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Adaptive Optical Phase Estimation Using Time-Symmetric quantum Smoothing

Physical Review Letters 2010年第9期104卷 093601-093601页

作者： T. A. Wheatley D. W. Berry H. Yonezawa D. Nakane H. Arao D. T. Pope T. C. Ralph H. M. Wiseman A. Furusawa E. H. Huntington Centre for Quantum Computer Technology Australian Research Council School of Engineering and Information Technology University College The University of New South Wales Canberra 2600 ACT Australia Department of Applied Physics and Quantum Phase Electronics Center School of Engineering The University of Tokyo 7-3-1 Hongo Bunkyo-ku Tokyo 113-8656 Japan Institute for Quantum Computing University of Waterloo Waterloo ON Canada Perimeter Institute for Theoretical Physics 31 Caroline Street N Waterloo ON N2L 2Y5 Canada Department of Physics University of Queensland Brisbane 4072 QLD Australia Centre for Quantum Dynamics Griffith University Brisbane 4111 QLD Australia

quantum parameter estimation has many applications, from gravitational wave detection to quantum key distribution. The most commonly used technique for this type of estimation is quantum filtering, using only past observations. We present the first experimental demonstration of quantum smoothing, a time-symmetric technique that uses past and future observations, for quantum parameter estimation. We consider both adaptive and nonadaptive quantum smoothing, and show that both are better than their filtered counterparts. For the problem of estimating a stochastically varying phase shift on a coherent beam, our theory predicts that adaptive quantum smoothing (the best scheme) gives an estimate with a mean-square error up to 22 times smaller than nonadaptive filtering (the standard quantum limit). The experimentally measured improvement is 2.24±0.14.

关键词： Parameter estimation

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Robust Decoupling Techniques to Extend quantum Coherence in Diamond

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Physical Review Letters 2010年第20期105卷 200402-200402页

作者： C. A. Ryan J. S. Hodges D. G. Cory Department of Nuclear Science and Engineering Massachusetts Institute of Technology Cambridge Massachusetts 02139 USA Department of Electrical Engineering Columbia University New York New York 10027 USA Institute for Quantum Computing and Department of Chemistry University of Waterloo ON N2L 3G1 Canada Perimeter Institute for Theoretical Physics Waterloo ON N2J 2W9 Canada

We experimentally demonstrate over 2 orders of magnitude increase in the room-temperature coherence time of nitrogen-vacancy centers in diamond by implementing decoupling techniques. We show that equal pulse spacing decoupling performs just as well as nonperiodic Uhrig decoupling and also allows us to take advantage of revivals in the echo to explore the longest coherence times. At short times, we can extend the coherence of particular quantum states out from T2*=2.7 μs out to an effective T2>340 μs. For preserving arbitrary states we show the experimental importance of using pulse sequences that compensate the imperfections of individual pulses for all input states through judicious choice of the phase of the pulses. We use these compensated sequences to enhance the echo revivals and show a coherence time of over 1.6 ms in ultrapure natural abundance C13 diamond.

关键词： Diamonds

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Experimental Feedback Control of quantum Systems Using Weak Measurements

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Physical Review Letters 2010年第8期104卷 080503-080503页

作者： G. G. Gillett R. B. Dalton B. P. Lanyon M. P. Almeida M. Barbieri G. J. Pryde J. L. O’Brien K. J. Resch S. D. Bartlett A. G. White Department of Physics and Centre for Quantum Computer Technology The University of Queensland Brisbane 4072 Australia Laboratoire Charles Fabry Institut d’Optique Palaiseau 91127 France Centre for Quantum Dynamics and Centre for Quantum Computer Technology Griffith University Brisbane 4111 Australia Centre for Quantum Photonics H. H. Wills Physics Laboratory and Department of Electrical and Electronic Engineering University of Bristol Bristol BS8 1UB United Kingdom Institute for Quantum Computing and Department of Physics and Astronomy University of Waterloo Waterloo Ontario N2L 3G1 Canada School of Physics The University of Sydney Sydney 2006 Australia

A goal of the emerging field of quantum control is to develop methods for quantum technologies to function robustly in the presence of noise. Central issues are the fundamental limitations on the available information about quantum systems and the disturbance they suffer in the process of measurement. In the context of a simple quantum control scenario—the stabilization of nonorthogonal states of a qubit against dephasing—we experimentally explore the use of weak measurements in feedback control. We find that, despite the intrinsic difficultly of implementing them, weak measurements allow us to control the qubit better in practice than is even theoretically possible without them. Our work shows that these more general quantum measurements can play an important role for feedback control of quantum systems.

