Virtual Journal of Quantum Information

Complete Characterization of Quantum-Optical Processes

Mirko Lobino, Dmitry Korystov, Connor Kupchak, Eden Figueroa, Barry C. Sanders et al.
The technologies of quantum information and quantum control are rapidly improving, but full exploitation of their capabilities requires complete characterization and assessment of processes that occur within quantum devices. We present a method for characterizing, with arbitrarily high accuracy, any ... [Science 322, 563 (2008)] published Fri Oct 24, 2008.

Single shot measurement of a silicon single electron transistor

D. G. Hasko, T. Ferrus, Q. R. Morrissey, S. R. Burge, E. J. Freeman et al.
We have fabricated a custom cryogenic complementary metal-oxide-semiconductor integrated circuit that has a higher measurement bandwidth compared to conventional room temperature electronics. This allowed implementing single shot operations and observe the real-time evolution of the current of a pho ... [Appl. Phys. Lett. 93, 192116 (2008)] published Fri Nov 14, 2008.

Infrared emission of the NV centre in diamond: Zeeman and uniaxial stress studies

L Rogers, S Armstrong, M Sellars, and N Manson
An emission band in the infrared (IR) is shown to be associated with a transition within the negative nitrogen-vacancy centre in diamond. The band has a zero-phonon line at 1046 nm, and uniaxial stress and magnetic field measurements indicate that the emission is associated with a transition between ... [New J. Phys. 10, 103024 (2008)] published Thu Nov 6, 2008.

Prepositioned single quantum dot in a lateral electric field

M. E. Reimer, M. Korkusinski, D. Dalacu, J. Lefebvre, J. Lapointe et al.
We examine the effect of a lateral electric field on the optical properties of a single deterministically positioned InAs/InP quantum dot. We show experimentally that the ground-state excitonic Stark shift is significantly reduced in comparison with the single-particle picture and that the lateral e ... [Phys. Rev. B 78, 195301 (2008)] published Tue Nov 4, 2008.

Contrasting Behavior of the (5/2) and (7/3) Fractional Quantum Hall Effect in a Tilted Field

C. R. Dean, B. A. Piot, P. Hayden, S. Das Sarma, G. Gervais et al.
Using a tilted-field geometry, the effect of an in-plane magnetic field on the even denominator nu=(5/2) fractional quantum Hall state is studied. The energy gap of the nu=(5/2) state is found to collapse linearly with the in-plane magnetic field above ~0.5 T. In contrast, a strong enhancement of t ... [Phys. Rev. Lett. 101, 186806 (2008)] published Fri Oct 31, 2008.

Coherent generation of non-classical light on a chip via photon-induced tunnelling and blockade

Andrei Faraon, Ilya Fushman, Dirk Englund, Nick Stoltz, Pierre Petroff et al.
Quantum dots in photonic crystals are interesting because of their potential in quantum information processing and as a testbed for cavity quantum electrodynamics. Recent advances in controlling and coherent probing of such systems open the possibility of realizing quantum networks originally propo ... [Nature Phys. 4, 859 (2008)] published Thu Nov 6, 2008.

Holographic quantum eraser experiment reveals polarization structure of depolarized light

Gerhard K. Ackermann and Jurgen Eichler
It is shown that a holographic setup for real-time interferometry can be used to realize a quantum eraser (QE) experiment. Circular polarized light is used to distinguish between the photons of the reconstructed image of the object and the direct object wave consisting of scattered photons from the ... [Appl. Opt. 47, 5660 (2008)] published Fri Nov 14, 2008.

Complete quantum control of a single quantum dot spin using ultrafast optical pulses

David Press, Thaddeus D. Ladd, Bingyang Zhang, and Yoshihisa Yamamoto
A basic requirement for quantum information processing systems is the ability to completely control the state of a single qubit. For qubits based on electron spin, a universal single-qubit gate is realized by a rotation of the spin by any angle about an arbitrary axis. Driven, coherent Rabi oscilla ... [Nature 456, 218 (2008)] published Thu Nov 13, 2008.

Trapped Rydberg ions: from spin chains to fast quantum gates

Markus Muller, Linmei Liang, Igor Lesanovsky, and Peter Zoller
We study the dynamics of Rydberg ions trapped in a linear Paul trap, and discuss the properties of ionic Rydberg states in the presence of the static and time-dependent electric fields constituting the trap. The interactions in a system of many ions are investigated and coupled equations of the inte ... [New J. Phys. 10, 093009 (2008)] published Thu Oct 16, 2008.

Probing the quantum coherence of a nanomechanical resonator using a superconducting qubit: I. Echo scheme

A Armour and M Blencowe
We propose a scheme in which the quantum coherence of a nanomechanical resonator can be probed using a superconducting qubit. We consider a mechanical resonator coupled capacitively to a Cooper pair box and assume that the superconducting qubit is tuned to the degeneracy point so that its coherence ... [New J. Phys. 10, 095004 (2008)] published Thu Oct 16, 2008.

Probing the quantum coherence of a nanomechanical resonator using a superconducting qubit: II. Implementation

M Blencowe and A Armour
We describe a possible implementation of the nanomechanical quantum superposition generation and detection scheme described in the preceding, companion paper (Armour A D and Blencowe M P 2008 New. J. Phys. 10 095004). The implementation is based on the circuit quantum electrodynamics (QED) set-up, w ... [New J. Phys. 10, 095005 (2008)] published Thu Oct 16, 2008.

Dissipation in circuit quantum electrodynamics: lasing and cooling of a low-frequency oscillator

Julian Hauss, Arkady Fedorov, Stephan Andre, Valentina Brosco, Carsten Hutter et al.
Superconducting qubits coupled to electric or nanomechanical resonators display effects previously studied in quantum electrodynamics (QED) as well as extensions thereof. Here, we consider a driven qubit coupled to a low-frequency oscillator and study the influence of dissipation. When the qubit is ... [New J. Phys. 10, 095018 (2008)] published Thu Oct 16, 2008.

Ground-state cooling of a nanomechanical resonator via a Cooper-pair box qubit

Konstanze Jaehne, Klemens Hammerer, and Margareta Wallquist
In this paper, we present a scheme for ground-state cooling of a flexural mode of a nanomechanical beam incorporated in a loop-shaped Cooper-pair box (CPB) circuit. Via the Lorentz force coupling of the beam motion to circulating CPB-circuit currents, energy is transferred to the CPB qubit which act ... [New J. Phys. 10, 095019 (2008)] published Thu Oct 16, 2008.

Fast and simple characterization of a photon pair source

F. Bussieres, J. A. Slater, N. Godbout, and W. Tittel
We present an exact model of the detection statistics of a probabilistic source of photon pairs from which a fast, simple and precise method to measure the source???s brightness and photon channel transmissions is demonstrated. We measure such properties for a source based on spontaneous parametric ... [Opt. Express 16, 17060 (2008)] published Fri Oct 17, 2008.

Photon echoes generated by reversing magnetic field gradients in a rubidium vapor

G. Hetet, M. Hosseini, B. M. Sparkes, D. Oblak, P. K. Lam et al.
We propose a photon echo quantum memory scheme using detuned Raman coupling to long-lived ground states. In contrast to previous three-level schemes based on controlled reversible inhomogeneous broadening that use sequences of pi pulses, the scheme does not require accurate control of the coupling d ... [Opt. Lett. 33, 2323 (2008)] published Fri Oct 31, 2008.

Phase variance of squeezed vacuum states

E. Bagan, A. Monras, and R. Munoz-Tapia
We consider the problem of estimating the phase of squeezed vacuum states within a Bayesian framework. We derive bounds on the average Holevo variance for an arbitrary number N of uncorrelated copies. We find that it scales with the mean photon number n, as dictated by the Heisenberg limit, i.e., as ... [Phys. Rev. A 78, 043829 (2008)] published Fri Oct 31, 2008.

Phase-space-region operators and the Wigner function: Geometric constructions and tomography

Demosthenes Ellinas and Anthony J. Bracken
Quasiprobability measures on a canonical phase space give rise through the action of Weyl's quantization map to operator-valued measures and, in particular, to region operators. Spectral properties, transformations, and general construction methods of such operators are investigated. Geometric trace ... [Phys. Rev. A 78, 052106 (2008)] published Fri Nov 7, 2008.

Experimental continuous-variable cloning of partial quantum information

Metin Sabuncu, Gerd Leuchs, and Ulrik L. Andersen
The fidelity of a quantum transformation is strongly linked with the prior partial information of the state to be transformed. We illustrate this interesting point by proposing and demonstrating the superior cloning of coherent states with prior partial information. More specifically, we propose two ... [Phys. Rev. A 78, 052312 (2008)] published Fri Nov 7, 2008.

