Virtual Journal of Quantum Information

Sliding mode control of quantum systems

Daoyi Dong and Ian Petersen
This paper proposes a new robust control method for quantum systems with uncertainties involving sliding mode control (SMC). SMC is a widely used approach in classical control theory and industrial applications. We show that SMC is also a useful method for robust control of quantum systems. In this ... [New J. Phys. 11, 105033 (2009)] published Thu Nov 5, 2009.

A dipolar gas of ultracold molecules

K.-K. Ni, S. Ospelkaus, D. J. Nesbitt, J. Ye, and D. S. Jin
Ultracold polar molecular gases promise new directions and exciting applications in collisions and chemical reactions at ultralow energies, precision measurements, novel quantum phase transitions, and quantum information science. Here we briefly discuss key experimental requirements for observing st ... [Phys. Chem. Chem. Phys. 11, 9626 (2009)] published Wed Oct 28, 2009.

Breakdown of the dipole blockade with a zero-area phase-jump pulse

Jun Qian, Yong Qian, Min Ke, Xun-Li Feng, C. H. Oh et al.
We theoretically investigate the effect of a zero-area pulse on the excitations of interacting Rydberg atoms. The unexpected breakdown of dipole blockade occurs in the strong Rydberg blockade regime, which results from the nonadiabatic character of a phase-jump pulse interacting with the atoms. We a ... [Phys. Rev. A 80, 053413 (2009)] published Thu Nov 12, 2009.

Zheng Replies:

Shi-Biao Zheng
A Reply to the Comment by Ognyan Oreshkov and John Calsamiglia. ... [Phys. Rev. Lett. 103, 188902 (2009)] published Thu Oct 29, 2009.

Comment on Nongeometric Conditional Phase Shift via Adiabatic Evolution of Dark Eigenstates: A New Approach to Quantum Computation

Ognyan Oreshkov and John Calsamiglia
A Comment on the Letter by Shi-Biao Zheng, Phys. Rev. Lett. 95, 080502 (2005). The authors of the Letter offer a Reply. ... [Phys. Rev. Lett. 103, 188901 (2009)] published Thu Oct 29, 2009.

Erratum: Efficient Dynamic Nuclear Polarization at High Magnetic Fields [Phys. Rev. Lett. 98, 220501 (2007)]

Gavin W. Morley, Johan van Tol, Arzhang Ardavan, Kyriakos Porfyrakis, Jinying Zhang et al.
Abstract not available. [Phys. Rev. Lett. 103, 199902 (2009)] published Thu Nov 5, 2009.

A tunable few electron triple quantum dot

L. Gaudreau, A. Kam, G. Granger, S. A. Studenikin, P. Zawadzki et al.
In this paper, we report on a tunable few electron lateral triple quantum dot design. The quantum dot potentials are arranged in series. The device is aimed at studies of triple quantum dot properties where knowing the exact number of electrons is important as well as quantum information application ... [Appl. Phys. Lett. 95, 193101 (2009)] published Mon Nov 9, 2009.

Single-shot qubit readout in circuit quantum electrodynamics

Francois Mallet, Florian R. Ong, Agustin Palacios-Laloy, Francois Nguyen, Patrice Bertet et al.
The future development of quantum information using superconducting circuits requires Josephson qubits with long coherence times combined with a high-fidelity readout. Significant progress in the control of coherence has recently been achieved using circuit quantum electrodynamics architectures, wh ... [Nature Phys. 5, 791 (2009)] published Thu Nov 5, 2009.

A quantum gas microscope for detecting single atoms in a Hubbard-regime optical lattice

Waseem S. Bakr, Jonathon I. Gillen, Amy Peng, Simon Folling, and Markus Greiner
Recent years have seen tremendous progress in creating complex atomic many-body quantum systems. One approach is to use macroscopic, effectively thermodynamic ensembles of ultracold atoms to create quantum gases and strongly correlated states of matter, and to analyse the bulk properties of the ens ... [Nature 462, 74 (2009)] published Thu Nov 5, 2009.

Generalized filtering of laser fields in optimal control theory: application to symmetry filtering of quantum gate operations

Markus Schroder and Alex Brown
We present a modified version of a previously published algorithm (Gollub et al 2008 Phys. Rev. Lett.101 073002) for obtaining an optimized laser field with more general restrictions on the search space of the optimal field. The modification leads to enforcement of the constraints on the optimal fie ... [New J. Phys. 11, 105031 (2009)] published Thu Nov 5, 2009.

Dynamics of dispersive single-qubit readout in circuit quantum electrodynamics

R. Bianchetti, S. Filipp, M. Baur, J. M. Fink, M. Goppl et al.
The quantum state of a superconducting qubit nonresonantly coupled to a transmission line resonator can be determined by measuring the quadrature amplitudes of an electromagnetic field transmitted through the resonator. We present experiments in which we analyze in detail the dynamics of the transmi ... [Phys. Rev. A 80, 043840 (2009)] published Fri Oct 30, 2009.

Extracavity quantum vacuum radiation from a single qubit

S. De Liberato, D. Gerace, I. Carusotto, and C. Ciuti
We present a theory of the quantum vacuum radiation that is generated by a fast modulation of the vacuum Rabi frequency of a single two-level system strongly coupled to a single cavity mode. The dissipative dynamics of the Jaynes-Cummings model in the presence of antirotating-wave terms is described ... [Phys. Rev. A 80, 053810 (2009)] published Mon Nov 9, 2009.

Chirped Biphotons and their Compression in Optical Fibers

G. Brida, M. V. Chekhova, I. P. Degiovanni, M. Genovese, G. Kh. Kitaeva et al.
We show that broadband biphoton wave packets produced via sponateous parametric down-conversion in crystals with linearly aperiodic poling can be easily compressed in time using the effect of group-velocity dispersion in optical fibers. This result could foster important developments in quantum metr ... [Phys. Rev. Lett. 103, 193602 (2009)] published Thu Nov 5, 2009.

Observation of geometric phases for three-level systems using NMR interferometry

Hongwei Chen, Mingguang Hu, Jingling Chen, and Jiangfeng Du
Inspired by the optical experimental scheme [Phys. Rev. Lett. 86, 369 (2001)], we propose and implement a NMR experiment to observe an Abelian geometric phase shift arising from the cyclic evolution of U(2)-invariant states. Such a phase shift geometrically refers to the Bargmann invariant in a four ... [Phys. Rev. A 80, 054101 (2009)] published Tue Nov 10, 2009.

Magnetic trapping of ultracold Rydberg atoms in low angular momentum states

Michael Mayle, Igor Lesanovsky, and Peter Schmelcher
We theoretically investigate the quantum properties of nS, nP, and nD Rydberg atoms in a magnetic Ioffe-Pritchard trap. In particular, it is demonstrated that the two-body character of Rydberg atoms significantly alters the trapping properties opposed to pointlike particles with identical magnetic m ... [Phys. Rev. A 80, 053410 (2009)] published Thu Nov 12, 2009.

Measurement and compensation of optical Stark shifts for manipulating the terahertz-separated qubit in Ca

Shinsuke Haze, Rekishu Yamazaki, Kenji Toyoda, and Shinji Urabe
We describe the measurement and compensation of optical Stark shifts induced by an off-resonant pulse for manipulating the terahertz-separated states 3 D3 D, in a single trapped Ca ion. These states can be used as a quantum bit and are coupled by a two-photon stimulated Raman transition with phase-l ... [Phys. Rev. A 80, 053408 (2009)] published Wed Nov 11, 2009.

Geometric quantum gates in liquid-state NMR based on a cancellation of dynamical phases

Yukihiro Ota, Yoshito Goto, Yasushi Kondo, and Mikio Nakahara
A proposal for applying nonadiabatic geometric phases to quantum computing, called double-loop method [S.-L. Zhu and Z. D. Wang, Phys. Rev. A 67, 022319 (2003)], is demonstrated in a liquid-state nuclear magnetic-resonance quantum computer. Using a spin-echo technique, the original method is modifie ... [Phys. Rev. A 80, 052311 (2009)] published Tue Nov 10, 2009.

Geometric phase of interacting qubits: Mean-field analysis

J. Dajka, M. Mierzejewski, and J. Luczka
We consider a system consisting of a large number of interacting qubits and study the geometric phase of one of them. The many body interaction is analyzed within a mean-field approach when the reduced dynamics of a single qubit is described by a nonlinear Hartree equation. Generally, the geometric ... [Phys. Rev. A 80, 044303 (2009)] published Mon Oct 19, 2009.

