, title = {Relaxation timescales and decay of correlations in a long-range interacting quantum simulator}, journal = {New Journal of Physics}, publisher = {IOP Publishing}, year = {2013}, volume = {15}, number = {8}, pages = {083007} }

N. Yao, Z.-X. Gong, C. Laumann, S. Bennett, L.-M. Duan, M. Lukin, L. Jiang and A. Gorshkov “Quantum logic between remote quantum registers” Phys. Rev. A 87(2013) [BibTeX] [URL]

BibTeX: @article{Yao2013, author = {Yao, N. and Gong, Z.-X. and Laumann, C. and Bennett, S. and Duan, L.-M. and Lukin, M. and Jiang, L. and Gorshkov, A.}, title = {Quantum logic between remote quantum registers}, journal = {Phys. Rev. A}, publisher = {American Physical Society (APS)}, year = {2013}, volume = {87}, number = {2} }

D. F. Abasto, M. Mohseni, S. Lloyd and P. Zanardi “Exciton diffusion length in complex quantum systems: the effects of disorder and environmental fluctuations on symmetry-enhanced supertransfer” Philosophical Transactions of the Royal Society A: Mathematical,Physical and Engineering Sciences 370, 3750-3770 (2012) [Abstract] [BibTeX] [URL]

Abstract: Symmetric couplings among aggregates of n chromophores increase the transfer rate of excitons by a factor n2, a quantum-mechanical phenomenon called ‘supertransfer’. In this work, we demonstrate how supertransfer effects induced by geometrical symmetries can enhance the exciton diffusion length by a factor n along cylindrically symmetric structures, consisting of arrays of rings of chromophores, and along spiral arrays. We analyse both closed-system dynamics and open quantum dynamics, modelled by combining a random bosonic bath with static disorder. In the closed-system case, we use the symmetries of the system within a short-time approximation to obtain a closed analytical expression for the diffusion length that explicitly reveals the supertransfer contribution. When subject to disorder, we show that supertransfer can enhance excitonic diffusion lengths for small disorders and characterize the crossover from coherent to incoherent motion. Owing to the quasi-one-dimensional nature of the model, disorder ultimately localizes the excitons, diminishing but not destroying the effects of supertransfer. When dephasing effects are included, we study the scaling of diffusion with both time and number of chromophores and observe that the transition from a coherent, ballistic regime to an incoherent, random-walk regime occurs at the same point as the change from supertransfer to classical scaling.

BibTeX: @article{Abasto2012, author = {Abasto, D. F. and Mohseni, M. and Lloyd, S. and Zanardi, P.}, title = {Exciton diffusion length in complex quantum systems: the effects of disorder and environmental fluctuations on symmetry-enhanced supertransfer}, journal = {Philosophical Transactions of the Royal Society A: Mathematical,Physical and Engineering Sciences}, year = {2012}, volume = {370}, number = {1972}, pages = {3750-3770} }

A. Ajoy and P. Cappellaro “Stable three-axis nuclear-spin gyroscope in diamond” Phys. Rev. A 86, 062104 (2012) [Abstract] [BibTeX] [URL]

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 ??0.5(mdegs-1)/Hzmm3 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{Ajoy2012, author = {Ajoy, A. and Cappellaro, P.}, title = {Stable three-axis nuclear-spin gyroscope in diamond}, journal = {Phys. Rev. A}, year = {2012}, volume = {86}, number = {6}, pages = {062104} }

N. Bar-Gill, L. M. Pham, C. Belthangady, D. Le Sage, P. Cappellaro, J. R. Maze, M. D. Lukin, A. Yacoby and R. Walsworth “Suppression of spin-bath dynamics for improved coherence of multi-spin-qubit systems” Nature Communications 3(2012) [Abstract] [BibTeX] [URL]

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-Gill2012, 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 = {Nature Communications}, year = {2012}, volume = {3} }

J. W. Britton, B. C. Sawyer, A. C. Keith, C.-C. J. Wang, J. K. Freericks, H. Uys, M. J. Biercuk and J. J. Bollinger “Engineered two-dimensional Ising interactions in a trapped-ion quantum simulator with hundreds of spins” Nature 484, 489-492 (2012) [Abstract] [BibTeX] [URL]