Experimental asymmetric phase-covariant quantum cloning of polarization qubits

Jan Soubusta, Lucie Bartuskova, Antonin Cernoch, Miloslav Dusek, and Jaromir Fiurasek
We report on two optical realizations of the 1-->2 asymmetric phase-covariant cloning machines for polarization states of single photons. The experimental setups combine two-photon interference and tunable polarization filtering, which enables us to control the asymmetry of the cloners. The first sc ... [Phys. Rev. A 78, 052323 (2008)] published Thu Nov 13, 2008.

Multimodal Properties and Dynamics of Gradient Echo Quantum Memory

G. Hetet, J. J. Longdell, M. J. Sellars, P. K. Lam, and B. C. Buchler
We investigate the properties of a recently proposed gradient echo memory (GEM) scheme for information mapping between optical and atomic systems. We show that GEM can be described by the dynamic formation of polaritons in k space. This picture highlights the flexibility and robustness with regards ... [Phys. Rev. Lett. 101, 203601 (2008)] published Thu Nov 13, 2008.

Single-Photon Excitation of Surface Plasmon Polaritons

M. S. Tame, C. Lee, J. Lee, D. Ballester, M. Paternostro et al.
We provide the quantum-mechanical description of the excitation of surface plasmon polaritons on metal surfaces by single photons. An attenuated-reflection setup is described for the quantum excitation process in which we find remarkably efficient photon-to-surface plasmon wave-packet transfer. Usin ... [Phys. Rev. Lett. 101, 190504 (2008)] published Fri Nov 7, 2008.

Erasing Distinguishability Using Quantum Frequency Up-Conversion

Hiroki Takesue
The frequency distinguishability of two single photons was successfully erased using single photon frequency up-conversion. A frequency nondegenerate photon pair generated via spontaneous four-wave mixing in a dispersion shifted fiber was used to emulate two telecom-band single photons that were in ... [Phys. Rev. Lett. 101, 173901 (2008)] published Tue Oct 21, 2008.

Resonance Beating of Light Stored Using Atomic Spinor Polaritons

Leon Karpa, Frank Vewinger, and Martin Weitz
We investigate the storage of light in atomic rubidium vapor using a multilevel-tripod scheme. In the system, two collective dark polariton modes exist, forming an effective spinor quasiparticle. Storage of light is performed by dynamically reducing the optical group velocity to zero. After releasin ... [Phys. Rev. Lett. 101, 170406 (2008)] published Fri Oct 24, 2008.

Spin-orbit coupling and the singlet-triplet transition in lateral double quantum dots

L. Meza-Montes, Carlos F. Destefani, and Sergio E. Ulloa
We study the electronic level structure in lateral double quantum dots containing one and two electrons. The calculations consider the role of spin-orbit coupling arising from structure inversion asymmetry (Rashba) and bulk inversion asymmetry (Dresselhaus), as well as the competition with diamagnet ... [Phys. Rev. B 78, 205307 (2008)] published Fri Nov 7, 2008.

Determination of the total angular momentum of a paraxial beam

Gabriel Molina-Terriza
The multipolar solution of the electromagnetic field is useful in light-matter interactions. Here is presented a method to control the superpositions of multipolar electromagnetic fields using paraxial beams. This is applied to orbital angular momentum paraxial modes, which allow us to calculate the ... [Phys. Rev. A 78, 053819 (2008)] published Fri Nov 14, 2008.

Nuclear spin dynamics and Zeno effect in quantum dots and defect centers

D. Klauser, W. A. Coish, and Daniel Loss
We analyze nuclear-spin dynamics in quantum dots and defect centers with a bound electron under electron-mediated coupling between nuclear spins due to the hyperfine interaction (J coupling in NMR). Our analysis shows that the Overhauser field generated by the nuclei at the position of the electron ... [Phys. Rev. B 78, 205301 (2008)] published Mon Nov 3, 2008.

Vacuum Rabi splitting due to strong coupling of a flux qubit and a coplanar-waveguide resonator

Abdufarrukh A. Abdumalikov, Jr., Oleg Astafiev, Yasunobu Nakamura, Yuri A. Pashkin, and JawShen Tsai
We have experimentally studied a superconducting flux qubit strongly coupled to a single mode of a high-quality transmission-type coplanar-waveguide resonator. The qubit-resonator coupling is revealed in the resonator transmission spectrum as a vacuum Rabi splitting. In the dispersive regime the qub ... [Phys. Rev. B 78, 180502 (2008)] published Mon Nov 10, 2008.

Quantum efficiency of binary-outcome detectors of solid-state qubits

Alexander N. Korotkov
We introduce definitions of the quantum efficiency for binary-outcome qubit detectors with imperfect fidelity, focusing on the subclass of quantum nondemolition detectors. Quantum efficiency is analyzed for several models of detectors, including indirect projective measurement, linear detector in bi ... [Phys. Rev. B 78, 174512 (2008)] published Wed Nov 12, 2008.

Gate-controlled spin-spin interactions in lateral quantum dot molecules

Yun-Pil Shim and Pawel Hawrylak
We present a method of changing the effective spin-spin interactions of electrons localized in quantum dot molecules from antiferromagnetic to ferromagnetic using electrical means. We show that starting with antiferromagnetic interaction of two electrons localized in two quantum dots, the effective ... [Phys. Rev. B 78, 165317 (2008)] published Mon Oct 20, 2008.

Spin decoherence of a heavy hole coupled to nuclear spins in a quantum dot

Jan Fischer, W. A. Coish, D. V. Bulaev, and Daniel Loss
We theoretically study the interaction of a heavy hole with nuclear spins in a quasi-two-dimensional IIIV semiconductor quantum dot and the resulting dephasing of heavy-hole spin states. It has frequently been stated in the literature that heavy holes have a negligible interaction with nuclear spins ... [Phys. Rev. B 78, 155329 (2008)] published Fri Oct 31, 2008.

Photon statistics from a resonantly driven quantum dot

Ahsan Nazir
Photon correlations in the emission of a resonantly driven quantum dot are investigated, accounting for the influence of the solid-state phonon environment. An analytical expression is derived for the second-order fluorescence intensity correlation function, from which regimes of correlated and unco ... [Phys. Rev. B 78, 153309 (2008)] published Tue Oct 28, 2008.

Role of relaxation in the quantum measurement of a superconducting qubit using a nonlinear oscillator

T. Picot, A. Lupascu, S. Saito, C. J. P. M. Harmans, and J. E. Mooij
We analyze the relaxation of a superconducting flux qubit during measurement. The qubit state is measured with a nonlinear oscillator driven across the threshold of bifurcation, acting as a switching dispersive detector. This readout scheme is of quantum nondemolition type. Two successive readouts a ... [Phys. Rev. B 78, 132508 (2008)] published Tue Oct 21, 2008.

Planar Coulomb bicrystals

I. M. Buluta and S. Hasegawa
We numerically studied two-component planar Coulomb crystals (planar bicrystals) in rf traps. The number of ions in the inner shell was found to be limited by the mass difference between the two components and we calculated the maximum possible number of ions in the inner shells for various mass rat ... [Phys. Rev. A 78, 042340 (2008)] published Fri Oct 31, 2008.

Quantum walks on circles in phase space via superconducting circuit quantum electrodynamics

Peng Xue, Barry C. Sanders, Alexandre Blais, and Kevin Lalumiere
We show how a quantum walk can be implemented in a quantum quincunx created via superconducting circuit quantum electrodynamics (QED) and how interpolation from a quantum to a random walk is implemented by controllable decoherence using a two-resonator system. Direct control over the coin qubit is d ... [Phys. Rev. A 78, 042334 (2008)] published Tue Oct 28, 2008.

Optical realization of the unambiguous discriminator for unknown quantum states

Xiaohua Wu and Yixi Gong
With a single-photon representation of the quantum states and a multirail optical network for performing the unitary transformation, we give a scheme for an optical implementation of the universal programmable quantum state discriminator that unambiguously discriminates between two unknown states wi ... [Phys. Rev. A 78, 042315 (2008)] published Thu Oct 16, 2008.

Quantum Computing with Alkaline-Earth-Metal Atoms

Andrew J. Daley, Martin M. Boyd, Jun Ye, and Peter Zoller
We present a complete scheme for quantum information processing using the unique features of alkaline-earth-metal atoms. We show how two completely independent lattices can be formed for the S and P states, with one used as a storage lattice for qubits encoded on the nuclear spin, and the other as a ... [Phys. Rev. Lett. 101, 170504 (2008)] published Thu Oct 23, 2008.