关键词： Feedback control

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Multi-error-correcting amplitude damping codes

Multi-error-correcting amplitude damping codes

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IEEE International Symposium on Information Theory

作者： Runyao Duan Markus Grassl Zhengfeng Ji Bei Zeng State Key Laboratory of Intelligent Technology and Systems Tsinghua National Laboratory for Information Science and Technology Department of Computer Science and Technology Tsinghua University Beijing China Centre for Quantum Computation and Intelligent Systems (QCIS) Faculty of Engineering and Information Technology University of Technology Sydney NSW Australia Centre for Quantum Technologies National University of Singapore Singapore Perimeter Institute of Theoretical Physics Waterloo ONT Canada State Key Laboratory of Computer Science Institute of Software Chinese Academy and Sciences Beijing China Institute for Quantum Computing and the Department of Combinatorics and Optimization University of Waterloo Waterloo ONT Canada

ISBN: (纸本)9781424478903

We construct new families of multi-error-correcting quantum codes for the amplitude damping channel. Our key observation is that, with proper encoding, two uses of the amplitude damping channel simulate a quantum erasure channel. This allows us to use concatenated codes with quantum erasure-correcting codes as outer codes for correcting multiple amplitude damping errors. Our new codes are degenerate stabilizer codes and have parameters which are better than the amplitude damping codes obtained by any previously known construction.

关键词： Damping quantum mechanics quantum computing Error correction codes Noise level Laboratories Phase noise Physics computing Intelligent systems Computer science

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Entanglement and area law with a fractal boundary in a topologically ordered phase

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Physical Review A 2010年第1期81卷 010102(R)-010102(R)页

作者： Alioscia Hamma Daniel A. Lidar Simone Severini Perimeter Institute for Theoretical Physics 31 Caroline St. N N2L 2Y5 Waterloo Ontario Canada Departments of Chemistry Electrical Engineering and Physics and Center for Quantum Information Science and Technology University of Southern California Los Angeles California 90089 USA Institute for Quantum Computing and Department of Combinatorics & Optimization University of Waterloo 200 University Ave. W N2L 3G1 Waterloo Ontario Canada

quantum systems with short-range interactions are known to respect an area law for the entanglement entropy: The von Neumann entropy S associated to a bipartition scales with the boundary p between the two parts. Here we study the case in which the boundary is a fractal. We consider the topologically ordered phase of the toric code with a magnetic field. When the field vanishes it is possible to analytically compute the entanglement entropy for both regular and fractal bipartitions (A,B) of the system and this yields an upper bound for the entire topological phase. When the A−B boundary is regular we have S/p=1 for large p. When the boundary is a fractal of the Hausdorff dimension D, we show that the entanglement between the two parts scales as S/p=γ⩽1/D, and γ depends on the fractal considered.

关键词： Entropy

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Manipulating Biphotonic Qutrits

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Physical Review Letters 2008年第6期100卷 060504-060504页

作者： B. P. Lanyon T. J. Weinhold N. K. Langford J. L. O’Brien K. J. Resch A. Gilchrist A. G. White Department of Physics and Centre for Quantum Computer Technology University of Queensland Brisbane Australia Centre for Quantum Photonics H. H. Wills Physics Laboratory and Department of Electrical and Electronic Engineering University of Bristol Bristol United Kingdom Institute for Quantum Computing and Department of Physics & Astronomy University of Waterloo Waterloo Canada

quantum information carriers with higher dimension than the canonical qubit offer significant advantages. However, manipulating such systems is extremely difficult. We show how measurement-induced nonlinearities can dramatically extend the range of possible transforms on biphotonic qutrits—three-level quantum systems formed by the polarization of two photons in the same spatiotemporal mode. We fully characterize the biphoton-photon entanglement that underpins our technique, thereby realizing the first instance of qubit-qutrit entanglement. We discuss an extension of our technique to generate qutrit-qutrit entanglement and to manipulate any bosonic encoding of quantum information.