Abstract: The presence of long-range quantum spin correlations underlies a variety of physical phenomena in condensed-matter systems, potentially including high-temperature superconductivity. However, many properties of exotic, strongly correlated spin systems, such as spin liquids, have proved difficult to study, in part because calculations involving N-body entanglement become intractable for as few as N?~?30 particles. Feynman predicted that a quantum simulator -a special-purpose “analogue” processor built using quantum bits (qubits)-would be inherently suited to solving such problems. In the context of quantum magnetism, a number of experiments have demonstrated the feasibility of this approach, but simulations allowing controlled, tunable interactions between spins localized on two-or three-dimensional lattices of more than a few tens of qubits have yet to be demonstrated, in part because of the technical challenge of realizing large-scale qubit arrays. Here we demonstrate a variable-range Ising-type spin-spin interaction, Ji,j, on a naturally occurring, two-dimensional triangular crystal lattice of hundreds of spin-half particles (beryllium ions stored in a Penning trap). This is a computationally relevant scale more than an order of magnitude larger than previous experiments. We show that a spin-dependent optical dipole force can produce an antiferromagnetic interaction, where 0?<=a?<=3 and di,j is the distance between spin pairs. These power laws correspond physically to infinite-range (a = 0), Coulomb-like (a = 1), monopole-dipole (a = 2) and dipole-dipole (a = 3) couplings. Experimentally, we demonstrate excellent agreement with a theory for 0.05?<=a?<=?1.4. This demonstration, coupled with the high spin count, excellent quantum control and low technical complexity of the Penning trap, brings within reach the simulation of otherwise computationally intractable problems in quantum magnetism.

BibTeX: @article{Britton2012, author = {Britton, Joseph W. and Sawyer, Brian C. and Keith, Adam C. and Wang, C.-C. Joseph and Freericks, James K. and Uys, Hermann and Biercuk, Michael J. and Bollinger, John J.}, title = {Engineered two-dimensional Ising interactions in a trapped-ion quantum simulator with hundreds of spins}, journal = {Nature}, publisher = {Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.}, year = {2012}, volume = {484}, number = {7395}, pages = {489--492} }

P. Cappellaro, G. Goldstein, J. S. Hodges, L. Jiang, J. R. Maze, A. S. Sørensen and M. D. Lukin “Environment-assisted metrology with spin qubits” Phys. Rev. A 85, 032336 (2012) [Abstract] [BibTeX] [URL]

Abstract: We investigate the sensitivity of a recently proposed method for precision measurement [Phys. Rev. Lett.Physical Review LettersTAO0031-900710.1103/PhysRevLett.106.140502 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{Cappellaro2012, 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}, number = {3}, pages = {032336} }

P. Cappellaro “Spin-bath narrowing with adaptive parameter estimation” Phys. Rev. A 85, 030301 (2012) [Abstract] [BibTeX] [URL]

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{Cappellaro2012a, author = {Cappellaro, P.}, title = {Spin-bath narrowing with adaptive parameter estimation}, journal = {Phys. Rev. A}, year = {2012}, volume = {85}, number = {3}, pages = {030301} }

P. Cappellaro “Spin-bath narrowing with adaptive parameter estimation” Physical Review A 85, 030301(R) (2012) [Abstract] [BibTeX] [URL]

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{Cappellaro2012c, author = {Cappellaro, Paola}, title = {Spin-bath narrowing with adaptive parameter estimation}, journal = {Physical Review A}, year = {2012}, volume = {85}, pages = {030301(R)} }

R. Garcia-Patron, C. Navarrete-Benlloch, S. Lloyd, J. H. Shapiro and N. J. Cerf “Majorization Theory Approach to the Gaussian Channel Minimum Entropy Conjecture” Phys. Rev. Lett. 108, 110505 (2012) [Abstract] [BibTeX] [URL]

Abstract: A long-standing open problem in quantum information theory is to find the classical capacity of an optical communication link, modeled as a Gaussian bosonic channel. It has been conjectured that this capacity is achieved by a random coding of coherent states using an isotropic Gaussian distribution in phase space. We show that proving a Gaussian minimum entropy conjecture for a quantum-limited amplifier is actually sufficient to confirm this capacity conjecture, and we provide a strong argument towards this proof by exploiting a connection between quantum entanglement and majorization theory.