Time evolution of the one-dimensional Jaynes-Cummings-Hubbard Hamiltonian

M. I. Makin, Jared H. Cole, Charles D. Hill, Andrew D. Greentree, and Lloyd C. L. Hollenberg
The Jaynes-Cummings-Hubbard (JCH) system describes a network of single-mode photonic cavities connected via evanescent coupling. Each cavity contains a single two-level system which can be tuned in resonance with the cavity. Here, we explore the behavior of single excitations (where an excitation ca ... [Phys. Rev. A 80, 043842 (2009)] published Fri Oct 30, 2009.

Rovibrational controlled-NOT gates using optimized stimulated Raman adiabatic passage techniques and optimal control theory

D. Sugny, L. Bomble, T. Ribeyre, O. Dulieu, and M. Desouter-Lecomte
Implementation of quantum controlled-NOT (CNOT) gates in realistic molecular systems is studied using stimulated Raman adiabatic passage (STIRAP) techniques optimized in the time domain by genetic algorithms or coupled with optimal control theory. In the first case, with an adiabatic solution (a ser ... [Phys. Rev. A 80, 042325 (2009)] published Fri Oct 23, 2009.

Single atom-scale diamond defect allows a large Aharonov-Casher phase

D. Maclaurin, A. D. Greentree, J. H. Cole, L. C. L. Hollenberg, and A. M. Martin
We propose an experiment that would produce and measure a large Aharonov-Casher (AC) phase in a solid-state system under macroscopic motion. A diamond crystal is mounted on a spinning disk in the presence of a uniform electric field. Internal magnetic states of a single nitrogen-vacancy (N-V) defect ... [Phys. Rev. A 80, 040104 (2009)] published Wed Oct 21, 2009.

Deterministic reordering of Ca ions in a linear segmented Paul trap

F Splatt, M Harlander, M Brownnutt, F Zahringer, R Blatt et al.
In the endeavour to scale up the number of qubits in an ion-based quantum computer several groups have started to develop miniaturized ion traps for extended spatial control and manipulation of the ions. Shuttling and separation of ion strings have been the foremost issues in linear-trap arrangement ... [New J. Phys. 11, 103008 (2009)] published Thu Nov 5, 2009.

Scalable arrays of rf Paul traps in degenerate Si

J. Britton, D. Leibfried, J. A. Beall, R. B. Blakestad, J. H. Wesenberg et al.
We report techniques for the fabrication of multizone linear radio frequency Paul traps that exploit the machinability and electrical conductivity of degenerate silicon. The approach was tested by trapping and laser cooling Mg ions in the two following trap geometries: a single-zone two-layer trap a ... [Appl. Phys. Lett. 95, 173102 (2009)] published Mon Oct 26, 2009.

Autler-Townes Effect in a Superconducting Three-Level System

Mika A. Sillanpaa, Jian Li, Katarina Cicak, Fabio Altomare, Jae I. Park et al.
When a three-level quantum system is irradiated by an intense coupling field resonant with one of the three possible transitions, the absorption peak of an additional probe field involving the remaining level is split. This process is known in quantum optics as the Autler-Townes effect. We observe t ... [Phys. Rev. Lett. 103, 193601 (2009)] published Mon Nov 2, 2009.

Theory of Spin-Conserving Excitation of the N-V Center in Diamond

Adam Gali, Erik Janzen, Peter Deak, Georg Kresse, and Efthimios Kaxiras
The negatively charged nitrogen-vacancy defect in diamond is an important atomic-scale structure that can be used as a qubit in quantum computing and as a marker in biomedical applications. Its usefulness relies on the ability to optically excite electrons between well-defined gap states, which requ ... [Phys. Rev. Lett. 103, 186404 (2009)] published Tue Oct 27, 2009.

Relaxation of Josephson qubits due to strong coupling to two-level systems

Clemens Muller, Alexander Shnirman, and Yuriy Makhlin
We investigate the energy relaxation (T) process of a qubit coupled to a bath of dissipative two-level fluctuators (TLFs). We consider the fluctuators strongly coupled to the qubit both in the limit of spectrally sparse single TLFs as well as in the limit of spectrally dense TLFs. We conclude that t ... [Phys. Rev. B 80, 134517 (2009)] published Thu Oct 22, 2009.

Polarized light in quantum dot qubit under an applied external magnetic field

V. N. Stavrou
Single self-assembled quantum dot (SAQD) dependence on an external magnetic field has been theoretically and numerically studied. The carrier wave functions and energy eigenvalues have been evaluated and the differences between small and large quantum dots have been explored. The current investigati ... [Phys. Rev. B 80, 153308 (2009)] published Mon Oct 26, 2009.

Power-dependent internal loss in Josephson bifurcation amplifiers

Michio Watanabe, Kunihiro Inomata, Tsuyoshi Yamamoto, and Jaw-Shen Tsai
We have studied nonlinear superconducting resonators: lambda/2 coplanar-waveguide (CPW) resonators with Josephson junctions (JJs) placed in the middle and lambda/4 CPW resonators terminated by JJs, which can be used for the qubit readout as bifurcation amplifiers. The nonlinearity of the resonators ... [Phys. Rev. B 80, 174502 (2009)] published Tue Nov 3, 2009.

Theory of two-dimensional macroscopic quantum tunneling in YBaCuO Josephson junctions coupled to an LC circuit

Shiro Kawabata, Thilo Bauch, and Takeo Kato
We investigate classical thermal activation (TA) and macroscopic quantum tunneling (MQT) for a YBaCuO (YBCO) Josephson junction coupled to an LC circuit theoretically. Due to the coupling between the junction and the LC circuit, the macroscopic phase dynamics can be described as the escape process o ... [Phys. Rev. B 80, 174513 (2009)] published Fri Nov 13, 2009.

Realizing singlet-triplet qubits in multivalley Si quantum dots

Dimitrie Culcer, Lukasz Cywinski, Qiuzi Li, Xuedong Hu, and S. Das Sarma
There has been significant progress in the implementation and manipulation of singlet-triplet qubits in GaAs quantum dots. Given the considerably longer spin coherence times measured in Si, considerable interest has been generated recently in Si quantum dots. The physics of these systems is consider ... [Phys. Rev. B 80, 205302 (2009)] published Thu Nov 5, 2009.

Decoherence Dynamics of Complex Photon States in a Superconducting Circuit

H. Wang, M. Hofheinz, M. Ansmann, R. C. Bialczak, Erik Lucero et al.
Quantum states inevitably decay with time into a probabilistic mixture of classical states due to their interaction with the environment and measurement instrumentation. We present the first measurement of the decoherence dynamics of complex photon states in a condensed-matter system. By controllabl ... [Phys. Rev. Lett. 103, 200404 (2009)] published Fri Nov 13, 2009.

Electrically detected magnetic resonance using radio-frequency reflectometry

H. Huebl, R. P. Starrett, D. R. McCamey, A. J. Ferguson, and L. H. Willems van Beveren
The authors demonstrate readout of electrically detected magnetic resonance at radio frequencies by means of a LCR tank circuit. Applied to a silicon field-effect transistor at millikelvin temperatures, this method shows a 25-fold increased signal-to-noise ratio of the conduction band electron spin ... [Rev. Sci. Instrum. 80, 114705 (2009)] published Wed Nov 11, 2009.

Localization of Metal-Induced Gap States at the Metal-Insulator Interface: Origin of Flux Noise in SQUIDs and Superconducting Qubits

SangKook Choi, Dung-Hai Lee, Steven G. Louie, and John Clarke
The origin of magnetic flux noise in superconducting quantum interference devices with a power spectrum scaling as 1/f (f is frequency) has been a puzzle for over 20 years. This noise limits the decoherence time of superconducting qubits. A consensus has emerged that the noise arises from fluctuatin ... [Phys. Rev. Lett. 103, 197001 (2009)] published Tue Nov 3, 2009.

Engineering single photon emitters by ion implantation in diamond

B. Naydenov, R. Kolesov, A. Batalov, J. Meijer, S. Pezzagna et al.
Diamond provides unique technological platform for quantum technologies including quantum computing and communication. Controlled fabrication of optically active defects is a key element for such quantum toolkit. Here we report the production of single color centers emitting in the blue spectral reg ... [Appl. Phys. Lett. 95, 181109 (2009)] published Thu Nov 5, 2009.

High-performance diamond-based single-photon sources for quantum communication

Chun-Hsu Su, Andrew D. Greentree, and Lloyd C. L. Hollenberg
Quantum communication places stringent requirements on single-photon sources. Here we report a theoretical study of the cavity Purcell enhancement of two diamond point defects, the nickel-nitrogen (NE8) and silicon-vacancy (SiV) centers, for high-performance, near on-demand single-photon generation. ... [Phys. Rev. A 80, 052308 (2009)] published Mon Nov 9, 2009.

