Virtual Journal of Quantum Information

Photonic analogue of Josephson effect in a dual-species optical-lattice cavity

Soi-Chan Lei, Tai-Kai Ng, and Ray-Kuang Lee
We extend the idea of quantum phase transitions of light in the photonic Bose-Hubbard model with interactions to two atomic species by a self-consistent mean field theory. The excitation of two-level atoms interacting with a coherent photon field is analyzed with a finite temperature dependence of t ... [Opt. Express 18, 14586 (2010)] published Thu Aug 5, 2010.

Switching of a single propagating plasmon by two quantum dots system

Nam-Chol Kim, Jian-Bo Li, Zhong-Jian Yang, Zhong-Hua Hao, and Qu-Quan Wang
We investigated the scattering property of a single plasmon interacting with two quantum dots coupled to one-dimensional surface plasmonic waveguide. We demonstrated that the transmission of a single plasmon could be switched on or off by adjusting spectral detuning and controlling spatial separatio ... [Appl. Phys. Lett. 97, 061110 (2010)] published Thu Aug 12, 2010.

Dynamically reconfigurable directionality of plasmon-based single photon sources

Yuntian Chen, Peter Lodahl, and A. Femius Koenderink
We propose a plasmon-based reconfigurable antenna to controllably distribute emission from single quantum emitters in spatially separated channels. Our calculations show that crossed particle arrays can split the stream of photons from a single emitter into multiple narrow beams. We predict that bea ... [Phys. Rev. B 82, 081402 ] published .

Electron Spin Coherence and Electron Nuclear Double Resonance of Bi Donors in Natural Si

Richard E. George, Wayne Witzel, H. Riemann, N. V. Abrosimov, N. Notzel et al.
Donors in silicon hold considerable promise for emerging quantum technologies, due to their uniquely long electron spin coherence times. Bismuth donors in silicon differ from more widely studied group V donors, such as phosphorous, in several significant respects: They have the strongest binding ene ... [Phys. Rev. Lett. 105, 067601 ] published .

Efficient photon number detection with silicon avalanche photodiodes

O. Thomas, Z. L. Yuan, J. F. Dynes, A. W. Sharpe, and A. J. Shields
We demonstrate an efficient photon number detector for visible wavelengths using a silicon avalanche photodiode. Under subnanosecond gating, the device is able to resolve up to four photons in an incident optical pulse. The detection efficiency at 600 nm is measured to be 73.8%, corresponding to an ... [Appl. Phys. Lett. 97, 031102 (2010)] published Mon Jul 19, 2010.

Differential reflection spectroscopy of a single quantum dot strongly coupled to a photonic crystal cavity

Erik D. Kim, Arka Majumdar, Hyochul Kim, Pierre Petroff, and Jelena VuCkovic
We demonstrate the use of periodically modulated Coulomb shifts in quantum dot (QD) transition energies to obtain differential reflection spectra of a photonic crystal nanocavity containing strongly coupled dots. Measured spectra isolate the change in the empty cavity optical reflectivity spectrum d ... [Appl. Phys. Lett. 97, 053111 (2010)] published Wed Aug 4, 2010.

Fabrication of concave silicon micro-mirrors

Yueh Sheng Ow, Mark B. H. Breese, and Sara Azimi
We have fabricated spherical and cylindrical concave micro-mirrors in silicon with dimensions from 20 [mu]m to 100 [mu]m. The fabrication process involves standard photolithography followed by large area ion beam irradiation and electrochemical anodisation in a HF electrolyte. After thermal oxidatio ... [Opt. Express 18, 14511 (2010)] published Thu Aug 5, 2010.

Measurement of the effect of quantum phase slips in a Josephson junction chain

I. M. Pop, I. Protopopov, F. Lecocq, Z. Peng, B. Pannetier et al.
The interplay between superconductivity and Coulomb interactions has been studied for more than 20 years now. In low-dimensional systems, superconductivity degrades in the presence of Coulomb repulsion: interactions tend to suppress fluctuations of charge, thereby increasing fluctuations of phase. ... [Nature Phys. 6, 589 (2010)] published Wed Aug 4, 2010.

Effect of Mechanical Strain on the Optical Properties of Quantum Dots: Controlling Exciton Shape, Orientation, and Phase with a Mechanical Strain

Garnett W. Bryant, M. Zielinski, Natalia Malkova, James Sims, W. Jaskolski et al.
We show how a nanomechanical strain can be used to dynamically reengineer the optics of quantum dots, giving a tool to manipulate mechanoexciton shape, orientation, fine structure splitting, and optical transitions, transfer carriers between dots, and interact qubits for quantum processing. Most imp ... [Phys. Rev. Lett. 105, 067404 ] published .