Temperature Dependence of Electric Field Noise above Gold Surfaces

Jaroslaw Labaziewicz, Yufei Ge, David R. Leibrandt, Shannon X. Wang, Ruth Shewmon et al.
Electric field noise from fluctuating patch potentials is a significant problem for a broad range of precision experiments, including trapped ion quantum computation and single spin detection. Recent results demonstrated strong suppression of this noise by cryogenic cooling, suggesting an underlying ... [Phys. Rev. Lett. 101, 180602 (2008)] published Thu Oct 30, 2008.

Quantum Phase Tomography of a Strongly Driven Qubit

M. S. Rudner, A. V. Shytov, L. S. Levitov, D. M. Berns, W. D. Oliver et al.
The interference between repeated Landau-Zener transitions in a qubit swept through an avoided level crossing results in Stuckelberg oscillations in qubit magnetization, a hallmark of the coherent strongly driven regime in two-level systems. The two-dimensional Fourier transforms of the resulting os ... [Phys. Rev. Lett. 101, 190502 (2008)] published Fri Nov 7, 2008.

Reversal of the Weak Measurement of a Quantum State in a Superconducting Phase Qubit

Nadav Katz, Matthew Neeley, M. Ansmann, Radoslaw C. Bialczak, M. Hofheinz et al.
We demonstrate in a superconducting qubit the conditional recovery (uncollapsing) of a quantum state after a partial-collapse measurement. A weak measurement extracts information and results in a nonunitary transformation of the qubit state. However, by adding a rotation and a second partial measure ... [Phys. Rev. Lett. 101, 200401 (2008)] published Mon Nov 10, 2008.

Directional Coupling for Quantum Computing and Communication

Georgios M. Nikolopoulos
We introduce the concept of directional coupling, i.e., the selective transfer of a state between adjacent quantum wires, in the context of quantum computing and communication. Our analysis rests upon a mathematical analogy between a dual-channel directional coupler and a composite spin system. ... [Phys. Rev. Lett. 101, 200502 (2008)] published Thu Nov 13, 2008.

Long-Lived Spin Coherence in Silicon with an Electrical Spin Trap Readout

G. W. Morley, D. R. McCamey, H. A. Seipel, L.-C. Brunel, J. van Tol et al.
Pulsed electrically detected magnetic resonance of phosphorous (P) in bulk crystalline silicon at very high magnetic fields (B>8.5 T) and low temperatures (T=2.8 K) is presented. We find that the spin-dependent capture and reemission of highly polarized (>95%) conduction electrons by equally highl ... [Phys. Rev. Lett. 101, 207602 (2008)] published Fri Nov 14, 2008.

Implementation of a many-qubit Grover search with trapped ultracold ions

Wan-Li Yang, Hua Wei, Chang-Yong Chen, and Mang Feng
We propose a potentially practical scheme for realization of an n-qubit (n>2) conditional phase flip (CPF) gate and implementation of Grover's search algorithm in an ion-trap system. We demonstrate, both analytically and numerically, that our scheme could be achieved efficiently to find a marked sta ... [J. Opt. Soc. Am. B 25, 1720 (2008)] published Mon Oct 20, 2008.

Solid-state quantum memory using the [sup 31]P nuclear spin

John J. L. Morton, Alexei M. Tyryshkin, Richard M. Brown, Shyam Shankar, Brendon W. Lovett et al.
The transfer of information between different physical formsfor example processing entities and memoryis a central theme in communication and computation. This is crucial in quantum computation, where great effort must be taken to protect the integrity of a fragile quantum bit (qubit). However, tra ... [Nature 455, 1085 (2008)] published Fri Oct 24, 2008.

Erratum: Uncertainty relations for positive-operator-valued measures [Phys. Rev. A [bold 76], 042114 (2007)]

Serge Massar
... [Phys. Rev. A 78, 059901 (2008)] published Tue Nov 4, 2008.

Memory Effects in Quantum Channel Discrimination

Giulio Chiribella, Giacomo M. D'Ariano, and Paolo Perinotti
We consider quantum-memory assisted protocols for discriminating quantum channels. We show that for optimal discrimination of memory channels, memory assisted protocols are needed. This leads to a new notion of distance for channels with memory, based on the general theory of quantum testers. For di ... [Phys. Rev. Lett. 101, 180501 (2008)] published Mon Oct 27, 2008.

Optimal Cloning of Unitary Transformation

Giulio Chiribella, Giacomo Mauro D'Ariano, and Paolo Perinotti
After proving a general no-cloning theorem for black boxes, we derive the optimal universal cloning of unitary transformations, from one to two copies. The optimal cloner is realized by quantum channels with memory, and greatly outperforms the optimal measure-and-reprepare cloning strategy. Applicat ... [Phys. Rev. Lett. 101, 180504 (2008)] published Thu Oct 30, 2008.

Construction of bilinear control Hamiltonians using the series product and quantum feedback

J. E. Gough
We show that it is possible to construct closed quantum systems governed by a bilinear Hamiltonian depending on an arbitrary input signal. This is achieved by coupling the system to a quantum input field and performing a feedback of the output field back into the system to cancel out the stochastic ... [Phys. Rev. A 78, 052311 (2008)] published Fri Nov 7, 2008.

Repeatable quantum memory channels

Tomas Rybar and Mario Ziman
Within the framework of quantum memory channels we introduce the notion of repeatability of quantum channels. In particular, a quantum channel is called repeatable if there exists a memory device implementing the same channel on each individual input. We show that random unitary channels can be impl ... [Phys. Rev. A 78, 052114 (2008)] published Fri Nov 14, 2008.

Qubit phase space: SU(n) coherent-state P representations

D. W. Barry and P. D. Drummond
We introduce a phase-space representation for qubits and spin models. The technique uses an SU(n) coherent-state basis and can equally be used for either static or dynamical simulations. We review previously known definitions and operator identities, and show how these can be used to define an off-d ... [Phys. Rev. A 78, 052108 (2008)] published Tue Nov 11, 2008.

Ab initio approach to the optimization of qubit manipulation

Carlos F. Destefani, Chris McDonald, Ramin M. Abolfath, Pawel Hawrylak, and Thomas Brabec
We investigate the multiconfiguration time-dependent Hartree-Fock approach and show that it relaxes computational requirements and makes accurate modeling of few-electron dynamics in nanodevices computationally feasible. The method is applied to the optimization of the manipulation of a two-electron ... [Phys. Rev. B 78, 165331 (2008)] published Fri Oct 31, 2008.

Quantum simulation of interacting high-dimensional systems: The influence of noise

Wolfgang Dur, Michael J. Bremner, and Hans J. Briegel
We consider the simulation of interacting high-dimensional systems using pairwise interacting qubits. The main tool in this context is the generation of effective many-body interactions, and we examine a number of different protocols for obtaining them. These methods include the usage of higher-orde ... [Phys. Rev. A 78, 052325 (2008)] published Fri Nov 14, 2008.

Multispin errors in the optical control of a spin quantum lattice

Michal Grochol and Carlo Piermarocchi
We study a spin lattice realized with an array of charged quantum dots and embedded in a cavity. Optically excited polaritons, i.e., exciton-cavity-mixed states, interact with the electron spins in the dots. Linearly polarized excitation induces two-spin and multispin interactions. We discuss how th ... [Phys. Rev. B 78, 165324 (2008)] published Mon Oct 27, 2008.

Sequential Implementation of Global Quantum Operations

L. Lamata, J. Leon, D. Perez-Garcia, D. Salgado, and E. Solano
We study the possibility for a global unitary applied on an arbitrary number of qubits to be decomposed in a sequential unitary procedure, where an ancillary system is allowed to interact only once with each qubit. We prove that sequential unitary decompositions are in general impossible for genuine ... [Phys. Rev. Lett. 101, 180506 (2008)] published Fri Oct 31, 2008.

Universality of Uhrig Dynamical Decoupling for Suppressing Qubit Pure Dephasing and Relaxation

Wen Yang and Ren-Bao Liu
The optimal N-pulse dynamical decoupling discovered by Uhrig for a spin-boson model [Phys. Rev. Lett. 98, 100504 (2007)] is proved to be universal in suppressing to O(T) the pure dephasing or the longitudinal relaxation of a qubit (or spin 1/2) coupled to a generic bath in a short-time evolution of ... [Phys. Rev. Lett. 101, 180403 (2008)] published Wed Oct 29, 2008.

Minimization of environment-induced decoherence in quantum subsystems and application to solid-state-based quantum gates

M. Wenin and W. Potz
A general formulation of optimal control theory for open quantum systems (quantum subsystems) based on a superoperator method is presented. This approach is applied to a computation of optimized time-dependent fields for solid-state-based realizations of quantum gates. Decoherence is incorporated wi ... [Phys. Rev. B 78, 165118 (2008)] published Tue Oct 21, 2008.