Whispering-gallery mode excitation in a microdroplet illuminated by a train of chirped ultrashort laser pulses

Yurii E. Geints, Alexander A. Zemlyanov, and Ekaterina K. Panina
The peculiarities of resonant optical field excitation inside a water microdroplet under illumination by a spatially bounded Gaussian beam with a temporal regime of a single chirped ultrashort laser pulse and a chirped pulse train are considered. It is established that the coupling efficiency of inc ... [Appl. Opt. 48, 5842 (2009)] published Tue Nov 10, 2009.

Proposals of nuclear spin quantum memory in group-IV elemental and II-VI semiconductors

Ozgur Cakır and Toshihide Takagahara
Schemes for the nuclear spin quantum memory are proposed based on a system composed of two electrons or one electron coupled to a single nuclear spin in isotopically purified group-IV elemental and II-VI compound semiconductors. The qubit consists of the singlet state and one of the triplet states o ... [Phys. Rev. B 80, 155323 (2009)] published Fri Oct 23, 2009.

Coherent control of multiple vibrational excitations for optimal detection

S McGrane, R Scharff, M Greenfield, and D Moore
While the means to selectively excite a single vibrational mode using ultrafast pulse shaping are well established, the subsequent problem of selectively exciting multiple vibrational modes simultaneously has been largely neglected. The coherent control of multiple vibrational excitations has applic ... [New J. Phys. 11, 105047 (2009)] published Thu Nov 5, 2009.

Unambiguous comparison of quantum measurements

Mario Ziman, Teiko Heinosaari, and Michal Sedlak
The goal of comparison is to reveal the difference of compared objects as fast and reliably as possible. In this paper we formulate and investigate the unambiguous comparison of unknown quantum measurements represented by nondegenerate sharp positive operator valued measures. We distinguish between ... [Phys. Rev. A 80, 052102 (2009)] published Thu Nov 5, 2009.

Forgetfulness of continuous Markovian quantum channels

Cosmo Lupo, Laleh Memarzadeh, and Stefano Mancini
The notion of forgetfulness, used in discrete quantum memory channels, is slightly weakened in order to be applied to the case of continuous channels. This is done in the context of quantum memory channels with Markovian noise. As a case study, we apply the notion of weak forgetfulness to a bosonic ... [Phys. Rev. A 80, 042328 (2009)] published Tue Oct 27, 2009.

Optimum unambiguous discrimination of linearly independent pure states

Shengshi Pang (庞盛世) and Shengjun Wu (吴盛俊)
Given n linearly independent pure states and their prior probabilities, we study the optimum unambiguous state discrimination problem. We derive the conditions for the optimum measurement strategy to achieve the maximum average success probability and establish two sets of equations that must be sat ... [Phys. Rev. A 80, 052320 (2009)] published Thu Nov 12, 2009.

Information causality as a physical principle

Marcin Pawlowski, Tomasz Paterek, Dagomir Kaszlikowski, Valerio Scarani, Andreas Winter et al.
Quantum physics has remarkable distinguishing characteristics. For example, it gives only probabilistic predictions (non-determinism) and does not allow copying of unknown states (no-cloning). Quantum correlations may be stronger than any classical ones, but information cannot be transmitted faster ... [Nature 461, 1101 (2009)] published Thu Oct 22, 2009.

Modeling two-spin dynamics in a noisy environment

M. J. Testolin, J. H. Cole, and L. C. L. Hollenberg
We describe how the effect of charge noise on a pair of spins coupled via the exchange interaction can be calculated by modeling charge fluctuations as a random telegraph noise process using probability density functions. We develop analytic expressions for the time-dependent superoperator of a pair ... [Phys. Rev. A 80, 042326 (2009)] published Fri Oct 23, 2009.

Universal existence of exact quantum state transmissions in interacting media

Lian-Ao Wu, Yu-xi Liu, and Franco Nori
We consider an exact state transmission, where a density matrix in one information processor A at time t=0 is exactly equal to that in another processor B at a later time. We demonstrate that there always exists a complete set of orthogonal states, which can be employed to perform the exact state tr ... [Phys. Rev. A 80, 042315 (2009)] published Mon Oct 19, 2009.

Non-Markovian dissipative dynamics of two coupled qubits in independent reservoirs: Comparison between exact solutions and master-equation approaches

E. Ferraro, M. Scala, R. Migliore, and A. Napoli
The reduced dynamics of two interacting qubits coupled to two independent bosonic baths is investigated. The one-excitation dynamics is derived and compared with that based on the resolution of appropriate non-Markovian master equations. The Nakajima-Zwanzig and the time-convolutionless projection o ... [Phys. Rev. A 80, 042112 (2009)] published Tue Oct 27, 2009.

Quantum discord between relatively accelerated observers

Animesh Datta
We calculate the quantum discord between two free modes of a scalar field, which start in a maximally entangled state and then undergo a relative constant acceleration. In a regime where there is no distillable entanglement due to the Unruh effect, we show that there is a finite amount of quantum di ... [Phys. Rev. A 80, 052304 (2009)] published Wed Nov 4, 2009.

Concavity of the set of quantum probabilities for any given dimension

Karoly F. Pal and Tamas Vertesi
Let us consider the set of all probabilities generated by local binary measurements on two separated quantum systems of a given local dimension d. We address the question of whether the shape of this quantum body is convex or not. We construct a point in the space of probabilities, which is on the c ... [Phys. Rev. A 80, 042114 (2009)] published Fri Oct 30, 2009.

Time-reversal frameness and superselection

Gilad Gour, Barry C. Sanders, and Peter S. Turner
We show that appropriate superpositions of motional states are a reference frame resource that enables breaking of time-reversal superselection so that two parties lacking knowledge about the other's direction of time can still communicate. We identify the time-reversal reference frame resource stat ... [J. Math. Phys. 50, 102105 (2009)] published Mon Oct 19, 2009.

Implementation of fault-tolerant quantum logic gates via optimal control

R Nigmatullin and S Schirmer
The implementation of fault-tolerant quantum gates on encoded logic qubits is considered. It is shown that transversal implementation of logic gates based on simple geometric control ideas is problematic for realistic physical systems suffering from imperfections such as qubit inhomogeneity or uncon ... [New J. Phys. 11, 105032 (2009)] published Thu Nov 5, 2009.

Continuous quantum error correction as classical hybrid control

Hideo Mabuchi
The standard formulation of quantum error correction (QEC) comprises repeated cycles of error estimation and corrective intervention in the free dynamics of a qubit register. QEC can thus be seen as a form of feedback control, and it is of interest to seek a deeper understanding of the connection be ... [New J. Phys. 11, 105044 (2009)] published Thu Nov 5, 2009.

Constructions of new families of nonbinary quantum codes

Giuliano G. La Guardia
Three code constructions generating new families of good nonbinary quantum codes are presented in this paper. The first two ones are derived from Hermitian self-orthogonal non-narrow-sense Bose-Chaudhuri-Hocquenghem (BCH) codes. The third one is derived from q-ary (q[not-equal]2 is a prime power) St ... [Phys. Rev. A 80, 042331 (2009)] published Wed Oct 28, 2009.

Tight Noise Thresholds for Quantum Computation with Perfect Stabilizer Operations

Wim van Dam and Mark Howard
We study how much noise can be tolerated by a universal gate set before it loses its quantum-computational power. Specifically we look at circuits with perfect stabilizer operations in addition to imperfect nonstabilizer gates. We prove that for all unitary single-qubit gates there exists a tight de ... [Phys. Rev. Lett. 103, 170504 (2009)] published Fri Oct 23, 2009.

Driving quantum systems into decoherence-free subspaces by Lyapunov control

X. X. Yi, X. L. Huang, Chunfeng Wu, and C. H. Oh
We present a scheme to drive a finite-dimensional quantum system into the decoherence-free subspaces (DFS) by Lyapunov control. Control fields are established by Lyapunov function. This proposal works well for both closed and open quantum systems, with replacing the DFS by desired subspaces for clos ... [Phys. Rev. A 80, 052316 (2009)] published Thu Nov 12, 2009.

High-threshold universal quantum computation on the surface code

Austin G. Fowler, Ashley M. Stephens, and Peter Groszkowski
We present a comprehensive and self-contained simplified review of the quantum computing scheme of Raussendorf et al. [Phys. Rev. Lett. 98, 190504 (2007); N. J. Phys. 9, 199 (2007)], which features a two-dimensional nearest-neighbor coupled lattice of qubits, a threshold error rate approaching 1%, n ... [Phys. Rev. A 80, 052312 (2009)] published Wed Nov 11, 2009.

Minimum error discrimination problem for pure qubit states

Boris F. Samsonov
The necessary and sufficient conditions for minimization of the generalized rate error for discriminating among N pure qubit states are reformulated in terms of Bloch vectors representing the states. For the direct optimization problem, an algorithmic solution to these conditions is indicated. A sol ... [Phys. Rev. A 80, 052305 (2009)] published Wed Nov 4, 2009.