BibTeX: @article{Garcia-Patron2012, author = {Garcia-Patron, Raul and Navarrete-Benlloch, Carlos and Lloyd, Seth and Shapiro, Jeffrey H. and Cerf, Nicolas J.}, title = {Majorization Theory Approach to the Gaussian Channel Minimum Entropy Conjecture}, journal = {Phys. Rev. Lett.}, publisher = {American Physical Society}, year = {2012}, volume = {108}, pages = {110505} }

V. Giovannetti, S. Lloyd and L. Maccone “Quantum Measurement Bounds beyond the Uncertainty Relations” Physical Review Letters 108, 260405 (2012) [Abstract] [BibTeX] [URL]

Abstract: In quantum mechanics, the Heisenberg uncertainty relations and the Cramér-Rao inequalities typically limit the precision in the estimation of a parameter through the standard deviation of a conjugate observable. Here we extend these relations by giving a bound to the precision of a parameter in terms of the expectation value of the conjugate observable. This has both foundational and Physical Review Actical consequences: in quantum optics it resolves a controversy over which is the ultimate precision in interferometry.

BibTeX: @article{Giovannetti2012, author = {Giovannetti, V. and Lloyd, S. and Maccone, L.}, title = {Quantum Measurement Bounds beyond the Uncertainty Relations}, journal = {Physical Review Letters}, year = {2012}, volume = {108}, number = {26}, pages = {260405} }

M. Gullans, T. G. Tiecke, D. E. Chang, J. Feist, J. D. Thompson, J. I. Cirac, P. Zoller and M. D. Lukin “Nanoplasmonic Lattices for Ultracold Atoms” Physical Review Letters 109, 235309 (2012) [Abstract] [BibTeX] [URL]

Abstract: We propose to use subwavelength confinement of light associated with the near field of plasmonic systems to create nanoscale optical lattices for ultracold atoms. Our approach combines the unique coherence properties of isolated atoms with the subwavelength manipulation and strong light-matter interaction associated with nanoplasmonic systems. It allows one to considerably increase the energy scales in the realization of Hubbard models and to engineer effective long-range interactions in coherent and dissipative many-body dynamics. Realistic imperfections and potential applications are discussed.

BibTeX: @article{Gullans2012, author = {Gullans, M. and Tiecke, T.~G. and Chang, D.~E. and Feist, J. and Thompson, J.~D. and Cirac, J.~I. and Zoller, P. and Lukin, M.~D. }, title = {Nanoplasmonic Lattices for Ultracold Atoms}, journal = {Physical Review Letters}, year = {2012}, volume = {109}, number = {23}, pages = {235309} }

M. Hirose, C. D. Aiello and P. Cappellaro “Continuous dynamical decoupling magnetometry” Phys. Rev. A 86, 062320 (2012) [Abstract] [BibTeX] [URL]

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{Hirose2012, author = {Hirose, M. and Aiello, C.~D. and Cappellaro, P.}, title = {Continuous dynamical decoupling magnetometry}, journal = {Phys. Rev. A}, year = {2012}, volume = {86}, number = {6}, pages = {062320} }

D. A. Hite, Y. Colombe, A. C. Wilson, K. R. Brown, U. Warring, R. Jördens, J. D. Jost, K. S. McKay, D. P. Pappas, D. Leibfried and D. J. Wineland “100-Fold Reduction of Electric-Field Noise in an Ion Trap Cleaned with In Situ Argon-Ion-Beam Bombardment” Physical Review Letters 109, 103001 (2012) [Abstract] [BibTeX] [URL]

Abstract: Motional heating of trapped atomic ions is a major obstacle to their use as quantum bits in a scalable quantum computer. The detailed physical origin of this heating is not well understood, but experimental evidence suggests that it is caused by electric-field noise emanating from the surface of the trap electrodes. In this study, we have investigated the role of adsorbates on the electrodes by identifying contaminant overlayers, implementing an in situ argon-ion-beam cleaning treatment, and measuring ion heating rates before and after treating the trap electrodes? surfaces. We find a 100-fold reduction in heating rate after treatment. The experiments described here are sensitive to low levels of electric-field noise in the MHz frequency range. Therefore, this approach could become a useful tool in surface science that complements established techniques.