Stability of quantum concatenated-code Hamiltonians

Dave Bacon
Protecting quantum information from the detrimental effects of decoherence and lack of precise quantum control is a central challenge that must be overcome if a large robust quantum computer is to be constructed. The traditional approach to achieving this is via active quantum error correction using ... [Phys. Rev. A 78, 042324 (2008)] published Thu Oct 23, 2008.

Constructing functional braids for low-leakage topological quantum computing

Haitan Xu and Xin Wan
We discuss how to significantly reduce leakage errors in topological quantum computation by introducing an irrelevant error in phase, using the construction of a controlled-NOT (CNOT) gate in the Fibonacci anyon model as a concrete example. To be specific, we construct a functional braid in a six-an ... [Phys. Rev. A 78, 042325 (2008)] published Thu Oct 23, 2008.

Purifying Greenberger-Horne-Zeilinger states using degenerate quantum codes

K. H. Ho and H. F. Chau
Degenerate quantum codes are codes that do not reveal the complete error syndrome. Their ability to conceal the complete error syndrome makes them powerful resources in certain quantum-information processing tasks. In particular, the most error-tolerant way to purify depolarized Bell states using on ... [Phys. Rev. A 78, 042329 (2008)] published Mon Oct 27, 2008.

Quantum capacity of a class of compound channels

Igor Bjelakovic, Holger Boche, and Janis Notzel
In this paper we address the issue of universal or robust communication over quantum channels. Specifically, we consider a memoryless communication scenario with channel uncertainty which is an analog of the compound channel in classical information theory. We determine the quantum capacity of finit ... [Phys. Rev. A 78, 042331 (2008)] published Tue Oct 28, 2008.

Quantum information processing in a spin-bus system of coupled chains

Xiang Hao and Shiqun Zhu
The effective Heisenberg interaction is generated by weakly interacting qubits with two strongly coupled chains. The system of two coupled chains can be regarded as a quantum bus. Universal quantum computation is enabled by restricting the bus to its ground state. The effective interaction is primar ... [Phys. Rev. A 78, 044302 (2008)] published Wed Oct 29, 2008.

Transverse spin relaxation time in organic molecules

B. Kanchibotla, S. Pramanik, S. Bandyopadhyay, and M. Cahay
We report a measurement of the ensemble-averaged transverse spin relaxation time (T) in bulk and few molecules of the organic semiconductor tris-(8-hydroxyquinolinolato aluminum) or Alq. This system exhibits two characteristic T times: the longer of which is temperature independent and the shorter i ... [Phys. Rev. B 78, 193306 (2008)] published Thu Nov 13, 2008.

Generalized parity measurements

Radu Ionicioiu, Anca E. Popescu, William J. Munro, and Timothy P. Spiller
Measurements play an important role in quantum computing (QC), either by providing the nonlinearity required for two-qubit gates (linear optics QC), or by implementing a quantum algorithm using single-qubit measurements on a highly entangled initial state (cluster state QC). Parity measurements can ... [Phys. Rev. A 78, 052326 (2008)] published Fri Nov 14, 2008.

Erratum: Entanglement invariant for the double Jaynes-Cummings model [Phys. Rev. A [bold 76], 042313 (2007)]

Isabel Sainz and Gunnar Bjork
... [Phys. Rev. A 78, 049905 (2008)] published Tue Oct 28, 2008.

Chaos, entanglement, and decoherence in the quantum kicked top

Shohini Ghose, Rene Stock, Poul Jessen, Roshan Lal, and Andrew Silberfarb
We analyze the interplay of chaos, entanglement, and decoherence in a system of qubits whose collective behavior is that of a quantum kicked top. The dynamical entanglement between a single qubit and the rest can be calculated from the mean of the collective spin operators. This allows the possibili ... [Phys. Rev. A 78, 042318 (2008)] published Mon Oct 20, 2008.

Entanglement-redistribution boxes

Andrzej Grudka, Michal Horodecki, Pawel Horodecki, Ryszard Horodecki, and Marco Piani
We establish a framework to study the classical-communication properties of primitive local operations assisted by classical communication which realize various redistributions of entanglement, like, e.g., entanglement swapping. On the one hand, we analyze what local operations and how much classica ... [Phys. Rev. A 78, 042317 (2008)] published Mon Oct 20, 2008.

Multiparty quantum states stabilized by the diagonal subgroup of the local unitary group

David W. Lyons and Scott N. Walck
We classify, up to local unitary equivalence, the set of n-qubit states that is stabilized by the diagonal subgroup of the local unitary group. We exhibit a basis for this set, parametrized by diagrams of nonintersecting chords connecting pairs of points on a circle, and give a criterion for when th ... [Phys. Rev. A 78, 042314 (2008)] published Thu Oct 16, 2008.

Decoherence and disentanglement for two qubits in a common squeezed reservoir

Maritza Hernandez and Miguel Orszag
We study the relation between the sudden death and revival of the entanglement of two qubits in a common squeezed bath and the decoherence. By getting close to the decoherence free subspace, we show that the death time shows a decrease and subsequent increase until the sudden death disappears and th ... [Phys. Rev. A 78, 042114 (2008)] published Fri Oct 31, 2008.

Classicality of spin states

Olivier Giraud, Petr Braun, and Daniel Braun
We extend the concept of classicality in quantum optics to spin states. We call a state classical if its density matrix can be decomposed as a weighted sum of angular momentum coherent states with positive weights. Classical spin states form a convex set [script C], which we fully characterize for a ... [Phys. Rev. A 78, 042112 (2008)] published Mon Oct 27, 2008.

Incomplete entanglement: Consequences for the quantum Zeno effect

R. Rossi, Jr., A. R. Bosco de Magalhaes, and M. C. Nemes
We show that the quantum Zeno effect (QZE) prevails even if the entanglement with the measuring probe is not complete. The dynamics toward the asymptotic regime as a function of N, the number of measurements, reveals surprising results: the transition probability, for some values of the coupling to ... [Phys. Rev. A 78, 042111 (2008)] published Thu Oct 23, 2008.

Time-resolved single-photon detection by femtosecond upconversion

Onur Kuzucu, Franco N. C. Wong, Sunao Kurimura, and Sergey Tovstonog
We demonstrate a time-resolved single-photon detection technique based on ultrafast sum-frequency generation, providing femtosecond measurement capability for single photons in photonic quantum information processing. Noncollinear broadband upconversion in periodically poled MgO-doped stoichiometric ... [Opt. Lett. 33, 2257 (2008)] published Fri Oct 17, 2008.

Spontaneous emission of quantum dot excitons into surface plasmons in a nanowire

Guang-Yin Chen, Yueh-Nan Chen, and Der-San Chuu
The spontaneous emission (SE) of quantum dot (QD) excitons into surface plasmons in a cylindrical nanowire is investigated theoretically. Maxwell's equations with appropriate boundary conditions are solved numerically to obtain the dispersion relations of surface plasmons. The SE rate of QD excitons ... [Opt. Lett. 33, 2212 (2008)] published Fri Oct 17, 2008.

Observation of time correlation function of multimode two-photon pairs on a rubidium D[sub 2] line

Fu-Yuan Wang, Bao-Sen Shi, and Guang-Can Guo
We report on the generation of a type-I multimode two-photon state on a rubidium D line (780 nm) using periodically poled KTiOPO crystals. With a degenerate optical parametric oscillator far below threshold, we observe an oscillatory correlation function; the cross correlation between two photons sh ... [Opt. Lett. 33, 2191 (2008)] published Fri Oct 17, 2008.

On the distinguishability of downconverted modes with orbital angular momentum

Geraldo Alexandre Barbosa
Assuming two quantum states of spontaneous parametric downconversion carrying orbital angular momentum, one may ask the question what is the minimum probability of error in identifying between two of these biphoton states by an arbitrary physical measurement over the biphoton state generated. While ... [Opt. Lett. 33, 2119 (2008)] published Fri Oct 17, 2008.

Unusual entanglement transformation properties of the quantum radiation through one-dimensional random system containing left-handed-materials

Yunxia Dong and Xiangdong Zhang
The quantum radiation through the multilayer structures containing the left-handed materials is investigated based on the Green-function approach to the quantization of the phenomenological Maxwell theory. Emphasis is placed on the effect of randomness on the generation and transmission of entangled ... [Opt. Express 16, 16950 (2008)] published Fri Oct 17, 2008.

Amplitude and phase modulation of time-energy entangled two-photon states

F. Zah, M. Halder, and T. Feurer
We experimentally demonstrate amplitude and phase modulation of a time-energy entangled two-photon wave function. The entangled photons are produced by spontaneous parametric down-conversion, spectrally dispersed in an prism compressor, modulated in amplitude and/or phase, and detected in coincidenc ... [Opt. Express 16, 16452 (2008)] published Fri Oct 17, 2008.