Realization of Universal Ion-Trap Quantum Computation with Decoherence-Free Qubits

T. Monz, K. Kim, A. S. Villar, P. Schindler, M. Chwalla et al.
Any residual coupling of a quantum computer to the environment results in computational errors. Encoding quantum information in a so-called decoherence-free subspace provides means to avoid these errors. Despite tremendous progress in employing this technique to extend memory storage times by orders ... [Phys. Rev. Lett. 103, 200503 (2009)] published Fri Nov 13, 2009.

Soft-pulse dynamical decoupling with Markovian decoherence

Leonid P. Pryadko and Gregory Quiroz
We consider the effect of broadband decoherence on the performance of refocusing sequences, having in mind applications of dynamical decoupling in concatenation with quantum error correcting codes as the first stage of coherence protection. Specifically, we construct cumulant expansions of effective ... [Phys. Rev. A 80, 042317 (2009)] published Tue Oct 20, 2009.

Monte Carlo analysis of critical phenomenon of the Ising model on memory stabilizer structures

C. Ricardo Viteri, Yu Tomita, and Kenneth R. Brown
We calculate the critical temperature of the Ising model on a set of graphs representing a concatenated three-bit error-correction code. The graphs are derived from the stabilizer formalism used in quantum error correction. The stabilizer for a subspace is defined as the group of Pauli operators who ... [Phys. Rev. A 80, 042313 (2009)] published Mon Oct 19, 2009.

Adiabatic approximation with exponential accuracy for many-body systems and quantum computation

Daniel A. Lidar, Ali T. Rezakhani, and Alioscia Hamma
We derive a version of the adiabatic theorem that is especially suited for applications in adiabatic quantum computation, where it is reasonable to assume that the adiabatic interpolation between the initial and final Hamiltonians is controllable. Assuming that the Hamiltonian is analytic in a finit ... [J. Math. Phys. 50, 102106 (2009)] published Tue Oct 20, 2009.

Preserving electron spin coherence in solids by optimal dynamical decoupling

Jiangfeng Du, Xing Rong, Nan Zhao, Ya Wang, Jiahui Yang et al.
To exploit the quantum coherence of electron spins in solids in future technologies such as quantum computing, it is first vital to overcome the problem of spin decoherence due to their coupling to the noisy environment. Dynamical decoupling, which uses stroboscopic spin flips to give an average co ... [Nature 461, 1265 (2009)] published Thu Oct 29, 2009.

Fault-tolerant quantum computation with probabilistic two-qubit gates

Hayato Goto and Kouichi Ichimura
A scheme for fault-tolerant quantum computation (FTQC) with probabilistic two-qubit gates is proposed. The error thresholds for FTQC are estimated by numerical simulations, where the conditional error probability (the error probability under the condition of success) is taken into account in additio ... [Phys. Rev. A 80, 040303 (2009)] published Tue Oct 20, 2009.

Information propagation through quantum chains with fluctuating disorder

Christian K. Burrell, Jens Eisert, and Tobias J. Osborne
We investigate the propagation of information through one-dimensional nearest-neighbor interacting quantum spin chains in the presence of external fields which fluctuate independently on each site. We study two fundamentally different models: (i) a model with general nearest-neighbor interactions in ... [Phys. Rev. A 80, 052319 (2009)] published Thu Nov 12, 2009.

Erratum: Linear optical protocol for preparation of N-photon GreenbergerHorneZeilinger state with conventional photon detectors [Appl. Phys. Lett. 92, 021127 (2008)]

Yan Xia, Jie Song, and He-Shan Song
Abstract not available. [Appl. Phys. Lett. 95, 169901 (2009)] published Mon Oct 19, 2009.

SU(N) Heisenberg model on the square lattice: A continuous-N quantum Monte Carlo study

K. S. D. Beach, Fabien Alet, Matthieu Mambrini, and Sylvain Capponi
A quantum phase transition is typically induced by tuning an external parameter that appears as a coupling constant in the Hamiltonian. Another route is to vary the global symmetry of the system, generalizing, e.g., SU(2) to SU(N). In that case, however, the discrete nature of the control parameter ... [Phys. Rev. B 80, 184401 (2009)] published Mon Nov 2, 2009.

Topological order in paired states of fermions in two dimensions with breaking of parity and time-reversal symmetries

Noah Bray-Ali, Letian Ding, and Stephan Haas
We numerically evaluate the entanglement spectrum (singular value decomposition of the wave function) of paired states of fermions in two dimensions that break parity and time-reversal symmetries, focusing on the spin-polarized p+ip case. The entanglement spectrum of the weak-pairing (BCS) phase con ... [Phys. Rev. B 80, 180504 (2009)] published Wed Nov 11, 2009.

Large-J approach to strongly coupled spin-orbital systems

Gia-Wei Chern and Natalia Perkins
We present an approach to study the ground-state and elementary excitations in compounds where spins and orbitals are entangled by on-site relativistic spin-orbit interaction. The appropriate degrees of freedom are localized states with an effective angular momentum J. We generalize J to arbitrary l ... [Phys. Rev. B 80, 180409 (2009)] published Wed Nov 11, 2009.

Fermionic multiscale entanglement renormalization ansatz

Philippe Corboz and Guifre Vidal
In a recent contribution [P. Corboz, G. Evenbly, F. Verstraete, and G. Vidal, arXiv:0904.4151 (unpublished)] entanglement renormalization was generalized to fermionic lattice systems in two spatial dimensions. Entanglement renormalization is a real-space coarse-graining transformation for lattice sy ... [Phys. Rev. B 80, 165129 (2009)] published Thu Oct 29, 2009.

Measuring the entanglement between double quantum dot charge qubits

Clive Emary
We present a scheme for creating and measuring entanglement between two double quantum dot charge qubits in a transport setup in which voltage pulses can modify system parameters. Detection of entanglement is performed via the construction of a Bell inequality with current correlation measurements. ... [Phys. Rev. B 80, 161309 (2009)] published Thu Oct 22, 2009.

In(Ga)As/GaAs quantum dots grown on a (111) surface as ideal sources of entangled photon pairs

Andrei Schliwa, Momme Winkelnkemper, Anatol Lochmann, Erik Stock, and Dieter Bimberg
Self-organized In(Ga)As/GaAs quantum dots (QDs) grown on (111) substrate are proposed as ideal sources for the generation of entangled photon pairs. Due to the threefold rotational symmetry of the (111) surface, QDs with C symmetry or higher are expected to develop during growth. In contrast to QDs ... [Phys. Rev. B 80, 161307 (2009)] published Wed Oct 21, 2009.

Cavity-assisted fast generation of entangled photon pairs through the biexciton-exciton cascade

P. K. Pathak and S. Hughes
We present a scheme for the fast cavity-assisted generation of entangled photon pairs from a single quantum dot coupled to a planar photonic crystal that support two orthogonally polarized cavity modes. We discuss within generation and across generation of entangled photons when both the biexciton t ... [Phys. Rev. B 80, 155325 (2009)] published Mon Oct 26, 2009.

Ab initio procedure for constructing effective models of correlated materials with entangled band structure

Takashi Miyake, Ferdi Aryasetiawan, and Masatoshi Imada
In a previous work [Phys. Rev. B 77, 085122 (2008)], a procedure for constructing low-energy models of electrons in solids was proposed. The procedure starts with dividing the Hilbert space into two subspaces: the low-energy part (d space) and the rest of the space (r space). The low-energy model is ... [Phys. Rev. B 80, 155134 (2009)] published Tue Oct 27, 2009.

Tensor-entanglement-filtering renormalization approach and symmetry-protected topological order

Zheng-Cheng Gu and Xiao-Gang Wen
We study the renormalization group flow of the Lagrangian for statistical and quantum systems by representing their path integral in terms of a tensor network. Using a tensor-entanglement-filtering renormalization approach that removes local entanglement and produces a coarse-grained lattice, we sho ... [Phys. Rev. B 80, 155131 (2009)] published Mon Oct 26, 2009.

Near-field correlations in the two-photon field

H. Di Lorenzo Pires and M. P. van Exter
In experiments with spontaneous parametric down conversion the near-field plane is usually defined as the output facet of the generating crystal. Experimentally, however, the spatial correlations are accessed via an imaging system and observed in a region around the image of the output facet. We sho ... [Phys. Rev. A 80, 053820 (2009)] published Fri Nov 13, 2009.

How colors influence numbers: Photon statistics of parametric down-conversion

Wolfgang Mauerer, Malte Avenhaus, Wolfram Helwig, and Christine Silberhorn
Parametric down-conversion (PDC) is a technique of ubiquitous experimental significance in the production of nonclassical, photon-number-correlated twin beams. Standard theory of PDC as a two-mode squeezing process predicts and homodyne measurements observe a thermal photon number distribution per b ... [Phys. Rev. A 80, 053815 (2009)] published Tue Nov 10, 2009.