Improving Quantum State Estimation with Mutually Unbiased Bases

R. B. A. Adamson and A. M. Steinberg
When used in quantum state estimation, projections onto mutually unbiased bases have the ability to maximize information extraction per measurement and to minimize redundancy. We present the first experimental demonstration of quantum state tomography of two-qubit polarization states to take advanta ... [Phys. Rev. Lett. 105, 030406 ] published .

Efficient multimode quantum memory based on photon echo in an optimal QED cavity

Sergey A. Moiseev, Sergey N. Andrianov, and Firdus F. Gubaidullin
Effective multimode photon echo quantum memory on multiatomic ensemble in the QED cavity is proposed. We obtain the analytical solution for the quantum memory efficiency that can be equal to unity when optimal conditions for the cavity and atomic parameters are held. Detailed analysis of the optimal ... [Phys. Rev. A 82, 022311 ] published .

High efficient loading of two atoms into a microscopic optical trap by dynamically reshaping the trap with a spatial light modulator

Xiaodong He, Peng Xu, Jin Wang, and Mingsheng Zhan
We demonstrated trapping two neutral Rb atoms in a two site optical ring lattice generated by reflecting a single laser beam from a computer controlled spatial light modulator directly. The ring lattice was transformed into a Gaussian trap by dynamically displaying the holograms animation movi ... [Opt. Express 18, 13586 (2010)] published Thu Aug 5, 2010.

Coherent control of a single exciton qubit by optoelectronic manipulation

S. Michaelis de Vasconcellos, S. Gordon, M. Bichler, T. Meier, and A. Zrenner
The coherent state manipulation of single quantum systems is a fundamental requirement for the implementation of quantum information processors. Exciton qubits are of particular interest for coherent optoelectronic applications, in particular due to their excellent coupling to photons. Until now, co ... [Nat. Photonics 4, 545 (2010)] published Wed Aug 4, 2010.

Bootstrap Tomography of the Pulses for Quantum Control

V. V. Dobrovitski, G. de Lange, D. Riste, and R. Hanson
Long-time dynamical decoupling and quantum control of qubits require high-precision control pulses. Full characterization (quantum tomography) of imperfect pulses presents a bootstrap problem: tomography requires initial states of a qubit which cannot be prepared without perfect pulses. We present a ... [Phys. Rev. Lett. 105, 077601 ] published .

Three-dimensional transport diagram of a triple quantum dot

G. Granger, L. Gaudreau, A. Kam, M. Pioro-Ladriere, S. A. Studenikin et al.
We measure a triple quantum dot in the regime where three addition lines, corresponding to the addition of an electron to each of three dots, pass through each other. In particular, we probe the interplay between transport and the three-dimensional nature of the stability diagram. We choose the regi ... [Phys. Rev. B 82, 075304 ] published .

Quantum theory of surface-plasmon polariton scattering

D. Ballester, M. S. Tame, and M. S. Kim
We introduce the quantum mechanical formalism for treating surface plasmon polariton scattering at an interface. Our developed theorywhich differs fundamentally from the analogous photonic scenariois used to investigate the possibility of plasmonic beam splitters at the quantum level. Remarkably, we ... [Phys. Rev. A 82, 012325 ] published .

Spin-orbit coupling and anisotropic exchange in two-electron double quantum dots

Fabio Baruffa, Peter Stano, and Jaroslav Fabian
The influence of the spin-orbit interactions on the energy spectrum of two-electron laterally coupled quantum dots is investigated. The effective Hamiltonian for a spin qubit pair proposed in Baruffa et al. [Phys. Rev. Lett. 104, 126401 (2010)] is confronted with exact numerical results in single an ... [Phys. Rev. B 82, 045311 ] published .

Fluorescence interferometry

G. S. Paraoanu
We describe an interferometer based on fluorescent emission of radiation of two qubits in quasi-one-dimensional modes. Such a system can readily be realized with dipole emitters near conducting surface-plasmonic nanowires or with superconducting qubits coupled to coplanar waveguide transmission line ... [Phys. Rev. A 82, 023802 ] published .

Transition from weak to strong measurements by nonlinear quantum feedback control

Jing Zhang, Yu-xi Liu, Re-Bing Wu, Chun-Wen Li, and Tzyh-Jong Tarn
We find that feedback control may induce pseudo-nonlinear dynamics in a damped harmonic oscillator, whose centroid trajectory in the phase space behaves like a classical nonlinear system. Thus, similar to nonlinear amplifiers (e.g., rf-driven Josephson junctions), feedback control on the harmonic os ... [Phys. Rev. A 82, 022101 ] published .