Entangled-photon coincidence fluorescence imaging

Giuliano Scarcelli and Seok H. Yun
We describe fluorescence imaging using the second-order correlation of entangled photon pairs. The proposed method is based on the principle that one photon of the pair carries information on where the other photon has been absorbed and has produced fluorescence in a sample. Because fluorescent mole ... [Opt. Express 16, 16189 (2008)] published Fri Oct 17, 2008.

Broadband source of telecom-band polarization-entangled photon-pairs for wavelength-multiplexed entanglement distribution

Han Chuen Lim, Akio Yoshizawa, Hidemi Tsuchida, and Kazuro Kikuchi
Studies on telecom-band entangled photon-pair sources for entanglement distribution have so far focused on their narrowband operations. Fiber-based sources are seriously limited by spontaneous Raman scattering while sources based on quasi-phase-matched crystals or waveguides are usually narrowband b ... [Opt. Express 16, 16052 (2008)] published Fri Oct 17, 2008.

Route to ponderomotive entanglement of light via optically trapped mirrors

Christopher Wipf, Thomas Corbitt, Yanbei Chen, and Nergis Mavalvala
The radiation pressure of two laser beams detuned from resonance in an optical cavity can create a stable optical trap for a mechanical oscillation mode of a movable cavity mirror. Here it is shown that such a configuration entangles the output light fields via interaction with a mirror that is susp ... [New J. Phys. 10, 095017 (2008)] published Thu Oct 16, 2008.

Optomechanical to mechanical entanglement transformation

Giovanni Vacanti, Mauro Paternostro, G Massimo Palma, and Vlatko Vedral
We present a scheme for generating entanglement between two mechanical oscillators that have never interacted with each other by using an entanglement-swapping protocol. The system under study consists of a MichelsonMorley interferometer comprising mechanical systems embodied by two cantilevers. Eac ... [New J. Phys. 10, 095014 (2008)] published Thu Oct 16, 2008.

Simultaneous cooling and entanglement of mechanical modes of a micromirror in an optical cavity

Claudiu Genes, David Vitali, and Paolo Tombesi
Laser cooling of a mechanical mode of a resonator by the radiation pressure of a detuned optical cavity mode has been recently demonstrated by various groups in different experimental configurations. Here, we consider the effect of a second mechanical mode with a close but different resonance freque ... [New J. Phys. 10, 095009 (2008)] published Thu Oct 16, 2008.

Focus on Mechanical Systems at the Quantum Limit

Markus Aspelmeyer and Keith Schwab
The last five years have witnessed an amazing development in the field of nano- and micromechanics. What was widely considered fantasy ten years ago is about to become an experimental reality: the quantum regime of mechanical systems is within reach of current experiments. Two factors (among many) ... [New J. Phys. 10, 095001 (2008)] published Thu Oct 16, 2008.

Frustration of decoherence in Y-shaped superconducting Josephson networks

Domenico Giuliano and Pasquale Sodano
We examine the possibility that pertinent impurities in a condensed matter system may help in designing quantum devices with enhanced coherent behaviors. For this purpose, we analyze a field theory model describing Y-shaped superconducting Josephson networks. We show that a new finite coupling stabl ... [New J. Phys. 10, 093023 (2008)] published Thu Oct 16, 2008.

Entanglement of distant atoms by projective measurement: the role of detection efficiency

Stefano Zippilli, Georgina Olivares-Renteria, Giovanna Morigi, Carsten Schuck, Felix Rohde et al.
We assess proposals for entangling two distant atoms by measurement of emitted photons, analyzing how their performance depends on the photon detection efficiency. We consider schemes based on measurement of one or two photons and compare them in terms of the probability to obtain the detection eve ... [New J. Phys. 10, 103003 (2008)] published Thu Nov 6, 2008.

Squeezing and entanglement in a Bose--Einstein condensate

J. Esteve, C. Gross, A. Weller, S. Giovanazzi, and M. K. Oberthaler
Entanglement, a key feature of quantum mechanics, is a resource that allows the improvement of precision measurements beyond the conventional bound attainable by classical means. This results in the standard quantum limit, which is reached in today's best available sensors of various quantities suc ... [Nature 455, 1216 (2008)] published Thu Oct 30, 2008.

Entanglement theory and the second law of thermodynamics

Fernando G. S. L. Brandao and Martin B. Plenio
Entanglement is central both to the foundations of quantum theory and, as a novel resource, to quantum information science. The theory of entanglement establishes basic laws that govern its manipulation, in particular the non-increase of entanglement under local operations on the constituent particl ... [Nature Phys. 4, 873 (2008)] published Thu Nov 6, 2008.

Quantum-inspired interferometry with chirped laser pulses

R. Kaltenbaek, J. Lavoie, D. N. Biggerstaff, and K. J. Resch
Interference is a defining feature of both quantum and classical theories of light, enabling the most precise measurements of a wide range of physical quantities including length and time. Quantum metrology exploits fundamental differences between these theories for new measurement techniques and e ... [Nature Phys. 4, 864 (2008)] published Thu Nov 6, 2008.

Deterministic entanglement swapping with an ion-trap quantum computer

M. Riebe, T. Monz, K. Kim, A. S. Villar, P. Schindler et al.
Entanglementonce only a subject of disputes about the foundation of quantum mechanicshas today become an essential issue in the emerging field of quantum information processing, promising a number of applications, including secure communication, teleportation and powerful quantum computation. There ... [Nature Phys. 4, 839 (2008)] published Thu Nov 6, 2008.

Quantum entanglement: Reversible path to thermodynamics

Michal Horodecki
... [Nature Phys. 4, 833 (2008)] published Thu Nov 6, 2008.

Monogamy equality in 2 [direct-product] 2 [direct-product] d quantum systems

Dong Pyo Chi, Jeong Woon Choi, Kabgyun Jeong, Jeong San Kim, Taewan Kim et al.
There is an interesting property about multipartite entanglement, called the monogamy of entanglement. The property can be shown by the monogamy inequality, called the CoffmanKunduWootters inequality [Phys. Rev. A 61, 052306 (2000); Phys. Rev. Lett. 96, 220503 (2006)], and more explicitly by the mon ... [J. Math. Phys. 49, 112102 (2008)] published Thu Nov 13, 2008.

On states of perfect correlation

Siendong Huang
For finite-dimensional systems, states satisfying perfect correlation are just maximally entangled states that play an essential role in quantum information theory. Thus perfect correlation may be a key to entanglement phenomena. For general systems, EinsteinPodolskyRosen type states are characteriz ... [J. Math. Phys. 49, 112101 (2008)] published Thu Nov 6, 2008.

Announcement: Special issue on ``Integrable Quantum Systems and Solvable Statistical Mechanics Models''

Bruno L. Z. Nachtergaele
... [J. Math. Phys. 49, 110201 (2008)] published Thu Nov 13, 2008.

Announcement: Special issue on ``Integrable Quantum Systems and Solvable Statistical Mechanics Models''

Bruno L. Z. Nachtergaele
... [J. Math. Phys. 49, 100201 (2008)] published Wed Oct 22, 2008.

Excess entropy scaling of transport properties of Lennard-Jones chains

Teena Goel, Chandra Nath Patra, Tulsi Mukherjee, and Charusita Chakravarty
Excess-entropy scaling relationships for diffusivity and viscosity of Lennard-Jones chain fluids are tested using molecular dynamics simulations for chain sizes that are sufficiently small that chain entanglement effects are insignificant. The thermodynamic excess entropy S is estimated using self-a ... [J. Chem. Phys. 129, 164904 (2008)] published Tue Oct 28, 2008.

Exact computation and large angular momentum asymptotics of 3nj symbols: Semiclassical disentangling of spin networks

Roger W. Anderson, Vincenzo Aquilanti, and Cristiane da Silva Ferreira
Spin networks, namely, the 3nj symbols of quantum angular momentum theory and their generalizations to groups other than SU(2) and to quantum groups, permeate many areas of pure and applied science. The issues of their computation and characterization for large values of their entries are a challeng ... [J. Chem. Phys. 129, 161101 (2008)] published Thu Oct 23, 2008.

Steady State Entanglement in the Mechanical Vibrations of Two Dielectric Membranes

Michael J. Hartmann and Martin B. Plenio
We consider two dielectric membranes suspended inside a Fabry-Perot cavity, which are cooled to a steady state via a drive by suitable classical lasers. We show that the vibrations of the membranes can be entangled in this steady state. They thus form two mechanical, macroscopic degrees of freedom t ... [Phys. Rev. Lett. 101, 200503 (2008)] published Thu Nov 13, 2008.