Quantum many-body dynamics of dark solitons in optical lattices

R. V. Mishmash, I. Danshita, Charles W. Clark, and L. D. Carr
We present a fully quantum many-body treatment of dark solitons formed by ultracold bosonic atoms in one-dimensional optical lattices. Using time-evolving block decimation to simulate the single-band Bose-Hubbard Hamiltonian, we consider the quantum dynamics of density and phase engineered dark soli ... [Phys. Rev. A 80, 053612 (2009)] published Fri Nov 13, 2009.

Vortex nucleation in a mesoscopic Bose superfluid and breaking of the parity symmetry

D. Dagnino, N. Barberan, and M. Lewenstein
We analyze vortex nucleation in mesoscopic two-dimensional Bose superfluid in a rotating trap. We explicitly include a weakly anisotropic stirring potential, breaking thus explicitly the axial symmetry. As the rotation frequency passes the critical value Omega, the system undergoes an extra symmetry ... [Phys. Rev. A 80, 053611 (2009)] published Fri Nov 13, 2009.

Generation of polarization-entangled photons using type-II doubly periodically poled lithium niobate waveguides

K. Thyagarajan, J. Lugani, S. Ghosh, K. Sinha, A. Martin et al.
In this paper, we address the issue of the generation of nondegenerate cross-polarization-entangled photon pairs using type-II periodically poled lithium niobate. We show that, by an appropriate engineering of the quasi-phase-matching grating, it is possible to simultaneously satisfy the conditions ... [Phys. Rev. A 80, 052321 (2009)] published Fri Nov 13, 2009.

Maximally entangled three-qubit states via geometric measure of entanglement

Sayatnova Tamaryan, Tzu-Chieh Wei, and DaeKil Park
Bipartite maximally entangled states have the property that the largest Schmidt coefficient reaches its lower bound. However, for multipartite states, the standard Schmidt decomposition generally does not exist. We use a generalized Schmidt decomposition and the geometric measure of entanglement to ... [Phys. Rev. A 80, 052315 (2009)] published Wed Nov 11, 2009.

Optimal Lewenstein-Sanpera decomposition of two-qubit states using semidefinite programming

Guo Chuan Thiang, Philippe Raynal, and Berthold-Georg Englert
We use the language of semidefinite programming and duality to derive necessary and sufficient conditions for the optimal Lewenstein-Sanpera decomposition (LSD) of two-qubit states. We first provide a simple and natural derivation of the Wellens-Kuś equations for full-rank states. Then, we ob ... [Phys. Rev. A 80, 052313 (2009)] published Wed Nov 11, 2009.

Weight of quadratic forms and graph states

Alessandro Cosentino and Simone Severini
We prove a connection between Schmidt rank and weight of quadratic forms. This provides a new tool for the classification of graph states based on entanglement. Our main tool arises from a reformulation of previously known results concerning the weight of quadratic forms in terms of graph states pro ... [Phys. Rev. A 80, 052309 (2009)] published Mon Nov 9, 2009.

General algorithm for manipulating nonlinear and linear entanglement witnesses by using exact convex optimization

M. A. Jafarizadeh, K. Aghayar, and A. Heshmati
We consider the problem of obtaining the entanglement witnesses for a given quantum system. We present a method to convert this problem to a standard convex optimization problem by defining a feasible region. Then we develop a generic two-step algorithm for this problem which can be applied to the e ... [Phys. Rev. A 80, 052307 (2009)] published Mon Nov 9, 2009.

Three-Color Entanglement

A. S. Coelho, F. A. S. Barbosa, K. N. Cassemiro, A. S. Villar, M. Martinelli et al.
Entanglement is an essential quantum resource for the acceleration of information processing as well as for sophisticated quantum communication protocols. Quantum information networks are expected to convey information from one place to another by using entangled light beams. We demonstrated the gen ... [Science 326, 823 (2009)] published Thu Nov 5, 2009.

Non-Gaussian State Generation from Linear Elements via Feedback

Masahiro Yanagisawa
We present a feedback scheme to produce quantum non-Gaussian states from linear optical elements. Instead of traditional additive control methods, we introduce a mutiplicative feedback control and apply it to the deterministic generation of quantum superposition states. The protocol of feedback cont ... [Phys. Rev. Lett. 103, 203601 (2009)] published Wed Nov 11, 2009.

Natural Mode Entanglement as a Resource for Quantum Communication

Libby Heaney and Vlatko Vedral
Natural particle-number entanglement resides between spatial modes in coherent ultracold atomic gases. However, operations on the modes are restricted by a superselection rule that forbids coherent superpositions of different particle numbers. This seemingly prevents mode entanglement being used as ... [Phys. Rev. Lett. 103, 200502 (2009)] published Wed Nov 11, 2009.

Geometric Entangling Gates in Decoherence-Free Subspaces with Minimal Requirements

Xun-Li Feng, Chunfeng Wu, Hui Sun, and C. H. Oh
We introduce a new strongly driven dispersive atom-cavity interaction and develop a new scheme for implementing the nontrivial entangling gates for two logical qubits in decoherence-free subspaces (DFSs). Our scheme combines the robust advantages of DFS and the geometric phase. Moreover, only two ne ... [Phys. Rev. Lett. 103, 200501 (2009)] published Tue Nov 10, 2009.

Testing for Multipartite Quantum Nonlocality Using Functional Bell Inequalities

Q. Y. He, E. G. Cavalcanti, M. D. Reid, and P. D. Drummond
We show that arbitrary functions of continuous variables, e.g., position and momentum, can be used to generate tests that distinguish quantum theory from local hidden variable theories. By optimizing these functions, we obtain more robust violations of local causality than obtained previously. We an ... [Phys. Rev. Lett. 103, 180402 (2009)] published Thu Oct 29, 2009.

Large Pressure Effect on the Angular Distribution of Two Lyman-alpha Photons Emitted by an Entangled Pair of H(2p) Atoms in the Photodissociation of H

Takehiko Tanabe, Takeshi Odagiri, Motoyoshi Nakano, Isao H Suzuki, and Noriyuki Kouchi
The angular distribution of two Lyman-alpha photons, i.e., the probability density that two Lyman-alpha photons are emitted in given directions, in the photodissociation of a hydrogen molecule have been measured at the hydrogen gas pressures of 0.40 and 0.13 Pa. We have found that the experimental a ... [Phys. Rev. Lett. 103, 173002 (2009)] published Fri Oct 23, 2009.

State-Independent Quantum Contextuality with Single Photons

Elias Amselem, Magnus Radmark, Mohamed Bourennane, and Adan Cabello
We present an experimental state-independent violation of an inequality for noncontextual theories on single particles. We show that 20 different single-photon states violate an inequality which involves correlations between results of sequential compatible measurements by at least 419 standard devi ... [Phys. Rev. Lett. 103, 160405 (2009)] published Fri Oct 16, 2009.

Complete Criterion for Separability Detection

Miguel Navascues, Masaki Owari, and Martin B. Plenio
Using new results on the separability properties of bosonic systems, we provide a new complete criterion for separability. This criterion aims at characterizing the set of separable states from the inside by means of a sequence of efficiently solvable semidefinite programs. We apply this method to d ... [Phys. Rev. Lett. 103, 160404 (2009)] published Fri Oct 16, 2009.

Interaction-tuned compressible-to-incompressible phase transitions in quantum Hall systems

Z. Papic, N. Regnault, and S. Das Sarma
We analyze transitions between quantum Hall ground states at prominent filling factors nu in the spherical geometry by tuning the width parameter of the Zhang-Das Sarma interaction potential. We find that incompressible ground states evolve adiabatically under this tuning, whereas the compressible o ... [Phys. Rev. B 80, 201303 (2009)] published Mon Nov 2, 2009.

Effects of squeezing on quantum nonlocality of superpositions of coherent states

Chang-Woo Lee and Hyunseok Jeong
We analyze effects of squeezing upon superpositions of coherent states (SCSs) and entangled coherent states for Bell-inequality tests. We find that external squeezing can always increase the degrees of Bell violations, if the squeezing direction is properly chosen, for the case of photon parity meas ... [Phys. Rev. A 80, 052105 (2009)] published Tue Nov 10, 2009.

Area law for the entropy of low-energy states

Lluis Masanes
It is often observed in the ground state of quantum lattice systems with local interactions that the entropy of a large region is proportional to its surface area. In some cases, this area law is corrected with a logarithmic factor. This contrasts with the fact that in almost all states of the Hilbe ... [Phys. Rev. A 80, 052104 (2009)] published Tue Nov 10, 2009.