BibTeX: @article{Hite2012, author = {Hite, D.~A. and Colombe, Y. and Wilson, A.~C. and Brown, K.~R. and Warring, U. and Jördens, R. and Jost, J.~D. and McKay, K.~S. and Pappas, D.~P. and Leibfried, D. and Wineland, D.~J.}, title = {100-Fold Reduction of Electric-Field Noise in an Ion Trap Cleaned with In Situ Argon-Ion-Beam Bombardment}, journal = {Physical Review Letters}, year = {2012}, volume = {109}, number = {10}, pages = {103001} }

C. Lupo, V. Giovannetti, S. Pirandola, S. Mancini and S. Lloyd “Capacities of linear quantum optical systems” Physical Review A 85, 062314 (2012) [Abstract] [BibTeX] [URL]

Abstract: A wide variety of communication channels employ the quantized electromagnetic field to convey information. Their communication capacity crucially depends on losses associated to spatial characteristics of the channel such as diffraction and antenna design. Here we focus on the communication via a finite pupil, showing that diffraction is formally described as a memory channel. By exploiting this equivalence we then compute the communication capacity of an optical refocusing system, modeled as a converging lens. Even though loss of information originates from the finite pupil of the lens, we show that the presence of the refocusing system can substantially enhance the communication capacity. We mainly concentrate on communication of classical information, the extension to quantum information being straightforward.

BibTeX: @article{Lupo2012, author = {Lupo, C. and Giovannetti, V. and Pirandola, S. and Mancini, S. and Lloyd, S.}, title = {Capacities of linear quantum optical systems}, journal = {Physical Review A}, year = {2012}, volume = {85}, number = {6}, pages = {062314} }

B. C. Sawyer, J. W. Britton, A. C. Keith, C.-C. J. Wang, J. K. Freericks, H. Uys, M. J. Biercuk and J. J. Bollinger “Spectroscopy and Thermometry of Drumhead Modes in a Mesoscopic Trapped-Ion Crystal Using Entanglement” Physical Review Letters 108, 213003 (2012) [Abstract] [BibTeX] [URL]

Abstract: We demonstrate spectroscopy and thermometry of individual motional modes in a mesoscopic 2D ion array using entanglement-induced decoherence as a method of transduction. Our system is a tilde400$m-diameter planar crystal of several hundred Be+9 ions exhibiting complex drumhead modes in the confining potential of a Penning trap. Exploiting precise control over the Be+9 valence electron spins, we apply a homogeneous spin-dependent optical dipole force to excite arbitrary transverse modes with an effective wavelength approaching the interparticle spacing (tilde20$m). Center-of-mass displacements below 1 nm are detected via the entanglement of spin and motional degrees of freedom.

BibTeX: @article{Sawyer2012, author = {Sawyer, B.~C. and Britton, J.~W. and Keith, A.~C. and Wang, C.-C.~J. and Freericks, J.~K. and Uys, H. and Biercuk, M.~J. and Bollinger, J.~J.}, title = {Spectroscopy and Thermometry of Drumhead Modes in a Mesoscopic Trapped-Ion Crystal Using Entanglement}, journal = {Physical Review Letters}, year = {2012}, volume = {108}, number = {21}, pages = {213003} }

C. Weedbrook, S. Pirandola, R. Garca-Patrón, N. J. Cerf, T. C. Ralph, J. H. Shapiro and S. Lloyd “Gaussian quantum information” Review of Modern Physics 84, 621-669 (2012) [Abstract] [BibTeX] [URL]