Experimental Quantum Computing without Entanglement

B. P. Lanyon, M. Barbieri, M. P. Almeida, and A. G. White
Deterministic quantum computation with one pure qubit (DQC1) is an efficient model of computation that uses highly mixed states. Unlike pure-state models, its power is not derived from the generation of a large amount of entanglement. Instead it has been proposed that other nonclassical correlations ... [Phys. Rev. Lett. 101, 200501 (2008)] published Thu Nov 13, 2008.

Dissipation Induced Coherence of a Two-Mode Bose-Einstein Condensate

D. Witthaut, F. Trimborn, and S. Wimberger
We discuss the dynamics of a Bose-Einstein condensate in a double-well trap subject to phase noise and particle loss. The phase coherence of a weakly interacting condensate as well as the response to an external driving show a pronounced stochastic resonance effect: Both quantities become maximal fo ... [Phys. Rev. Lett. 101, 200402 (2008)] published Mon Nov 10, 2008.

Coherent Particle Transfer in an On-Demand Single-Electron Source

J. Keeling, A. V. Shytov, and L. S. Levitov
Electron transfer from a localized state in a quantum dot into a ballistic conductor generally results in particle-hole excitations. We study this effect, considering a resonance level with time-dependent energy coupled to particle states in the Fermi sea. We find that, as the resonance level is dri ... [Phys. Rev. Lett. 101, 196404 (2008)] published Fri Nov 7, 2008.

Novel Schemes for Directly Measuring Entanglement of General States

Jianming Cai and Wei Song
An intrinsic relation between maximally entangled states and entanglement measures is revealed, which plays a role in establishing connections for different entanglement quantifiers. We exploit the basic idea and propose a framework to construct schemes for directly measuring entanglement of general ... [Phys. Rev. Lett. 101, 190503 (2008)] published Fri Nov 7, 2008.

Generation of Narrow-Band Polarization-Entangled Photon Pairs for Atomic Quantum Memories

Xiao-Hui Bao, Yong Qian, Jian Yang, Han Zhang, Zeng-Bing Chen et al.
We report an experimental realization of a narrow band polarization-entangled photon source with a linewidth of 9.6 MHz through cavity-enhanced spontaneous parametric down-conversion. This linewidth is comparable to the typical linewidth of atomic ensemble-based quantum memories. Single-mode output ... [Phys. Rev. Lett. 101, 190501 (2008)] published Mon Nov 3, 2008.

Magnetic Dipolar Interaction in a Bose-Einstein Condensate Atomic Interferometer

M. Fattori, G. Roati, B. Deissler, C. D'Errico, M. Zaccanti et al.
We study the role played by the magnetic dipole interaction in the decoherence of a lattice-based interferometer that employs an alkali Bose-Einstein condensate with a tunable scattering length. The different behavior we observe for two different orientations of the dipoles gives us evidence of the ... [Phys. Rev. Lett. 101, 190405 (2008)] published Fri Nov 7, 2008.

Efficient Hidden-Variable Simulation of Measurements in Quantum Experiments

Borivoje Dakic, Milovan Suvakov, Tomasz Paterek, and Caslav Brukner
We prove that the results of a finite set of general quantum measurements on an arbitrary dimensional quantum system can be simulated using a polynomial (in measurements) number of hidden-variable states. In the limit of infinitely many measurements, our method gives models with the minimal number o ... [Phys. Rev. Lett. 101, 190402 (2008)] published Tue Nov 4, 2008.

Frustration, Area Law, and Interference in Quantum Spin Models

Aditi Sen(De), Ujjwal Sen, Jacek Dziarmaga, Anna Sanpera, and Maciej Lewenstein
We study frustrated quantum systems from a quantum information perspective. Within this approach, we find that highly frustrated systems do not follow any general area law of block entanglement, while weakly frustrated ones have area laws similar to those of nonfrustrated systems away from criticali ... [Phys. Rev. Lett. 101, 187202 (2008)] published Wed Oct 29, 2008.

Irreducible Multiparty Correlations in Quantum States without Maximal Rank

D. L. Zhou
The correlations of an n-partite quantum state are classified into a series of irreducible k-party ones (2<=k<=n), with the irreducible k-party correlation being the correlation in the states of k parties but nonexistent in the states of (k-1) parties. A measure of the degree of irreducible k-party ... [Phys. Rev. Lett. 101, 180505 (2008)] published Thu Oct 30, 2008.

Quantum Multiscale Entanglement Renormalization Ansatz Channels

V. Giovannetti, S. Montangero, and Rosario Fazio
Tensor network representations of many-body quantum systems can be described in terms of quantum channels. We focus on channels associated with the multiscale entanglement renormalization ansatz tensor network that has been recently introduced to efficiently describe critical systems. Our approach a ... [Phys. Rev. Lett. 101, 180503 (2008)] published Thu Oct 30, 2008.

Exploring Energy-Time Entanglement Using Geometric Phase

Anand Kumar Jha, Mehul Malik, and Robert W. Boyd
Using the signal and idler photons produced by parametric down-conversion, we report an experimental observation of a violation of the Bell inequality for energy and time based purely on the geometric phases of the signal and idler photons. We thus show that energy-time entanglement between the sign ... [Phys. Rev. Lett. 101, 180405 (2008)] published Fri Oct 31, 2008.

Entanglement Evolution in Finite Dimensions

Markus Tiersch, Fernando de Melo, and Andreas Buchleitner
We provide a relation which describes how the entanglement of two d-level systems evolves as either system undergoes an arbitrary physical process. The dynamics of the entanglement turns out to be of a simple form and is fully captured by a single quantity. ... [Phys. Rev. Lett. 101, 170502 (2008)] published Tue Oct 21, 2008.

Evolution of Entanglement Between Distinguishable Light States

R. Mark Stevenson, Andrew J. Hudson, Anthony J. Bennett, Robert J. Young, Christine A. Nicoll et al.
We investigate the evolution of quantum correlations over the lifetime of a multiphoton state. Measurements reveal time-dependent oscillations of the entanglement fidelity for photon pairs created by a single semiconductor quantum dot. The oscillations are attributed to the phase acquired in the int ... [Phys. Rev. Lett. 101, 170501 (2008)] published Mon Oct 20, 2008.

Spatial Symmetry and Conservation of Orbital Angular Momentum in Spontaneous Parametric Down-Conversion

Sheng Feng and Prem Kumar
Directly contradictory arguments coexist regarding the conservation rule of orbital angular momentum in spontaneous parametric down-conversion. We analytically show how this rule is decided by spatial symmetry. We discover that the down-converted photon pairs can carry non-negligible extrinsic orbit ... [Phys. Rev. Lett. 101, 163602 (2008)] published Fri Oct 17, 2008.

p[sub x] + ip[sub y] Superfluid from s-Wave Interactions of Fermionic Cold Atoms

Chuanwei Zhang, Sumanta Tewari, Roman M. Lutchyn, and S. Das Sarma
Two-dimensional (p+ip) superfluids or superconductors offer a playground for studying intriguing physics such as quantum teleportation, non-Abelian statistics, and topological quantum computation. Creating such a superfluid in cold fermionic atom optical traps using p-wave Feshbach resonance is turn ... [Phys. Rev. Lett. 101, 160401 (2008)] published Fri Oct 17, 2008.

Entanglement and the generation of random states in the quantum chaotic dynamics of kicked coupled tops

Collin M. Trail, Vaibhav Madhok, and Ivan H. Deutsch
We study the dynamical generation of entanglement as a signature of chaos in a system of periodically kicked coupled tops, where chaos and entanglement arise from the same physical mechanism. The long-time-averaged entanglement as a function of the position of an initially localized wave packet very ... [Phys. Rev. E 78, 046211 (2008)] published Tue Oct 21, 2008.

Entangled photon sources based on semiconductor quantum dots: The role of pure dephasing

Gernot Pfanner, Marek Seliger, and Ulrich Hohenester
We theoretically investigate the production of polarization-entangled photons through the biexciton cascade decay in a single semiconductor quantum dot. To accomplish a high degree of entanglement, despite the exciton fine-structure splitting, one must either energetically align the two exciton stat ... [Phys. Rev. B 78, 195410 (2008)] published Tue Nov 11, 2008.

Quantum phase transitions in the S = (1/2) distorted diamond chain

Yan-Chao Li and Shu-Shen Li
By means of the second derivative of the ground-state and first-excited energy, the quantum phase transitions (QPTs) for the distorted diamond chain (DDC) with ferromagnetic and antiferromagnetic frustrated interactions and the trimerized case are investigated, respectively. Our results show the ple ... [Phys. Rev. B 78, 184412 (2008)] published Thu Nov 13, 2008.

Wave-packet formalism of full counting statistics

F. Hassler, M. V. Suslov, G. M. Graf, M. V. Lebedev, G. B. Lesovik et al.
We make use of the first-quantized wave-packet formulation of the full counting statistics to describe charge transport of noninteracting electrons in a mesoscopic device. We derive various expressions for the characteristic function generating the full counting statistics, accounting for both energ ... [Phys. Rev. B 78, 165330 (2008)] published Wed Oct 29, 2008.