Long-distance entanglement and quantum teleportation in coupled-cavity arrays

Salvatore M. Giampaolo and Fabrizio Illuminati
We introduce quantum spin models whose ground states allow for sizable entanglement between distant spins. We discuss how spin models with global end-to-end entanglement realize quantum teleportation channels with optimal compromise between scalability and resilience to thermal decoherence and can b ... [Phys. Rev. A 80, 050301 (2009)] published Fri Nov 6, 2009.

Emission of photon pairs at discontinuities of nonlinearity in spontaneous parametric down-conversion

Jan Perina, Jr., Antonin Luks, and Ondrej Haderka
In order to fulfill the continuity requirements for electric- and magnetic-field amplitudes at discontinuities of chi nonlinearity, additional photon pairs have to be emitted in the area of discontinuity. Generalized two-photon spectral amplitudes can be used to describe properties of photon pairs g ... [Phys. Rev. A 80, 043837 (2009)] published Tue Oct 27, 2009.

Discrete coherent and squeezed states of many-qudit systems

Andrei B. Klimov, Carlos Munoz, and Luis L. Sanchez-Soto
We consider the phase space for n identical qudits (each one of dimension d, with d a primer number) as a grid of d x d points and use the finite Galois field GF(d) to label the corresponding axes. The associated displacement operators permit to define s-parametrized quasidistributions on this grid, ... [Phys. Rev. A 80, 043836 (2009)] published Tue Oct 27, 2009.

Generation of squeezed vacuum pulses using cascaded second-order optical nonlinearity of periodically poled lithium niobate in a Sagnac interferometer

Kenichi Hirosawa, Yasuyuki Ito, Hidetake Ushio, Hisayuki Nakagome, and Fumihiko Kannari
Squeezed vacuum pulses up to 1.7 dB at telecom-band (~1550 nm) is generated with femtosecond laser pulses using cascaded chi optical nonlinearity in a periodically poled lithium niobate crystal placed in a Sagnac interferometer. In spite of group velocity mismatch at short laser pulse pumping, suf ... [Phys. Rev. A 80, 043832 (2009)] published Mon Oct 26, 2009.

Ground-state entanglement in a coupled-cavity model

E. K. Irish
Bipartite entanglement entropies are calculated for the ground state of the two-excitation subspace in a two-site coupled cavity model. Each region in the phase diagram (atomic insulator, polaritonic insulator, photonic superfluid, and polaritonic superfluid) is found to be characterized by unique e ... [Phys. Rev. A 80, 043825 (2009)] published Wed Oct 21, 2009.

Enhanced two-photon absorption using entangled states and small mode volumes

Hao You, S. M. Hendrickson, and J. D. Franson
We calculate the rate of two-photon absorption for frequency-entangled photons in a tapered optical fiber whose diameter is comparable to the wavelength of the light. The confinement of the electric field in the transverse direction increases the intensity associated with a single photon, while the ... [Phys. Rev. A 80, 043823 (2009)] published Mon Oct 19, 2009.

Second-order Talbot effect with entangled photon pairs

Kai-Hong Luo, Jianming Wen, Xi-Hao Chen, Qian Liu, Min Xiao et al.
The second-order Talbot effect is analyzed for a periodic object illuminated by entangled photon pairs in both the quantum imaging and quantum lithography configurations. The Klyshko picture is applied to describe the quantum imaging scheme, in which self-images of the object that may or may not be ... [Phys. Rev. A 80, 043820 (2009)] published Mon Oct 19, 2009.

Probing the quantum ground state of a spin-1 Bose-Einstein condensate with cavity transmission spectra

J. M. Zhang, S. Cui, H. Jing, D. L. Zhou, and W. M. Liu
We propose to probe the quantum ground state of a spin-1 Bose-Einstein condensate with the transmission spectra of an optical cavity. By choosing a circularly polarized cavity mode with an appropriate frequency, we can realize coupling between the cavity mode and the magnetization of the condensate. ... [Phys. Rev. A 80, 043623 (2009)] published Mon Oct 26, 2009.

Control of the dynamics of coupled atomic-molecular Bose-Einstein condensates: Modified Gross-Pitaevskii approach

Moumita Gupta and Krishna Rai Dastidar
We study the dynamics of the atomic and molecular Bose-Einstein condensates (BECs) of Rb in a spherically symmetric trap coupled by stimulated Raman photoassociation process. Considering the higher order nonlinearity in the atom-atom interaction we analyze the dynamics of the system using coupled mo ... [Phys. Rev. A 80, 043618 (2009)] published Tue Oct 20, 2009.

Steady-state two-atom entanglement in a pumped cavity

Hideomi Nihira and C. R. Stroud, Jr.
In this paper we explore the possibility of a steady-state entanglement of two two-level atoms inside a pumped cavity by taking into account cavity leakage and the spontaneous emission of photons by the atoms. We describe the system in the dressed state picture in which the coherence is built into t ... [Phys. Rev. A 80, 042329 (2009)] published Tue Oct 27, 2009.

Environment-induced entanglement with a single photon

M. Hor-Meyll, A. Auyuanet, C. V. S. Borges, A. Aragao, J. A. O. Huguenin et al.
We propose an all-optical setup, which couples different degrees of freedom of a single photon, to investigate entanglement generation by a common environment. The two qubits are represented by the photon polarization and Hermite-Gauss transverse modes, while the environment corresponds to the photo ... [Phys. Rev. A 80, 042327 (2009)] published Mon Oct 26, 2009.

Algebraic and the graphic depictions of stabilized multipartite unlockable bound entanglement

Li-Yi Hsu and Keng-Shuo Wu
In this paper, we investigate multipartite unlockable stabilized bound entanglement. First, the mathematical structure of these stabilized bound entangled states is studied. Second, since stabilizer states are the local equivalent to the graph states, we study such stabilized mixed states in the gra ... [Phys. Rev. A 80, 042324 (2009)] published Fri Oct 23, 2009.

Quantum energy teleportation with trapped ions

Masahiro Hotta
We analyze a protocol of quantum energy teleportation that transports energy from the left edge of a linear ion crystal to the right edge by local operations and classical communication at a speed considerably greater than the speed of a phonon in the crystal. A probe qubit is strongly coupled with ... [Phys. Rev. A 80, 042323 (2009)] published Thu Oct 22, 2009.

Hybrid photonic entanglement: Realization, characterization, and applications

Leonardo Neves, Gustavo Lima, Aldo Delgado, and Carlos Saavedra
We show that the quantum disentanglement eraser implemented on a two-photon system from parametric downconversion is a general method to create hybrid photonic entanglement, namely the entanglement between different degrees of freedom of the photon pair. To demonstrate this, we generate and characte ... [Phys. Rev. A 80, 042322 (2009)] published Thu Oct 22, 2009.

Experimental quantum teleportation and multiphoton entanglement via interfering narrowband photon sources

Jian Yang, Xiao-Hui Bao, Han Zhang, Shuai Chen, Cheng-Zhi Peng et al.
In this paper, we report a realization of synchronization-free quantum teleportation and narrowband three-photon entanglement through interfering narrowband photon sources. Since both the single-photon and the entangled photon pair utilized are completely autonomous, it removes the requirement of hi ... [Phys. Rev. A 80, 042321 (2009)] published Thu Oct 22, 2009.

Quantum communication network utilizing quadripartite entangled states of optical field

Heng Shen, Xiaolong Su, Xiaojun Jia, and Changde Xie
We propose two types of quantum dense coding communication networks with optical continuous variables, in which a quadripartite entangled state of the optical field with totally three-party correlations of quadrature amplitudes is utilized. In the networks, the exchange of information between any tw ... [Phys. Rev. A 80, 042320 (2009)] published Thu Oct 22, 2009.

Fermionic entanglement that survives a black hole

Eduardo Martin-Martinez and Juan Leon
We introduce an arbitrary number of accessible modes when analyzing bipartite entanglement degradation due to Unruh effect between two partners Alice and Rob. Under the single mode approximation (SMA) a fermion field only had a few accessible levels due to Pauli exclusion principle conversely to bos ... [Phys. Rev. A 80, 042318 (2009)] published Tue Oct 20, 2009.

Cluster-type entangled coherent states: Generation and application

Nguyen Ba An and Jaewan Kim
We consider a type of (M+N)-mode entangled coherent states and propose a simple deterministic scheme to generate these states that can fly freely in space. We then exploit such free-flying states to teleport certain kinds of superpositions of multimode coherent states. We also address the issue of m ... [Phys. Rev. A 80, 042316 (2009)] published Tue Oct 20, 2009.

Spectral conditions for entanglement witnesses versus bound entanglement

Dariusz Chruścinski, Andrzej Kossakowski, and Gniewomir Sarbicki
It is shown that entanglement witnesses constructed via the family of spectral conditions are decomposable, i.e., cannot be used to detect bound entanglement. It supports several observations that bound entanglement reveals highly nonspectral features. ... [Phys. Rev. A 80, 042314 (2009)] published Mon Oct 19, 2009.