Abstract: The science of quantum information has arisen over the last two decades centered on the manipulation of individual quanta of information, known as quantum bits or qubits. Quantum computers, quantum cryptography, and quantum teleportation are among the most celebrated ideas that have emerged from this new field. It was realized later on that using continuous-variable quantum information carriers, instead of qubits, constitutes an extremely powerful alternative approach to quantum information processing. This review focuses on continuous-variable quantum information processes that rely on any combination of Gaussian states, Gaussian operations, and Gaussian measurements. Interestingly, such a restriction to the Gaussian realm comes with various benefits, since on the theoretical side, simple analytical tools are available and, on the experimental side, optical components effecting Gaussian processes are readily available in the laboratory. Yet, Gaussian quantum information processing opens the way to a wide variety of tasks and applications, including quantum communication, quantum cryptography, quantum computation, quantum teleportation, and quantum state and channel discrimination. This review reports on the state of the art in this field, ranging from the basic theoretical tools and landmark experimental realizations to the most recent successful developments.

BibTeX: @article{Weedbrook2012, author = {Weedbrook, C. and Pirandola, S. and Garca-Patrón, R. and Cerf, N.~J. and Ralph, T.~C. and Shapiro, J.~H. and Lloyd, S. }, title = {Gaussian quantum information}, journal = {Review of Modern Physics}, year = {2012}, volume = {84}, pages = {621-669} }

N. Wiebe, D. Braun and S. Lloyd “Quantum Algorithm for Data Fitting” Phys. Rev. Lett. 109, 050505 (2012) [Abstract] [BibTeX] [URL]

Abstract: We provide a new quantum algorithm that efficiently determines the quality of a least-squares fit over an exponentially large data set by building upon an algorithm for solving systems of linear equations efficiently [Harrow et al., Phys. Rev. Lett. 103 150502 (2009)]. In many cases, our algorithm can also efficiently find a concise function that approximates the data to be fitted and bound the approximation error. In cases where the input data are pure quantum states, the algorithm can be used to provide an efficient parametric estimation of the quantum state and therefore can be applied as an alternative to full quantum-state tomography given a fault tolerant quantum computer.

BibTeX: @article{Wiebe2012, author = {Wiebe, Nathan and Braun, Daniel and Lloyd, Seth}, title = {Quantum Algorithm for Data Fitting}, journal = {Phys. Rev. Lett.}, publisher = {American Physical Society}, year = {2012}, volume = {109}, pages = {050505} }

M. van den Worm, B. C. Sawyer, J. J. Bollinger and M. Kastner “Relaxation timescales and decay of correlations in a long-range interacting quantum simulator” New J. Phys. 15,, 083007 (2012) [Abstract] [BibTeX] [URL]

Abstract: We study the time evolution of correlation functions in long-range interacting quantum Ising models. For a large class of initial conditions, exact analytic results are obtained in arbitrary lattice dimension, both for ferromagnetic and antiferromagnetic coupling, and hence also in the presence of geometric frustration. In contrast to the nearest-neighbour case, we find that correlations decay like stretched or compressed exponentials in time. Provided the long-range character of the interactions is sufficiently strong, pronounced prethermalization plateaus are observed and relaxation timescales are widely separated. Specializing to a triangular lattice in two spatial dimensions, we propose to utilize these results for benchmarking of a recently developed ion-trap based quantum simulator.

BibTeX: @article{Worm2012, author = {Mauritz van den Worm and Brian C. Sawyer and John J. Bollinger and Michael Kastner}, title = {Relaxation timescales and decay of correlations in a long-range interacting quantum simulator}, journal = {New J. Phys.}, year = {2012}, volume = {15,}, pages = {083007} }

H. Zhang, R. McConnell, S. Cuk, Q. Lin, M. H. Schleier-Smith, I. D. Leroux and V. Vuletic “Collective State Measurement of Mesoscopic Ensembles with Single-Atom Resolution” Physical Review Letters 109, 133603 (2012) [Abstract] [BibTeX] [URL]

Abstract: We demonstrate single-atom resolution, as well as detection sensitivity more than 20 dB below the quantum projection noise limit, for hyperfine-state-selective measurements on mesoscopic ensembles containing 100 or more atoms. The measurement detects the atom-induced shift of the resonance frequency of an optical cavity containing the ensemble. While spatially varying coupling of atoms to the cavity prevents the direct observation of a quantized signal, the demonstrated measurement resolution provides the readout capability necessary for atomic interferometry substantially below the standard quantum limit and down to the Heisenberg limit.