关键词： .encoding Qutrits dramatically Manipulating Biphotonic

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Deterministic optical quantum computer using photonic modules

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Physical Review A 2008年第3期78卷 032318-032318页

作者： Ashley M. Stephens Zachary W. E. Evans Simon J. Devitt Andrew D. Greentree Austin G. Fowler William J. Munro Jeremy L. O’Brien Kae Nemoto Lloyd C. L. Hollenberg Centre for Quantum Computer Technology The University of Melbourne Victoria 3010 Australia National Institute for Informatics 2-1-2 Hitotsubashi Chiyoda-ku Tokyo 101-8430 Japan Institute for Quantum Computing University of Waterloo Waterloo Ontario Canada N2L 3G1 Hewlett-Packard Laboratories Filton Road Stoke Gifford Bristol BS34 8QZ United Kingdom Centre for Quantum Photonics H. H. Wills Physics Laboratory and Department of Electrical and Electronic Engineering University of Bristol Merchant Venturers Building Woodland Road Bristol BS8 1UB United Kingdom

The optical quantum computer is one of the few experimental systems to have demonstrated small scale quantum information processing. Making use of cavity quantum electrodynamics approaches to operator measurements, we detail an optical network for the deterministic preparation of arbitrarily large two-dimensional cluster states. We show that this network can form the basis of a large scale deterministic optical quantum computer that can be fabricated entirely on chip.

关键词： quantum optics

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Fidelity Enhancement by Logical Qubit Encoding

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Physical Review Letters 2007年第22期99卷 220501-220501页

作者： Michael K. Henry Chandrasekhar Ramanathan Jonathan S. Hodges Colm A. Ryan Michael J. Ditty Raymond Laflamme David G. Cory Department of Nuclear Science and Engineering Massachusetts Institute of Technology Cambridge Massachusetts 02139 USA Institute for Quantum Computing University of Waterloo Waterloo Ontario Canada N2L 3G1

We demonstrate coherent control of two logical qubits encoded in a decoherence free subspace (DFS) of four dipolar-coupled protons in an NMR quantum information processor. A pseudopure fiducial state is created in the DFS, and a unitary logical qubit entangling operator evolves the system to a logical Bell state. The four-spin molecule is partially aligned by a liquid crystal solvent, which introduces strong dipolar couplings among the spins. Although the system Hamiltonian is never fully specified, we demonstrate high fidelity control over the logical degrees of freedom. In fact, the DFS encoding leads to higher fidelity control than is available in the full four-spin Hilbert space.

关键词： quantum theory

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Efficient error characterization in quantum information processing

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Physical Review A 2007年第2期75卷 022314-022314页

作者： Benjamin Lévi Cecilia C. López Joseph Emerson D. G. Cory Department of Nuclear Science and Engineering Massachusetts Institute of Technology Cambridge Massachusetts 02139 USA Department of Applied Mathematics and Institute for Quantum Computing University of Waterloo Waterloo ON N2L 3G1 Canada

We describe how to use the fidelity decay as a tool to characterize the errors affecting a quantum information processor through a noise generator Gτ. For weak noise, the initial decay rate of the fidelity proves to be a simple way to measure the magnitude of the different terms in Gτ. When the generator has only terms associated with few-body couplings, our proposal is scalable. We present the explicit protocol for estimating the magnitude of the noise generators when the noise consists of only one- and two-body terms, and describe a method for measuring the parameters of more general noise models. The protocol focuses on obtaining the magnitude with which these terms affect the system during a time step of length τ; measurement of this information has critical implications for assessing the scalability of fault-tolerant quantum computation in any physical setup.

关键词： UNITARY OPERATORS FIDELITY DECAY

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Time-Reversal and Super-Resolving Phase Measurements

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Physical Review Letters 2007年第22期98卷 223601-223601页

作者： K. J. Resch K. L. Pregnell R. Prevedel A. Gilchrist G. J. Pryde J. L. O’Brien A. G. White Department of Physics University of Queensland Brisbane QLD 4072 Australia Institute for Quantum Computing and Department of Physics & Astronomy University of Waterloo Waterloo ON N2L 3G1 Canada Centre for Quantum Computer Technology University of Queensland Brisbane QLD 4072 Australia Institute for Mathematical Sciences Imperial College London SW7 2PG Unied Kingdom Institute for Experimental Physics University of Vienna Vienna A-1090 Austria Centre for Quantum Dynamics & Centre for Quantum Computer Technology School of Biomolecular and Physical Sciences Griffith University Nathan Brisbane QLD 4111 Australia H. H. Wills Physics Laboratory & Department of Electrical and Electronic Engineering University of Bristol Bristol BS8 1UB United Kingdom

We demonstrate phase super-resolution in the absence of entangled states. The key insight is to use the inherent time-reversal symmetry of quantum mechanics: our theory shows that it is possible to measure, as opposed to prepare, entangled states. Our approach is robust, requiring only photons that exhibit classical interference: we experimentally demonstrate high-visibility phase super-resolution with three, four, and six photons using a standard laser and photon counters. Our six-photon experiment demonstrates the best phase super-resolution yet reported with high visibility and resolution.

关键词： .TIME-REVERSAL Centre for quantum entangled states phase super-resolution high-visibility

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