Implementation of superconductor/ferromagnet/ superconductor -shifters in superconducting digital and quantum circuits

A. K. Feofanov, V. A. Oboznov, V. V. Bolginov, J. Lisenfeld, S. Poletto et al.
High operation speed and low energy consumption may allow the superconducting digital single-flux-quantum circuits to outperform traditional complementary metaloxidesemiconductor logic. The remaining major obstacle towards high element densities on-chip is a relatively large cell size necessary to ... [Nature Phys. 6, 593 (2010)] published Wed Aug 4, 2010.

A quantum spin transducer based on nanoelectromechanical resonator arrays

P. Rabl, S. J. Kolkowitz, F. H. L. Koppens, J. G. E. Harris, P. Zoller et al.
Isolated electronic and nuclear spins in solids are at present being actively explored for potential quantum-computing applications. Spin degrees of freedom provide an excellent quantum memory, owing to their weak magnetic interactions with the environment. For the same reason, however, it is diffic ... [Nature Phys. 6, 602 (2010)] published Wed Aug 4, 2010.

Ultracold atoms in an optical lattice with dynamically variable periodicity

S. Al-Assam, R. A. Williams, and C. J. Foot
The use of a dynamic accordion lattice with ultracold atoms is demonstrated. Ultracold atoms of Rb are trapped in a two-dimensional optical lattice, and the spacing of the lattice is then increased in both directions from 2.2 to 5.5 [mu]m. Atoms remain bound for expansion times as short as a few mil ... [Phys. Rev. A 82, 021604 ] published .

Sensitivity of the population and the Pancharatnam phase for a trapped ion with Stark shift

M. A. Bouchene, M. Abdel-Aty, and S. Mandal
We demonstrate the strong dependence of the Pancharatnam phase and the excited-state population of a single trapped ion on the laser intensity and phase shift. The simulation demonstrates that there is an enhancement of the Pancharatnam phase sensitivity to the phase shift compared with imposition o ... [Phys. Rev. A 82, 023409 ] published .

Doubly Magic Conditions in Magic-Wavelength Trapping of Ultracold Alkali-Metal Atoms

Andrei Derevianko
In experiments with trapped atoms, atomic energy levels are shifted by the trapping optical and magnetic fields. Regardless of this strong perturbation, precision spectroscopy may be still carried out using specially crafted, magic trapping fields. Finding these conditions for particularly valuable ... [Phys. Rev. Lett. 105, 033002 ] published .

Majorana Bound States without Vortices in Topological Superconductors with Electrostatic Defects

M. Wimmer, A. R. Akhmerov, M. V. Medvedyeva, J. Tworzydlo, and C. W. J. Beenakker
Vortices in two-dimensional superconductors with broken time-reversal and spin-rotation symmetry can bind states at zero excitation energy. These so-called Majorana bound states transform a thermal insulator into a thermal metal and may be used to encode topologically protected qubits. We identify a ... [Phys. Rev. Lett. 105, 046803 ] published .

Process Tomography of Dynamical Decoupling in a Dense Cold Atomic Ensemble

Yoav Sagi, Ido Almog, and Nir Davidson
Atomic ensembles have many potential applications in quantum information science. Owing to collective enhancement, working with ensembles at high densities increases the efficiency of quantum operations, but at the same time also increases the collision rate and leads to decoherence. Here we report ... [Phys. Rev. Lett. 105, 053201 ] published .

Storage and Recall of Weak Coherent Optical Pulses with an Efficiency of 25%

M. Sabooni, F. Beaudoin, A. Walther, N. Lin, A. Amari et al.
We demonstrate experimentally an efficient coherent rephasing scheme for the storage and recall of weak coherent light pulses in an inhomogeneously broadened optical transition in a Pr:YSO crystal at 2.1 K. Precise optical pumping using a frequency stable ([approximate]1 kHz linewidth) laser is emp ... [Phys. Rev. Lett. 105, 060501 ] published .

Heisenberg-limited interferometry with pair coherent states and parity measurements

Christopher C. Gerry and Jihane Mimih
After reviewing parity-measurement-based interferometry with twin Fock states, which allows for supersensitivity (Heisenberg limited) and super-resolution, we consider interferometry with two different superpositions of twin Fock states, namely, two-mode squeezed vacuum states and pair coherent stat ... [Phys. Rev. A 82, 013831 ] published .

Distance growth of quantum states due to initial system-environment correlations

J. Dajka and J. Luczka
Intriguing features of the distance between two arbitrary states of an open quantum system are identified that are induced by initial system-environment correlations. As an example, we analyze a qubit dephasingly coupled to a bosonic environment. Within tailored parameter regimes, initial correlatio ... [Phys. Rev. A 82, 012341 ] published .