BibTeX: @article{Zhang2012, author = {Zhang, H. and McConnell, R. and Cuk, S. and Lin, Q. and Schleier-Smith, M.~H. and Leroux, I.~D. and Vuletic, V. }, title = {Collective State Measurement of Mesoscopic Ensembles with Single-Atom Resolution}, journal = {Physical Review Letters}, year = {2012}, volume = {109}, number = {13}, pages = {133603} }

W. S. Bakr, P. M. Preiss, M. E. Tai, R. Ma, J. Simon and M. Greiner “Orbital excitation blockade and algorithmic cooling in quantum gases” #nat# 480, 500-503 (2011) [Abstract] [BibTeX] [URL]

Abstract: Interaction blockade occurs when strong interactions in a confined, few-body system prevent a particle from occupying an otherwise accessible quantum state. Blockade phenomena reveal the underlying granular nature of quantum systems and allow for the detection and manipulation of the constituent particles, be they electrons1, spins2, atoms3, 4, 5 or photons6. Applications include single-electron transistors based on electronic Coulomb blockade7 and quantum logic gates in Rydberg atoms8, 9. Here we report a form of interaction blockade that occurs when transferring ultracold atoms between orbitals in an optical lattice. We call this orbital excitation blockade (OEB). In this system, atoms at the same lattice site undergo coherent collisions described by a contact interaction whose strength depends strongly on the orbital wavefunctions of the atoms. We induce coherent orbital excitations by modulating the lattice depth, and observe staircase-like excitation behaviour as we cross the interaction-split resonances by tuning the modulation frequency. As an application of OEB, we demonstrate algorithmic cooling10, 11 of quantum gases: a sequence of reversible OEB-based quantum operations isolates the entropy in one part of the system and then an irreversible step removes the entropy from the gas. This technique may make it possible to cool quantum gases to have the ultralow entropies required for quantum simulation12, 13 of strongly correlated electron systems. In addition, the close analogy between OEB and dipole blockade in Rydberg atoms provides a plan for the implementation of two-quantum-bit gates14 in a quantum computing architecture with natural

BibTeX: @article{Bakr2011, author = {Bakr, W.~S. and Preiss, P.~M. and Tai, M.~E. and Ma, R. and Simon, J. and Greiner, M.}, title = {Orbital excitation blockade and algorithmic cooling in quantum gases}, journal = {#nat#}, year = {2011}, volume = {480}, pages = {500-503} }

K. R. Brown, A. C. Wilson, Y. Colombe, C. Ospelkaus, A. M. Meier, E. Knill, D. Leibfried and D. J. Wineland “Single-qubit-gate error below 10$^-4$ in a trapped ion” Physical Review A 84, 030303 (2011) [Abstract] [BibTeX] [URL]

Abstract: With a $^9$Be$^+$ trapped-ion hyperfine-state qubit, we demonstrate an error probability per randomized single-qubit gate of 2.0(2)times10$^-5$, below the threshold estimate of 10$^-4$ commonly considered sufficient for fault-tolerant quantum computing. The $^9$Be$^+$ ion is trapped above a microfabricated surface-electrode ion trap and is manipulated with microwaves applied to a trap electrode. The achievement of low single-qubit-gate errors is an essential step toward the construction of a scalable quantum computer.

BibTeX: @article{Brown2011, author = {Brown, K.~R. and Wilson, A.~C. and Colombe, Y. and Ospelkaus, C. and Meier, A.~M. and Knill, E. and Leibfried, D. and Wineland, D.~J. }, title = {Single-qubit-gate error below 10$^-4$ in a trapped ion}, journal = {Physical Review A}, year = {2011}, volume = {84}, number = {3}, pages = {030303} }

V. Giovannetti, S. Lloyd and L. Maccone “Advances in quantum metrology” Nature Photonics 5, 222-229 (2011) [Abstract] [BibTeX] [URL]

Abstract: The statistical error in any estimation can be reduced by repeating the measurement and averaging the results. The central limit theorem implies that the reduction is proportional to the square root of the number of repetitions. Quantum metrology is the use of quantum techniques such as entanglement to yield higher statistical precision than purely classical approaches. In this Review, we analyse some of the most promising recent developments of this research field and point out some of the new experiments. We then look at one of the major new trends of the field: analyses of the effects of noise and experimental imperfections.