Non-Abelian statistics from an Abelian model

James R. Wootton, Ville Lahtinen, Zhenghan Wang, and Jiannis K. Pachos
It is well known that the Abelian Z anyonic model (toric code) can be realized on a highly entangled two-dimensional spin lattice, where the anyons are quasiparticles located at the end points of stringlike concatenations of Pauli operators. Here we show that the same entangled states of the same la ... [Phys. Rev. B 78, 161102 (2008)] published Thu Oct 30, 2008.

Proposal for geometric generation of a biexciton in a quantum dot using a chirped pulse

H. Y. Hui and R. B. Liu
We propose to create a biexciton in a quantum dot by a coherent optical process using a frequency-sweeping (chirped) laser pulse. In contrast to the two-photon Rabi flop scheme, the present method uses the adiabatic state transfer through avoided level crossing. As a geometric control, the proposed ... [Phys. Rev. B 78, 155315 (2008)] published Mon Oct 20, 2008.

Topological order in a three-dimensional toric code at finite temperature

Claudio Castelnovo and Claudio Chamon
We study topological order in a toric code in three spatial dimensions or a 3+1D [openface Z] gauge theory at finite temperature. We compute exactly the topological entropy of the system and show that it drops, for any infinitesimal temperature, to half its value at zero temperature. The remaining h ... [Phys. Rev. B 78, 155120 (2008)] published Tue Oct 21, 2008.

Momentum-space analysis of multipartite entanglement at quantum phase transitions

Alberto Anfossi, Paolo Giorda, and Arianna Montorsi
We investigate entanglement properties at quantum phase transitions (QPTs) of a one-dimensional integrable extended Hubbard model in the momentum-space representation. Two elementary subsystems are recognized: the single mode of an electron and the pair of modes (electrons coupled through the eta-pa ... [Phys. Rev. B 78, 144519 (2008)] published Wed Oct 29, 2008.

Bell-state preparation for fullerene-based electron spins in distant nanotubes

W. L. Yang, H. Wei, X. L. Zhang, and M. Feng
We propose a potentially practical scheme, in combination with the Bell-state analyzer [X. L. Zhang et al., Phys. Rev. A 73, 014301 (2006)], to generate Bell states for two electron spins confined, respectively, to two distant C fullerenes. To this end, we consider the endohedral fullerenes staying ... [Phys. Rev. A 78, 054301 (2008)] published Thu Nov 13, 2008.

Collective generation of quantum states of light by entangled atoms

D. Porras and J. I. Cirac
We present a theoretical framework to describe the collective emission of light by entangled atomic states. Our theory applies to the low-excitation regime, where most of the atoms are initially in the ground state, and relies on a bosonic description of the atomic excitations. In this way, the prob ... [Phys. Rev. A 78, 053816 (2008)] published Wed Nov 12, 2008.

Second-order interference with orthogonally polarized pseudothermal beams

I. Vidal, D. P. Caetano, C. Olindo, E. J. S. Fonseca, and J. M. Hickmann
We experimentally investigate interference pattern formation in second-order intensity correlation considering the polarization state of pseudothermal light beams. By placing a double slit in one of the beams it is shown that the interference pattern depends on the polarization state of the light af ... [Phys. Rev. A 78, 053815 (2008)] published Wed Nov 12, 2008.

Entanglement of the orbital angular momentum states of the photon pairs generated in a hot atomic ensemble

Qun-Feng Chen, Bao-Sen Shi, Yong-Sheng Zhang, and Guang-Can Guo
In this paper, we experimentally demonstrate that the photon pairs generated via spontaneous four-wave mixing in a hot atomic ensemble are in entangled orbital angular momentum (OAM) states. The density matrix of the OAM states of the photon pair is reconstructed, from which the fidelity to the maxi ... [Phys. Rev. A 78, 053810 (2008)] published Thu Nov 6, 2008.

Exact solution of a Yang-Baxter spin-1/2 chain model and quantum entanglement

Ming-Guang Hu, Kang Xue, and Mo-Lin Ge
Entanglement is believed to be crucial in macroscopic physical systems for understanding the collective quantum phenomena such as quantum phase transitions. We start from and solve exactly a Yang-Baxter spin-1/2 chain model with inhomogeneous and anisotropic short-range interactions. For the ground ... [Phys. Rev. A 78, 052324 (2008)] published Thu Nov 13, 2008.

Generating entanglement between quantum dots with different resonant frequencies based on dipole-induced transparency

Deepak Sridharan and Edo Waks
We describe a method for generating entanglement between two spatially separated dipoles coupled to optical microcavities. The protocol works even when the dipoles have different resonant frequencies and radiative lifetimes. This method is particularly important for solid-state emitters, such as qua ... [Phys. Rev. A 78, 052321 (2008)] published Wed Nov 12, 2008.

Unifying several separability conditions using the covariance matrix criterion

O. Gittsovich, O. Guhne, P. Hyllus, and J. Eisert
We present a framework for deciding whether a quantum state is separable or entangled using covariance matrices of locally measurable observables. This leads to the covariance matrix criterion as a general separability criterion. We demonstrate that this criterion allows us to detect many states whe ... [Phys. Rev. A 78, 052319 (2008)] published Wed Nov 12, 2008.

Increasing entanglement of local qubits by applying electron current

Ye Xiong
We investigate the evolution of entanglement between local qubits in the presence of electron current which is flowing through and interacting with them. We find that if the electrons in the current are passing through one by one (sequentially), it is possible to generate or increase the entanglemen ... [Phys. Rev. A 78, 052318 (2008)] published Wed Nov 12, 2008.

Multipartite entanglement criterion from uncertainty relations

J. Gillet, T. Bastin, and G. S. Agarwal
We formulate an entanglement criterion using Peres-Horodecki positive partial transpose operations combined with the Schrodinger-Robertson uncertainty relation. We show that any pure entangled bipartite and tripartite state can be detected by experimentally measuring mean values and variances of spe ... [Phys. Rev. A 78, 052317 (2008)] published Tue Nov 11, 2008.

Entanglement swapping between spacelike-separated atoms

Juan Leon and Carlos Sabin
We show a mechanism that projects a pair of neutral two-level atoms from an initially uncorrelated state to a maximally entangled state while they remain spacelike separated. The atoms begin both excited in a common electromagnetic vacuum, and the radiation is collected with a partial Bell-state ana ... [Phys. Rev. A 78, 052314 (2008)] published Mon Nov 10, 2008.

Environment-assisted creation and enhancement of two-mode entanglement in a joint decoherent model

Shao-Hua Xiang, Bin Shao, and Ke-Hui Song
We investigate the entanglement dynamics of two modes in a common squeezed thermal environment by means of the characteristic function method and the logarithmic negativity. When the two modes are prepared initially in separable squeezed states, we find that the external environment can not only ind ... [Phys. Rev. A 78, 052313 (2008)] published Mon Nov 10, 2008.

Architectures for a quantum random access memory

Vittorio Giovannetti, Seth Lloyd, and Lorenzo Maccone
A random access memory, or RAM, is a device that, when interrogated, returns the content of a memory location in a memory array. A quantum RAM, or qRAM, allows one to access superpositions of memory sites, which may contain either quantum or classical information. RAMs and qRAMs with n-bit addresses ... [Phys. Rev. A 78, 052310 (2008)] published Wed Nov 5, 2008.

Comparison of the relative entropy of entanglement and negativity

Adam Miranowicz, Satoshi Ishizaka, Bohdan Horst, and Andrzej Grudka
It is well known that for two qubits the upper bounds of the relative entropy of entanglement (REE) for a given concurrence as well as the negativity for a given concurrence are reached by pure states. We show that, by contrast, there are two-qubit mixed states for which the REE for some range of a ... [Phys. Rev. A 78, 052308 (2008)] published Wed Nov 5, 2008.

Graphical description of local Gaussian operations for continuous-variable weighted graph states

Jing Zhang (张靖)
The form of a local Clifford [LC, also called local Gaussian (LG)] operation for the continuous-variable (CV) weighted graph states is presented in this paper, which is the counterpart of the LC operation of local complementation for qubit graph states. The property of the CV weighted graph states i ... [Phys. Rev. A 78, 052307 (2008)] published Tue Nov 4, 2008.

Scalable ellipsoidal classification for bipartite quantum states

David A. Herrera-Marti
The separability problem is approached from the perspective of ellipsoidal classification. A density operator of dimension N can be represented as a vector in a real vector space of dimension N1, whose components are the projections of the matrix onto some selected basis. We suggest a method to test ... [Phys. Rev. A 78, 052306 (2008)] published Tue Nov 4, 2008.