Heralded generation of entangled photon pairs

Stefanie Barz, Gunther Cronenberg, Anton Zeilinger, and Philip Walther
Entangled photons are a crucial resource for quantum communication and linear optical quantum computation. Unfortunately, the applicability of many photon-based schemes is limited due to the stochastic character of the photon sources. Therefore, a worldwide effort has focused on overcoming the limit ... [Nat. Photonics 4, 553 (2010)] published Wed Aug 4, 2010.

Quantum Hall FabryPerot interferometer: Logic gate responses

S. Bellucci and P. Onorato
We discuss the electron transport through a quantum Hall FabryPerot interferometer (QHFPI) obtained with two quantum point contacts (QPCs) in series along a ballistic quantum wire by focusing on the effects due to quantum interference and to quantum Hall effect. We calculate the conductance-energy a ... [J. Appl. Phys. 108, 033710 (2010)] published Fri Aug 6, 2010.

Bismuth Qubits in Silicon: The Role of EPR Cancellation Resonances

M. H. Mohammady, G. W. Morley, and T. S. Monteiro
We investigate electron paramagnetic resonance spectra of bismuth-doped silicon, at intermediate magnetic fields B~=0.10.6 T, theoretically and experimentally (with 9.7 GHz X-band spectra). We identify a previously unexplored regime of cancellation resonances, where a component of the hyperfine cou ... [Phys. Rev. Lett. 105, 067602 ] published .

Performance engineering of semiconductor spin qubit systems

Ramin M. Abolfath and Thomas Brabec
The performance of a quantum computation system is investigated with qubits represented by magnetic impurities in coupled quantum dots filled with two electrons. Magnetic impurities are electrically manipulated by electrons. The dominant noise source is the electron-mediated indirect coupling betwee ... [Phys. Rev. B 82, 075311 ] published .

Spin electric effects in molecular antiferromagnets

Mircea Trif, Filippo Troiani, Dimitrije Stepanenko, and Daniel Loss
Molecular nanomagnets show clear signatures of coherent behavior and have a wide variety of effective low-energy spin Hamiltonians suitable for encoding qubits and implementing spin-based quantum information processing. At the nanoscale, the preferred mechanism for the control of a quantum systems i ... [Phys. Rev. B 82, 045429 ] published .

Quantum Nondemolition Measurement of a Superconducting Qubit in the Weakly Projective Regime

T. Picot, R. Schouten, C. J. P. M. Harmans, and J. E. Mooij
Quantum state detectors based on switching of hysteretic Josephson junctions biased close to their critical current are simple to use but have strong backaction. We show that the backaction of a dc-switching detector can be considerably reduced by limiting the switching voltage and using a fast cryo ... [Phys. Rev. Lett. 105, 040506 ] published .

Strong Coupling of a Quantum Oscillator to a Flux Qubit at Its Symmetry Point

A. Fedorov, A. K. Feofanov, P. Macha, P. Forn-Diaz, C. J. P. M. Harmans et al.
A flux qubit biased at its symmetry point shows a minimum in the energy splitting (the gap), providing protection against flux noise. We have fabricated a qubit of which the gap can be tuned fast and have coupled this qubit strongly to an LC oscillator. We show full spectroscopy of the qubit-oscilla ... [Phys. Rev. Lett. 105, 060503 ] published .

Mapping spin-orbit splitting in strained (In,Ga)As epilayers

B. M. Norman, C. J. Trowbridge, J. Stephens, A. C. Gossard, D. D. Awschalom et al.
Time-resolved and spatially resolved Faraday rotation spectroscopy is used to measure the magnitude and direction of the momentum-dependent spin splitting in strained InGaAs epilayers. The epilayers are coherently strained and therefore designed to reduce inhomogeneous effects related to strain rela ... [Phys. Rev. B 82, 081304 ] published .

Electron spin coherence in metallofullerenes: Y, Sc, and La@C

Richard M. Brown, Yasuhiro Ito, Jamie H. Warner, Arzhang Ardavan, Hisanori Shinohara et al.
Endohedral fullerenes encapsulating a spin-active atom or ion within a carbon cage offer a route to self-assembled arrays such as spin chains. In the case of metallofullerenes the charge transfer between the atom and the fullerene cage has been thought to limit the electron spin phase coherence time ... [Phys. Rev. B 82, 033410 ] published .

Impedance-matched cavity quantum memory

Mikael Afzelius and Christoph Simon
We consider an atomic frequency comb based quantum memory inside an asymmetric optical cavity. In this configuration it is possible to absorb the input light completely in a system with an effective optical depth of one, provided that the absorption per cavity round trip exactly matches the transmis ... [Phys. Rev. A 82, 022310 ] published .