BibTeX: @article{Giovannetti2011, author = {Giovannetti, V. and Lloyd, S. and Maccone, L.}, title = {Advances in quantum metrology}, journal = {Nature Photonics}, year = {2011}, volume = {5}, pages = {222-229} }

D. B. Hume, C. W. Chou, D. R. Leibrandt, M. J. Thorpe, D. J. Wineland and T. Rosenband “Trapped-Ion State Detection through Coherent Motion” Physical Review Letters 107, 243902 (2011) [Abstract] [BibTeX] [URL]

Abstract: We demonstrate a general method for state detection of trapped ions that can be applied to a large class of atomic and molecular species. We couple a spectroscopy ion (Al+27) to a control ion (Mg+25) in the same trap and perform state detection through off-resonant laser excitation of the spectroscopy ion that induces coherent motion. The motional amplitude, dependent on the spectroscopy ion state, is measured either by time-resolved photon counting or by resolved sideband excitations on the control ion. The first method provides a simplified way to distinguish clock states in Al+27, which avoids ground-state cooling and sideband transitions. The second method reduces spontaneous emission and optical pumping on the spectroscopy ion, which we demonstrate by nondestructively distinguishing Zeeman sublevels in the S01 ground state of Al+27.

BibTeX: @article{Hume2011, author = {Hume, D.~B. and Chou, C.~W. and Leibrandt, D.~R. and Thorpe, M.~J. and Wineland, D.~J. and Rosenband, T.}, title = {Trapped-Ion State Detection through Coherent Motion}, journal = {Physical Review Letters}, year = {2011}, volume = {107}, number = {24}, pages = {243902} }

S. Lloyd, V. Giovannetti and L. Maccone “Sequential Projective Measurements for Channel Decoding” Physical Review Letters 106, 250501 (2011) [Abstract] [BibTeX] [URL]

Abstract: We study the transmission of classical information in quantum channels. We present a decoding procedure that is very simple but still achieves the channel capacity. It is used to give an alternative straightforward proof that the classical capacity is given by the regularized Holevo bound. This procedure uses only projective measurements and is based on successive ldquoyes-nordquo tests only.

BibTeX: @article{Lloyd2011, author = {Lloyd, S. and Giovannetti, V. and Maccone, L.}, title = {Sequential Projective Measurements for Channel Decoding}, journal = {Physical Review Letters}, year = {2011}, volume = {106}, number = {25}, pages = {250501} }

C. Lupo, V. Giovannetti, S. Pirandola, S. Mancini and S. Lloyd “Enhanced quantum communication via optical refocusing” Physical Review A 84, 010303 (2011) [Abstract] [BibTeX] [URL]

Abstract: We consider the problem of quantum communication mediated by a passive optical refocusing system. The model captures the basic features of all those situations in which a signal is either refocused by a repeater for long-distance communication, or it is focused on a detector prior to the information decoding process. Introducing a general method for linear passive optical systems, we determine the conditions under which optical refocusing implies information transmission gain. Although the finite aperture of the repeater may cause loss of information, we show that the presence of the refocusing system can substantially enhance the rate of reliable communication with respect to the free-space propagation. We explicitly address the transferring of classical messages over the quantum channel, but the results can be easily extended to include the case of transferring quantum messages as well.

BibTeX: @article{Lupo2011, author = {Lupo, C. and Giovannetti, V. and Pirandola, S. and Mancini, S. and Lloyd, S.}, title = {Enhanced quantum communication via optical refocusing}, journal = {Physical Review A}, year = {2011}, volume = {84}, number = {1}, pages = {010303} }

R. Ma, M. E. Tai, P. M. Preiss, W. S. Bakr, J. Simon and M. Greiner “Photon-Assisted Tunneling in a Biased Strongly Correlated Bose Gas” Physical Review Letters 107, 095301 (2011) [Abstract] [BibTeX] [URL]

Abstract: We study the impact of coherently generated lattice photons on an atomic Mott insulator subjected to a uniform force. Analogous to an array of tunnel-coupled and biased quantum dots, we observe sharp, interaction-shifted photon-assisted tunneling resonances corresponding to tunneling one and two lattice sites either with or against the force and resolve multiorbital shifts of these resonances. By driving a Landau-Zener sweep across such a resonance, we realize a quantum phase transition between a paramagnet and an antiferromagnet and observe quench dynamics when the system is tuned to the critical point. Direct extensions will produce gauge fields and site-resolved spin flips, for topological physics and quantum computing.