Entangling characterization of SWAP[sup 1/m] and controlled unitary gates

S. Balakrishnan and R. Sankaranarayanan
We study the entangling power and perfect entangler nature of SWAP for m>=1 and controlled-unitary (CU) gates. It is shown that SWAP is the only perfect entangler in the SWAP family. On the other hand, a subset of CU gates which is locally equivalent to controlled-NOT is identified. It is shown that ... [Phys. Rev. A 78, 052305 (2008)] published Tue Nov 4, 2008.

Teleportation using squeezed single photons

Agata M. Branczyk and T. C. Ralph
We present an analysis of squeezed single photon states as a resource for teleportation of coherent state qubits and propose proof-of-principle experiments for the demonstration of coherent state teleportation and entanglement swapping. We include an analysis of the squeezed vacuum as a simpler appr ... [Phys. Rev. A 78, 052304 (2008)] published Tue Nov 4, 2008.

Entanglement of solitons in the Frenkel-Kontorova model

S. Marcovitch and B. Reznik
We investigate entanglement of solitons in the continuum limit of the nonlinear Frenkel-Kontorova chain. We find that the entanglement of solitons manifests particlelike behavior as they are characterized by localization of entanglement. The von Neumann entropy of solitons mixes critical with noncri ... [Phys. Rev. A 78, 052303 (2008)] published Mon Nov 3, 2008.

Entanglement of two blocks of spins in the critical Ising model

P. Facchi, G. Florio, C. Invernizzi, and S. Pascazio
We compute the entropy of entanglement of two blocks of L spins at a distance d in the ground state of an Ising chain in an external transverse magnetic field. We numerically study the von Neumann entropy for different values of the transverse field. At the critical point we obtain analytical result ... [Phys. Rev. A 78, 052302 (2008)] published Mon Nov 3, 2008.

Spatial entanglement of paired photons generated in cold atomic ensembles

Clara I. Osorio, Sergio Barreiro, Morgan W. Mitchell, and Juan P. Torres
Cold atomic ensembles can mediate the generation of entanglement between pairs of photons. Photons with specific directions of propagation are detected, and the entanglement can reside in any of the degrees of freedom that describe the whole quantum state of the photons: polarization, spatial shape, ... [Phys. Rev. A 78, 052301 (2008)] published Mon Nov 3, 2008.

Completely positive Markovian quantum dynamics in the weak-coupling limit

David Taj and Fausto Rossi
We obtain exponential decay laws for solutions of density-matrix master equations in the weak-coupling limit: after comparing with results already present in the literature and developing the necessary techniques, we study the crucial aspect of complete positivity under fairly general conditions. We ... [Phys. Rev. A 78, 052113 (2008)] published Thu Nov 13, 2008.

Effect of feedback on the control of a two-level dissipative quantum system

L. C. Wang, X. L. Huang, and X. X. Yi
We show that it is possible to modify the stationary state by a feedback control in a two-level dissipative quantum system. Based on the geometric control theory, we also analyze the effect of the feedback on the time-optimal control in the dissipative system governed by the Lindblad master equation ... [Phys. Rev. A 78, 052112 (2008)] published Thu Nov 13, 2008.

Simulation of partial entanglement with nonsignaling resources

Nicolas Brunner, Nicolas Gisin, Sandu Popescu, and Valerio Scarani
With the goal of gaining a deeper understanding of quantum nonlocality, we decompose quantum correlations into more elementary nonlocal correlations. We show that the correlations of all pure entangled states of two qubits can be simulated without communication, hence using only no-signaling resourc ... [Phys. Rev. A 78, 052111 (2008)] published Thu Nov 13, 2008.

Possible entanglement detection with the naked eye

Nicolas Brunner, Cyril Branciard, and Nicolas Gisin
The human eye can detect optical signals containing only a few photons. We investigate the possibility to demonstrate entanglement with such biological detectors. While one person could not detect entanglement by simply observing photons, we discuss the possibility for several observers to demonstra ... [Phys. Rev. A 78, 052110 (2008)] published Wed Nov 12, 2008.

Enhancement of motional entanglement of cold atoms by pairwise scattering of photons

Rui Guo, Xiaoji Zhou, and Xuzong Chen
The mechanism of pairwise scattering of photons with cold atoms is investigated. We show that considerable motional correlation can be established between atoms during the scattering process, which can effectively squeeze the scattering spectrum and can be utilized to produce highly entangled atomic ... [Phys. Rev. A 78, 052107 (2008)] published Fri Nov 7, 2008.

Exchange symmetry and multipartite entanglement

Tsubasa Ichikawa, Toshihiko Sasaki, Izumi Tsutsui, and Nobuhiro Yonezawa
Entanglement of multipartite systems is studied based on exchange symmetry under the permutation group S. With the observation that symmetric property under the exchange of two constituent states and their separability are intimately linked, we show that antisymmetric (fermionic) states are necessar ... [Phys. Rev. A 78, 052105 (2008)] published Thu Nov 6, 2008.

Quantum brachistochrone evolution of systems of two identical particles: The role of entanglement

A. Borras, A. R. Plastino, M. Casas, and A. Plastino
Entanglement plays a fundamental role in the brachistochrone evolution of composite quantum systems. In the case of composite systems with distinguishable subsystems quantum brachistochrone evolutions cannot be implemented without entanglement, excepting trivial cases in which only one of the subsys ... [Phys. Rev. A 78, 052104 (2008)] published Thu Nov 6, 2008.

Multisetting Bell inequality for qudits

Se-Wan Ji, Jinhyoung Lee, James Lim, Koji Nagata, and Hai-Woong Lee
We propose a generalized Bell inequality for two three-dimensional systems with three settings in each local measurement. It is shown that this inequality is maximally violated if local measurements are configured to be mutually unbiased and a composite state is maximally entangled. This feature is ... [Phys. Rev. A 78, 052103 (2008)] published Thu Nov 6, 2008.

Collective processes of an ensemble of spin-1/2 particles

Bradley A. Chase and J. M. Geremia
When the dynamics of a spin ensemble are expressible solely in terms of symmetric processes and collective spin operators, the symmetric collective states of the ensemble are preserved. These many-body states, which are invariant under particle relabeling, can be efficiently simulated since they spa ... [Phys. Rev. A 78, 052101 (2008)] published Tue Nov 4, 2008.

Numerically exact solution to the finite-size Dicke model

Qing-Hu Chen, Yu-Yu Zhang, Tao Liu, and Ke-Lin Wang
By using extended bosonic coherent states, a technique to solve the Dicke model exactly is proposed in the numerical sense. The accessible system size is two orders of magnitude higher than that reported in the literature. Finite-size scaling for several observables, such as the ground-state energy, ... [Phys. Rev. A 78, 051801 (2008)] published Mon Nov 3, 2008.

Macroscopic superpositions of phase states with Bose-Einstein condensates

F. Piazza, L. Pezze, and A. Smerzi
Quantum superpositions of macroscopically distinguishable states having distinct phases can be created with a Bose-Einstein condensate trapped in a periodic potential. The experimental signature is contained in the phase distribution of the interference patterns obtained after releasing the traps. M ... [Phys. Rev. A 78, 051601 (2008)] published Wed Nov 5, 2008.

Purification of single-photon entanglement with linear optics

Nicolas Sangouard, Christoph Simon, Thomas Coudreau, and Nicolas Gisin
We show that single-photon entangled states of the form |0>|1>+|1>|0> can be purified with a simple linear-optics-based protocol, which is eminently feasible with current technology. Besides its conceptual interest, this result is relevant for attractive quantum repeater protocols. ... [Phys. Rev. A 78, 050301 (2008)] published Thu Nov 13, 2008.

Protecting, enhancing, and reviving entanglement

J. G. Oliveira, Jr., R. Rossi, Jr., and M. C. Nemes
We propose a strategies not only to protect but also to enhance and revive the entanglement in a double Jaynes-Cummings model. We show that such surprising features arise when Zeno-like measurements are performed during the dynamical process. ... [Phys. Rev. A 78, 044301 (2008)] published Fri Oct 17, 2008.

Ultrabroadband photon pair preparation by spontaneous four-wave mixing in a dispersion-engineered optical fiber

Karina Garay-Palmett, Alfred B. U'Ren, Raul Rangel-Rojo, Rodger Evans, and Santiago Camacho-Lopez
We present a study of the spectral properties of photon pairs generated through the process of spontaneous four-wave mixing (SFWM) in single mode fiber. Our analysis assumes narrowband pumps, which are allowed to be frequency degenerate or nondegenerate. Based on this analysis, we derive conditions ... [Phys. Rev. A 78, 043827 (2008)] published Thu Oct 30, 2008.