Optimal two-copy discrimination of quantum measurements

Jaromir Fiurasek and Michal MiCuda
We investigate optimal discrimination between two projective quantum measurements on a single qubit. We consider a scenario where the measurement that should be identified can be performed twice and we show that adaptive discrimination strategy, entangled probe states, and feed forward all help to i ... [Phys. Rev. A 80, 042312 (2009)] published Fri Oct 16, 2009.

Redundancy of classical and quantum correlations during decoherence

Juan Pablo Paz and Augusto J. Roncaglia
We analyze the time dependence of entanglement and total correlations between a system and fractions of its environment in the course of decoherence. For the quantum Brownian motion model, we show that the entanglement and total correlations have rather different dependence on the size of the enviro ... [Phys. Rev. A 80, 042111 (2009)] published Thu Oct 22, 2009.

Quantum decoherence of two qubits

Julius Helm and Walter T. Strunz
It is commonly stated that decoherence in open quantum systems is due to growing entanglement with an environment. In practice, however, surprisingly often decoherence may equally well be described by random unitary dynamics without invoking a quantum environment at all. For a single qubit, for inst ... [Phys. Rev. A 80, 042108 (2009)] published Mon Oct 19, 2009.

Optimal control of interacting particles: a multi-configuration time-dependent HartreeFock approach

Michael Mundt and David Tannor
We combine optimal control theory with the multi-configuration time-dependent HartreeFock method to control the dynamics of interacting particles. We use the resulting scheme to optimize state-to-state transitions in a one-dimensional (1D) model of helium and to entangle the external degrees-of-free ... [New J. Phys. 11, 105038 (2009)] published Thu Nov 5, 2009.

Anti-symmetrization reveals hidden entanglement

Alessandro Fedrizzi, Thomas Herbst, Markus Aspelmeyer, Marco Barbieri, Thomas Jennewein et al.
Two-photon anti-bunching at a beamsplitter is only possible if the photons are entangled in a specific state, anti-symmetric in the spatial modes. Thus, observation of anti-bunching is an indication of entanglement in a degree of freedom, which might not be easily accessible in an experiment. We exp ... [New J. Phys. 11, 103052 (2009)] published Thu Nov 5, 2009.

Dynamics of entanglement and `attractor' states in the TavisCummings model

C Jarvis, D Rodrigues, B Gyorffy, T Spiller, A Short et al.
We study the time evolution of N two-level atoms (or qubits) interacting with a single mode of a quantized radiation field. In the case of two qubits, we show that for a set of initial conditions the reduced density matrix of the atomic system approaches that of a pure state at [Formula: see text], ... [New J. Phys. 11, 103047 (2009)] published Thu Nov 5, 2009.

Entangling nanomechanical oscillators in a ring cavity by feeding squeezed light

Sumei Huang and G Agarwal
A scheme is presented for entangling two separated nanomechanical oscillators by injecting broad band squeezed vacuum light and laser light into the ring cavity. We work in the resolved sideband regime. We find that in order to obtain the maximum entanglement of the two oscillators, the squeezing p ... [New J. Phys. 11, 103044 (2009)] published Thu Nov 5, 2009.

Precise quantum tomography of photon pairs with entangled orbital angular momentum

B Jack, J Leach, H Ritsch, S Barnett, M Padgett et al.
We report a high fidelity tomographic reconstruction of the quantum state of photon pairs generated by parametric down-conversion with orbital angular momentum (OAM) entanglement. Our tomography method allows us to estimate an upper and lower bound for the entanglement between the down-converted pho ... [New J. Phys. 11, 103024 (2009)] published Thu Nov 5, 2009.

Non-locality of non-Abelian anyons

G Brennen, S Iblisdir, J Pachos, and J Slingerland
Entangled states of quantum systems can give rise to measurement correlations of separated observers that cannot be described by local hidden variable theories. Usually, it is assumed that entanglement between particles is generated due to some distance-dependent interaction. Yet anyonic particles i ... [New J. Phys. 11, 103023 (2009)] published Thu Nov 5, 2009.

Experimental high fidelity six-photon entangled state for telecloning protocols

Magnus Radmark, Marek Zukowski, and Mohamed Bourennane
We experimentally generate and characterize a six-photon polarization entangled state, which is usually called `Psi '. This is realized with a filtering procedure of triple emissions of entangled photon pairs from a single source, which does not use any interferometric overlaps. The setup is very st ... [New J. Phys. 11, 103016 (2009)] published Thu Nov 5, 2009.

Encoding orbital angular momentum onto multiple spin states based on a Huffman tree

Lixiang Chen and Weilong She
Based on single-photon spinorbit entanglement, we propose in this paper an experimental scheme to sort orbital angular momentum (OAM) by cascading conventional polarizing beam splitters. An economical algorithm, taking advantage of the pair's complementary notation of OAM numbers, is designed to sep ... [New J. Phys. 11, 103002 (2009)] published Thu Nov 5, 2009.

Fluxon-Based Generation of Graph States in Josephson Qubits

Toshiyuki Fujii, Masaru Fukunaga, and Noriyuki Hatakenaka
Graph states are a special kind of multiparticle entangled state with great potential for applications in quantum information technologies, especially in measurement-based quantum computers. These states cause significant reductions of the number of qubits needed for a given computation, leading ... [J. Phys. Soc. Jpn. 78, 103801 (2009)] published Wed Oct 28, 2009.

Characterization of partial positive transposition states and measures of entanglement

Wladyslaw A. Majewski, Takashi Matsuoka, and Masanori Ohya
A detailed characterization of partial positive transposition (PPT) states, both in the Heisenberg and in the Schrodinger picture, is given. Measures of entanglement are defined and discussed in details. Illustrative examples are provided. ... [J. Math. Phys. 50, 113509 (2009)] published Thu Nov 12, 2009.

Polarization entanglement with graded-index lenses

Giuseppe Vallone, Gaia Donati, Francesco De Martini, and Paolo Mataloni
By using an optical device based on the integration of a graded-index (GRIN) rod lens and a single-mode optical fiber we efficiently collected photon pairs generated by spontaneous parametric down-conversion. We show that this system preserves the polarization entanglement of the two-photon states. ... [Appl. Phys. Lett. 95, 181110 (2009)] published Fri Nov 6, 2009.

Power of symmetric extensions for entanglement detection

Miguel Navascues, Masaki Owari, and Martin B. Plenio
In this paper, we present progress on the study of the symmetric extension criterion for separability. First, we show that a perturbation of order O(1/N) is sufficient and, in general, necessary to destroy the entanglement of any state admitting an N Bose-symmetric extension. On the other hand, the ... [Phys. Rev. A 80, 052306 (2009)] published Fri Nov 6, 2009.

Inseparability criteria based on matrices of moments

Adam Miranowicz, Marco Piani, Pawel Horodecki, and Ryszard Horodecki
Inseparability criteria for continuous and discrete bipartite quantum states based on moments of annihilation and creation operators are studied by developing the idea of Shchukin-Vogel criterion [Phys. Rev. Lett. 95, 230502 (2005)]. If a state is separable, then the corresponding matrix of moments ... [Phys. Rev. A 80, 052303 (2009)] published Tue Nov 3, 2009.

Preparation of Dicke states in an ion chain

D. B. Hume, C. W. Chou, T. Rosenband, and D. J. Wineland
We have investigated theoretically and experimentally a method for preparing Dicke states in trapped atomic ions. We consider a linear chain of N ion qubits that is prepared in a particular Fock state of motion |m>. The m phonons are removed by applying a laser pulse globally to the N qubits and con ... [Phys. Rev. A 80, 052302 (2009)] published Mon Nov 2, 2009.

Genuine tripartite entanglement in quantum brachistochrone evolution of a three-qubit system

Bao-Kui Zhao, Fu-Guo Deng, Feng-Shou Zhang, and Hong-Yu Zhou
We explore the connection between quantum brachistochrone (time-optimal) evolution of a three-qubit system and its residual entanglement called three-tangle. The result shows that the entanglement between two qubits is not required for some brachistochrone evolutions of a three-qubit system. However ... [Phys. Rev. A 80, 052106 (2009)] published Wed Nov 11, 2009.

Spatial reflection and associated string order in quantum spin chains

Li-Xiang Cen
We investigate spatial reflection and associated nonlocal order in spin-chain quantum systems. The proposed string order parameters, e.g., reflected via operations of the spatial reflection or combinations of it with spin reflection, are able to characterize a variety of physical systems and allow u ... [Phys. Rev. B 80, 132405 (2009)] published Tue Oct 20, 2009.