Photonic Phase Gate via an Exchange of Fermionic Spin Waves in a Spin Chain

Alexey V. Gorshkov, Johannes Otterbach, Eugene Demler, Michael Fleischhauer, and Mikhail D. Lukin
We propose a new protocol for implementing the two-qubit photonic phase gate. In our approach, the pi phase is acquired by mapping two single photons into atomic excitations with fermionic character and exchanging their positions. The fermionic excitations are realized as spin waves in a spin chain, ... [Phys. Rev. Lett. 105, 060502 ] published .

Optical Continuous-Variable Qubit

Jonas S. Neergaard-Nielsen, Makoto Takeuchi, Kentaro Wakui, Hiroki Takahashi, Kazuhiro Hayasaka et al.
In a new branch of quantum computing, information is encoded into coherent states, the primary carriers of optical communication. To exploit it, quantum bits of these coherent states are needed, but it is notoriously hard to make superpositions of such continuous-variable states. We have realized th ... [Phys. Rev. Lett. 105, 053602 ] published .

Proposal for Manipulating and Detecting Spin and Orbital States of Trapped Electrons on Helium Using Cavity Quantum Electrodynamics

D. I. Schuster, A. Fragner, M. I. Dykman, S. A. Lyon, and R. J. Schoelkopf
We propose a hybrid architecture in which an on-chip high finesse superconducting cavity is coupled to the lateral motion and spin state of a single electron trapped on the surface of superfluid helium. We estimate the motional coherence times to exceed 15 [mu]s, while energy will be coherently exc ... [Phys. Rev. Lett. 105, 040503 ] published .

Coherent spin manipulation in an exchange-only qubit

E. A. Laird, J. M. Taylor, D. P. DiVincenzo, C. M. Marcus, M. P. Hanson et al.
Initialization, two-spin coherent manipulation, and readout of a three-spin qubit are demonstrated using a few-electron triple quantum dot. The three-spin qubit is designed to allow all operations for full qubit control to be tuned via nearest-neighbor exchange interaction. Fast readout of charge st ... [Phys. Rev. B 82, 075403 ] published .

Landau-Zener-Stuckelberg interferometry in the presence of quantum noise

Lingjie Du, Minjie Wang, and Yang Yu
We proposed an unconventional Landau-Zener-Stuckelberg interferometry in strongly driven two-level systems subjected to quantum noise. A general transition rate for consecutive Landau-Zener transitions was obtained, from which we could analytically calculate interference fringes. The strong low-freq ... [Phys. Rev. B 82, 045128 ] published .

Resonant quantum gates in circuit quantum electrodynamics

G. Haack, F. Helmer, M. Mariantoni, F. Marquardt, and E. Solano
We propose the implementation of fast resonant gates in circuit quantum electrodynamics for quantum information processing. We show how a suitable utilization of three-level superconducting qubits inside a resonator constitutes a key tool to perform diverse two-qubit resonant gates, improving the op ... [Phys. Rev. B 82, 024514 ] published .

Quantum logical gates with four-level superconducting quantum interference devices coupled to a superconducting resonator

Xiao-Ling He, Chui-Ping Yang, Sheng Li, Jun-Yan Luo, and Siyuan Han
We propose a way for realizing a two-qubit controlled phase gate with superconducting quantum interference devices (SQUIDs) coupled to a superconducting resonator. In this proposal, the two lowest levels of each SQUID serve as the logical states and two intermediate levels of each SQUID are used for ... [Phys. Rev. A 82, 024301 ] published .

Proposal for high-speed and high-fidelity electron-spin initialization in a negatively charged quantum dot coupled to a microcavity in a weak external magnetic field

Arka Majumdar, Ziliang Lin, Andrei Faraon, and Jelena VuCkovic
We describe a proposal for fast electron-spin initialization in a negatively charged quantum dot coupled to a microcavity without the need for a strong magnetic field. We employ two-photon excitation to access trion states that are spin forbidden by one-photon excitation. Our simulation shows a maxi ... [Phys. Rev. A 82, 022301 ] published .

Experimental evidence of quantum randomness incomputability

Cristian S. Calude, Michael J. Dinneen, Monica Dumitrescu, and Karl Svozil
In contrast with software-generated randomness (called pseudo-randomness), quantum randomness can be proven incomputable; that is, it is not exactly reproducible by any algorithm. We provide experimental evidence of incomputabilityan asymptotic propertyof quantum randomness by performing finite test ... [Phys. Rev. A 82, 022102 ] published .