BibTeX: @article{Ma2011, author = {Ma, R. and Tai, M.~E. and Preiss, P.~M. and Bakr, W.~S. and Simon, J. and Greiner, M.}, title = {Photon-Assisted Tunneling in a Biased Strongly Correlated Bose Gas}, journal = {Physical Review Letters}, year = {2011}, volume = {107}, number = {9}, pages = {095301} }

J. Simon, W. S. Bakr, R. Ma, M. E. Tai, P. M. Preiss and M. Greiner “Quantum simulation of antiferromagnetic spin chains in an optical lattice” Nature 472, 307-312 (2011) [Abstract] [BibTeX] [URL]

Abstract: Understanding exotic forms of magnetism in quantum mechanical systems is a central goal of modern condensed matter physics, with implications for systems ranging from high-temperature superconductors to spintronic devices. Simulating magnetic materials in the vicinity of a quantum phase transition is computationally intractable on classical computers, owing to the extreme complexity arising from quantum entanglement between the constituent magnetic spins. Here we use a degenerate Bose gas of rubidium atoms confined in an optical lattice to simulate a chain of interacting quantum Ising spins as they undergo a phase transition. Strong spin interactions are achieved through a site-occupation to pseudo-spin mapping. As we vary a magnetic field, quantum fluctuations drive a phase transition from a paramagnetic phase into an antiferromagnetic phase. In the paramagnetic phase, the interaction between the spins is overwhelmed by the applied field, which aligns the spins. In the antiferromagnetic phase, the interaction dominates and produces staggered magnetic ordering. Magnetic domain formation is observed through both in situ site-resolved imaging and noise correlation measurements. By demonstrating a route to quantum magnetism in an optical lattice, this work should facilitate further investigations of magnetic models using ultracold atoms, thereby improving our understanding of real magnetic materials.

BibTeX: @article{Simon2011, author = {Simon, Jonathan and Bakr, Waseem S. and Ma, Ruichao and Tai, M. Eric and Preiss, Philipp M. and Greiner, Markus}, title = {Quantum simulation of antiferromagnetic spin chains in an optical lattice}, journal = {Nature}, publisher = {Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.}, year = {2011}, volume = {472}, number = {7343}, pages = {307--312} }

E. Togan, Y. Chu, A. Imamoglu and M. D. Lukin “Laser cooling and real-time measurement of the nuclear spin environment of a solid-state qubit” Nature 478, 497-501 (2011) [Abstract] [BibTeX] [URL]

Abstract: Control over quantum dynamics of open systems is one of the central challenges in quantum science and engineering. Coherent optical techniques, such as coherent population trapping involving dark resonances, are widely used to control quantum states of isolated atoms and ions. In conjunction with spontaneous emission, they allow for laser cooling of atomic motion, preparation and manipulation of atomic states and rapid quantum optical measurements that are essential for applications in metrology. Here we show that these techniques can be applied to monitor and control individual atom-like impurities, and their local environment in the solid state. Using all-optical manipulation of the electronic spin of an individual nitrogen-vacancy colour centre in diamond, we demonstrate optical cooling, real-time measurement and conditional preparation of its nuclear spin environment by post-selection. These methods offer potential applications ranging from all-optical nanomagnetometry to quantum feedback control of solid-state qubits, and may lead to new approaches for quantum information storage and processing

BibTeX: @article{Togan2011, author = {Togan, E. and Chu, Y. and Imamoglu, A. and Lukin, M. D.}, title = {Laser cooling and real-time measurement of the nuclear spin environment of a solid-state qubit}, journal = {Nature}, publisher = {Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.}, year = {2011}, volume = {478}, number = {7370}, pages = {497--501} }

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