Anomalous finite size effects on surface states in the topological insulator BiSe

Jacob Linder, Takehito Yokoyama, and Asle Sudbo
We study how the surface states in the strong topological insulator BiSe are influenced by finite size effects and compare our results with those recently obtained for two-dimensional topological insulator HgTe. We demonstrate two important distinctions: (i) contrary to HgTe, the surface states in B ... [Phys. Rev. B 80, 205401 (2009)] published Mon Nov 2, 2009.

Gaussian-state quantum-illumination receivers for target detection

Saikat Guha and Baris I. Erkmen
The signal half of an entangled twin beam, generated using spontaneous parametric downconversion, interrogates a region of space that is suspected of containing a target and has high loss and high (entanglement-breaking) background noise. A joint measurement is performed on the returned light and th ... [Phys. Rev. A 80, 052310 (2009)] published Tue Nov 10, 2009.

Ultranarrow bandwidth spectral filtering for long-range free-space quantum key distribution at daytime

David Hockel, Lars Koch, Eugen Martin, and Oliver Benson
We describe a FabryPerot-based spectral filter for free-space quantum key distribution (QKD). A multipass etalon filter was built, and its performance was studied. The whole filter setup was carefully optimized to add less than 2 dB attenuation to a signal beam but block stray light by 21 dB. Simula ... [Opt. Lett. 34, 3169 (2009)] published Tue Nov 3, 2009.

Device independent quantum key distribution secure against coherent attacks with memoryless measurement devices

Matthew McKague
Device independent quantum key distribution (QKD) aims to provide a higher degree of security than traditional QKD schemes by reducing the number of assumptions that need to be made about the physical devices used. The previous proof of security by Pironio et al (2009 New J. Phys. 11 045021) applies ... [New J. Phys. 11, 103037 (2009)] published Thu Nov 5, 2009.

Optical networking for quantum key distribution and quantum communications

T Chapuran, P Toliver, N Peters, J Jackel, M Goodman et al.
Modern optical networking techniques have the potential to greatly extend the applicability of quantum communications by moving beyond simple point-to-point optical links and by leveraging existing fibre infrastructures. We experimentally demonstrate many of the fundamental capabilities that are req ... [New J. Phys. 11, 105001 (2009)] published Thu Nov 5, 2009.

Non-Poissonian statistics from Poissonian light sources with application to passive decoy state quantum key distribution

Marcos Curty, Tobias Moroder, Xiongfeng Ma, and Norbert Lutkenhaus
We propose a method to prepare different non-Poissonian signal pulses from sources of Poissonian photon number distribution, using only linear optical elements and threshold photon detectors. This method allows a simple passive preparation of decoy states for quantum key distribution. We show that t ... [Opt. Lett. 34, 3238 (2009)] published Tue Nov 3, 2009.

Unconditional Security of Single-Photon Differential Phase Shift Quantum Key Distribution

Kai Wen, Kiyoshi Tamaki, and Yoshihisa Yamamoto
In this Letter, we prove the unconditional security of the single-photon differential phase shift quantum key distribution (DPS-QKD) protocol, based on the conversion to an equivalent entanglement-based protocol. We estimate the upper bound of the phase error rate from the bit error rate, and show t ... [Phys. Rev. Lett. 103, 170503 (2009)] published Thu Oct 22, 2009.

Limitations on continuous variable quantum algorithms with Fourier transforms

Mark Adcock, Peter Hoyer, and Barry Sanders
We study quantum algorithms implemented within a single harmonic oscillator, or equivalently within a single mode of the electromagnetic field. Logical states correspond to functions of the canonical position, and the Fourier transform to canonical momentum serves as the analogue of the Hadamard tra ... [New J. Phys. 11, 103035 (2009)] published Thu Nov 5, 2009.

Contraction of fermionic operator circuits and the simulation of strongly correlated fermions

Thomas Barthel, Carlos Pineda, and Jens Eisert
A fermionic operator circuit is a product of fermionic operators of usually different and partially overlapping support. Further elements of fermionic operator circuits (FOCs) are partial traces and partial projections. The presented framework allows for the introduction of fermionic versions of kno ... [Phys. Rev. A 80, 042333 (2009)] published Fri Oct 30, 2009.

Quantum hypothesis testing with group symmetry

Fumio Hiai, Milan Mosonyi, and Masahito Hayashi
The asymptotic discrimination problem of two quantum states is studied in the setting where measurements are required to be invariant under some symmetry group of the system. We consider various asymptotic error exponents in connection with the problems of the Chernoff bound, the Hoeffding bound, an ... [J. Math. Phys. 50, 103304 (2009)] published Thu Oct 22, 2009.

Can Closed Timelike Curves or Nonlinear Quantum Mechanics Improve Quantum State Discrimination or Help Solve Hard Problems?

Charles H. Bennett, Debbie Leung, Graeme Smith, and John A. Smolin
We study the power of closed timelike curves (CTCs) and other nonlinear extensions of quantum mechanics for distinguishing nonorthogonal states and speeding up hard computations. If a CTC-assisted computer is presented with a labeled mixture of states to be distinguishedthe most natural formulationw ... [Phys. Rev. Lett. 103, 170502 (2009)] published Wed Oct 21, 2009.

Erratum: Efficient quantum circuit implementation of quantum walks [Phys. Rev. A 79, 052335 (2009)]

B. L. Douglas and J. B. Wang
Abstract not available. [Phys. Rev. A 80, 059901 (2009)] published Tue Nov 10, 2009.

Globally controlled universal quantum computation with arbitrary subsystem dimension

Gerardo A. Paz-Silva, Gavin K. Brennen, and Jason Twamley
We present a scheme to perform universal quantum computation using global control with an arbitrary subsystem dimension (not necessarily finite). The scheme is developed over a one spatial dimension N-element array, using only mirror-symmetric logical encoding, global pulses for single subsystem ope ... [Phys. Rev. A 80, 052318 (2009)] published Thu Nov 12, 2009.

Realization of topological quantum computation with planar codes

Su-Peng Kou
In this paper, the degenerate ground states of Z topological order on a plane with holes (the so-called planar codes) are used as the protected code subspace to build a topological quantum computer by tuning their quantum tunneling effect. Using a designer Hamiltonianthe Kitaev toric-code model as a ... [Phys. Rev. A 80, 052317 (2009)] published Thu Nov 12, 2009.

Learning and testing algorithms for the Clifford group

Richard A. Low
Given oracle access to an unknown unitary C from the Clifford group and its conjugate, we give an exact algorithm for identifying C with O(n) queries, which we prove is optimal. We then extend this to all levels of the Gottesman-Chuang hierarchy (also known as the [script C] hierarchy). Further, for ... [Phys. Rev. A 80, 052314 (2009)] published Wed Nov 11, 2009.

Equivalence between discrete quantum walk models in arbitrary topologies

F. M. Andrade and M. G. E. da Luz
Coin and scattering are the two major formulations for discrete quantum walks models, each believed to have its own advantages in different applications. Although they are related in some cases, it was an open question their equivalence in arbitrary topologies. Here we present a general construction ... [Phys. Rev. A 80, 052301 (2009)] published Mon Nov 2, 2009.

Driving quantum-walk spreading with the coin operator

A. Romanelli
We generalize the discrete quantum walk on the line using a time-dependent unitary coin operator. We find an analytical relation between the long-time behaviors of the standard deviation and the coin operator. Selecting the coin time sequence allows to obtain a variety of predetermined asymptotic wa ... [Phys. Rev. A 80, 042332 (2009)] published Thu Oct 29, 2009.

Scalable quantum information processing with atomic ensembles and flying photons

Feng Mei, Mang Feng, Ya-Fei Yu, and Zhi-Ming Zhang
We present a scheme for scalable quantum information processing with atomic ensembles and flying photons. Using the Rydberg blockade, we encode the qubits in the collective atomic states, which could be manipulated fast and easily due to the enhanced interaction in comparison to the single-atom case ... [Phys. Rev. A 80, 042319 (2009)] published Tue Oct 20, 2009.

Quantum Walk on a Line for a Trapped Ion

Peng Xue, Barry C. Sanders, and Dietrich Leibfried
We show that a multistep quantum walk can be realized for a single trapped ion with an interpolation between a quantum and random walk achieved by randomizing the generalized Hadamard coin flip phase. The signature of the quantum walk is manifested not only in the ion's position but also in its phon ... [Phys. Rev. Lett. 103, 183602 (2009)] published Wed Oct 28, 2009.

Quantum Monte Carlo Simulations of Fidelity at Magnetic Quantum Phase Transitions

David Schwandt, Fabien Alet, and Sylvain Capponi
When a system undergoes a quantum phase transition, the ground-state wave function shows a change of nature, which can be monitored using the fidelity concept. We introduce two quantum Monte Carlo schemes that allow the computation of fidelity and its susceptibility for large interacting many-body s ... [Phys. Rev. Lett. 103, 170501 (2009)] published Mon Oct 19, 2009.