Fast quantum phase gate in a small-detuning circuit QED model

Chun-Wang Wu, Yang Han, Hong-Yi Li, Zhi-Jiao Deng, Ping-Xing Chen et al.
We propose a theoretical protocol for a quantum phase gate between two transmon qubits capacitively coupled to a superconducting transmission line resonator driven by a strong microwave field. Compared with previous protocols involving a dispersive regime, our protocol works at a small detuning, Del ... [Phys. Rev. A 82, 014303 ] published .

Asymmetrical two-dimensional magnetic lattices for ultracold atoms

A. Abdelrahman, M. Vasiliev, K. Alameh, and P. Hannaford
A simple method for implementing an asymmetrical two-dimensional magnetic lattice is proposed. The asymmetrical two-dimensional magnetic lattice is created by periodically distributing nonzero magnetic minima across the surface of a magnetic thin film, where the magnetic patterns are formed by milli ... [Phys. Rev. A 82, 012320 ] published .

Fiber-microsphere system at cryogenic temperatures toward cavity QED using diamond NV centers

Hideaki Takashima, Takeshi Asai, Kiyota Toubaru, Masazumi Fujiwara, Keiji Sasaki et al.
The coupling of a microsphere resonator to a tapered fiber was demonstrated at cryogenic temperatures (8 - 13 K) and investigated with a probe laser light whose frequency around the zero phonon line of nitrogen vacancy centers in diamond (638 nm). For this purpose, a liquid-helium-flow cryostat with ... [Opt. Express 18, 15169 (2010)] published Thu Aug 5, 2010.

Entanglement generation of nitrogen-vacancy centers via coupling to nanometer-sized resonators and a superconducting interference device

Qiong Chen, Zhenyu Xu, and Mang Feng
We present a potential scheme to entangle negatively charged nitrogen-vacancy (N-V) centers in distance using nanomechanical resonators (NAMRs) and a common superconducting quantum interference device (SQUID). By virtually exciting the vibrational modes of the NAMRs, we show the effective coupling b ... [Phys. Rev. A 82, 014302 ] published .

Single-photon frequency down-conversion experiment

Hiroki Takesue
We report a single-photon frequency down-conversion experiment. Using the difference frequency generation process in a periodically poled lithium niobate waveguide, we successfully observed the phase-preserved frequency down-conversion of a coherent pulse train with an average photon number per puls ... [Phys. Rev. A 82, 013833 ] published .

Experimental demonstration of a heralded entanglement source

Claudia Wagenknecht, Che-Ming Li, Andreas Reingruber, Xiao-Hui Bao, Alexander Goebel et al.
The heralded generation of entangled states is a long-standing goal in quantum information processing, because it is indispensable for a number of quantum protocols. Polarization entangled photon pairs are usually generated through spontaneous parametric down-conversion, but the emission is probabil ... [Nat. Photonics 4, 549 (2010)] published Wed Aug 4, 2010.

Large effects of boundaries on spin amplification in spin chains

Benoit Roubert, Petr Braun, and Daniel Braun
We investigate the effect of boundary conditions on spin amplification in spin chains. We show that the boundaries play a crucial role for the dynamics: A single additional coupling between the first and last spins can macroscopically modify the physical behavior compared to the open chain, even in ... [Phys. Rev. A 82, 022302 ] published .

An accurate high-speed single-electron quantum dot pump

S Giblin, S Wright, J Fletcher, M Kataoka, M Pepper et al.
Using standard microfabrication techniques, it is now possible to construct devices that appear to reliably manipulate electrons one at a time. These devices have potential use as building blocks in quantum computing devices, or as a standard of electrical current derived only from a frequency and t ... [New J. Phys. 12, 073013 (2010)] published Wed Aug 4, 2010.

Single-Shot Readout of a Single Nuclear Spin

Philipp Neumann, Johannes Beck, Matthias Steiner, Florian Rempp, Helmut Fedder et al.
Projective measurement of single electron and nuclear spins has evolved from a gedanken experiment to a problem relevant for applications in atomic-scale technologies like quantum computing. Although several approaches allow for detection of a spin of single atoms and molecules, multiple repetitions ... [Science 329, 542 ] published .

Fisher information in a quantum-critical environment

Zhe Sun, Jian Ma, Xiao-Ming Lu, and Xiaoguang Wang
We consider a process of parameter estimation in a spin-j system surrounded by a quantum-critical spin chain. Quantum Fisher information lies at the heart of the estimation task. We employ Ising spin chain in a transverse field as the environment which exhibits a quantum phase transition. Fisher inf ... [Phys. Rev. A 82, 022306 ] published .

Adiabatic Markovian Dynamics

Ognyan Oreshkov and John Calsamiglia
We propose a theory of adiabaticity in quantum Markovian dynamics based on a decomposition of the Hilbert space induced by the asymptotic behavior of the Lindblad semigroup. A central idea of our approach is that the natural generalization of the concept of eigenspace of the Hamiltonian in the case ... [Phys. Rev. Lett. 105, 050503 ] published .

Efficient Simulation of Strong System-Environment Interactions

Javier Prior, Alex W. Chin, Susana F. Huelga, and Martin B. Plenio
Multicomponent quantum systems in strong interaction with their environment are receiving increasing attention due to their importance in a variety of contexts, ranging from solid state quantum information processing to the quantum dynamics of biomolecular aggregates. Unfortunately, these systems ar ... [Phys. Rev. Lett. 105, 050404 ] published .

Hadamard matrices from mutually unbiased bases

P. Dita
An analytical method for getting new complex Hadamard matrices by using mutually unbiased bases and a nonlinear doubling formula is provided. The method is illustrated with the n=4 case that leads to a rich family of eight-dimensional Hadamard matrices that depend on five arbitrary parameters whose ... [J. Math. Phys. 51, 072202 (2010)] published Mon Jul 19, 2010.

Universal Bounds for the Holevo Quantity, Coherent Information, and the Jensen-Shannon Divergence

Wojciech Roga, Mark Fannes, and Karol Zyczkowski
The mutual information between the sender of a classical message encoded in quantum carriers and a receiver is fundamentally limited by the Holevo quantity. Using strong subadditivity of entropy, we prove that the Holevo quantity is not larger than an exchange entropy. This implies an upper bound fo ... [Phys. Rev. Lett. 105, 040505 ] published .

Emergence of equilibrium thermodynamic properties in quantum pure states. I. Theory

Barbara Fresch and Giorgio J. Moro
Investigation on foundational aspects of quantum statistical mechanics recently entered a renaissance period due to novel intuitions from quantum information theory and to increasing attention on the dynamical aspects of single quantum systems. In the present contribution a simple but effective theo ... [J. Chem. Phys. 133, 034509 (2010)] published Tue Jul 20, 2010.

Ruling Out Multi-Order Interference in Quantum Mechanics

Urbasi Sinha, Christophe Couteau, Thomas Jennewein, Raymond Laflamme, and Gregor Weihs
Quantum mechanics and gravitation are two pillars of modern physics. Despite their success in describing the physical world around us, they seem to be incompatible theories. There are suggestions that one of these theories must be generalized to achieve unification. For example, Born's ruleone of th ... [Science 329, 418 (2010)] published Mon Jul 26, 2010.

Elementary gates for quantum information with superposed coherent states

Petr Marek and Jaromir Fiurasek
We propose an alternative way of implementing several elementary quantum gates for qubits in the coherent-state basis. The operations are probabilistic and employ single-photon subtractions as the driving force. Our schemes for single-qubit phase gate and two-qubit controlled phase gate are capable ... [Phys. Rev. A 82, 014304 ] published .

Exact solution for quantum dynamics of a periodically driven two-level system

Anirban Gangopadhyay, Maxim Dzero, and Victor Galitski
We present a family of exact analytic solutions for nonlinear quantum dynamics of a two-level system (TLS) subject to a periodic-in-time external field. In constructing the exactly solvable models, we use a reverse engineering approach where the form of external perturbation is chosen to preserve an ... [Phys. Rev. B 82, 024303 ] published .

Rapid purification of quantum systems by measuring in a feedback-controlled unbiased basis

Joshua Combes, Howard M. Wiseman, Kurt Jacobs, and Anthony J. O'Connor
Rapid purification by feedbackspecifically, reducing the mean impurity faster than by measurement alonecan be achieved by choosing the eigenbasis of the density matrix to be unbiased relative to the measurement basis. Here we further examine the protocol introduced by Combes and Jacobs [Phys. Rev. L ... [Phys. Rev. A 82, 022307 ] published .

Quantum limits of thermometry

Thomas M. Stace
The precision of typical thermometers consisting of N particles scales as ~1/sqrt(N). For high-precision thermometry and thermometric standards, this presents an important theoretical noise floor. Here it is demonstrated that thermometry may be mapped onto the problem of phase estimation, and using ... [Phys. Rev. A 82, 011611 ] published .

The quantum mechanics of electric conduction in crystals

Raina J. Olsen and Giovanni Vignale
We introduce a model of electrons incident on a one-dimensional periodic potential and show that conduction is a result of the interference of different parts of an electron wave that bounce multiple times through a series of potentials, ultimately being transmitted or reflected. Simple iterative eq ... [Am. J. Phys. 78, 954 (2010)] published Wed Aug 11, 2010.

Local equivalence, surface-code states, and matroids

Pradeep Sarvepalli and Robert Raussendorf
Recently, Ji et al. disproved the local-unitarylocal Clifford (LU-LC) conjecture and showed that the local unitary (LU) and local Clifford (LC) equivalence classes of the stabilizer states are not always the same. Despite the fact that this settles the LU-LC conjecture, a sufficient condition for st ... [Phys. Rev. A 82, 022304 ] published .

Entanglement-assisted zero-error capacity is upper-bounded by the Lovasz theta function

Salman Beigi
The zero-error capacity of a classical channel is expressed in terms of the independence number of some graph and its tensor powers. This quantity is hard to compute even for small graphs such as the cycle of length seven, so upper bounds such as the Lovasz theta function play an important role in z ... [Phys. Rev. A 82, 010303 ] published .

Householder factorizations of unitary matrices

Jesus Urias
A method to construct all representations of finite dimensional unitary matrices as the product of Householder reflections is given. By arbitrarily severing the state space into orthogonal subspaces, the method may, e.g., identify the entangling and single-component quantum operations that are requi ... [J. Math. Phys. 51, 072204 (2010)] published Wed Jul 21, 2010.

Sufficient criterion for separability of bipartite states

Kil-Chan Ha
We observe a convex subset of the set of all M[direct-product]N separable states. On the basis of this observation, we give an alternative sufficient condition for separability of bipartite states. This criterion gives rise to a family of quantum channels which are entanglement breaking. ... [Phys. Rev. A 82, 014102 ] published .

Teleportation is necessary for faithful quantum state transfer through noisy channels of maximal rank

Raffaele Romano and Peter van Loock
Quantum teleportation enables deterministic and faithful transmission of quantum states, provided a maximally entangled state is preshared between sender and receiver, and a one-way classical channel is available. Here, we prove that these resources are not only sufficient, but also necessary, for d ... [Phys. Rev. A 82, 012334 ] published .

Thermodynamic entropy of a many-body energy eigenstate

J Deutsch
It is argued that a typical many-body energy eigenstate has a well-defined thermodynamic entropy and that individual eigenstates possess thermodynamic characteristics analogous to those of generic isolated systems. We examine large systems with eigenstate energies equivalent to finite temperatures. ... [New J. Phys. 12, 075021 (2010)] published Wed Aug 4, 2010.

Entanglement criteria via concave-function uncertainty relations

Yichen Huang
A general theorem as a necessary condition for the separability of quantum states in both finite and infinite dimensional systems, based on concave-function uncertainty relations, is derived. Two special cases of the general theorem are stronger than two known entanglement criteria based on the Shan ... [Phys. Rev. A 82, 012335 ] published .

Necessary condition for the thermalization of a quantum system coupled to a quantum bath

Oleg Lychkovskiy
A system put in contact with a large heat bath normally thermalizes. This means that the state of the system rho(t) approaches an equilibrium state rho, the latter depending only on macroscopic characteristics of the bath (e.g., temperature) but not on the initial state of the system. The above stat ... [Phys. Rev. E 82, 011123 ] published .

Quantum Annealing with the Jarzynski Equality

Masayuki Ohzeki
We show a practical application of the Jarzynski equality in quantum computation. Its implementation may open a way to solve combinatorial optimization problems, minimization of a real single-valued function, cost function, with many arguments. We consider to incorporate the Jarzynski equality into ... [Phys. Rev. Lett. 105, 050401 ] published .

Ultrasensitive diamond magnetometry using optimal dynamic decoupling

L. T. Hall, C. D. Hill, J. H. Cole, and L. C. L. Hollenberg
Magnetometry techniques based on nitrogen-vacancy (NV) defects in diamond have received much attention of late as a means to probe nanoscale magnetic environments. The sensitivity of a single NV magnetometer is primarily determined by the transverse spin-relaxation time, T. Current approaches to imp ... [Phys. Rev. B 82, 045208 ] published .

Self-correcting quantum memory in a thermal environment

Stefano Chesi, Beat Rothlisberger, and Daniel Loss
The ability to store information is of fundamental importance to any computer, be it classical or quantum. To identify systems for quantum memories, which rely, analogously to classical memories, on passive error protection (self-correction), is of greatest interest in quantum information science. W ... [Phys. Rev. A 82, 022305 ] published .

High-fidelity feedback-assisted parity measurement in circuit QED

L. Tornberg and G. Johansson
We analyze a two-qubit parity measurement based on dispersive readout in circuit quantum electrodynamics. The back action on the qubits has two qualitatively different contributions. One is an unavoidable dephasing in one of the parity subspaces, arising during the transient time of switching on the ... [Phys. Rev. A 82, 012329 ] published .

Implementation of a three-qubit quantum error-correction code in a cavity-QED setup

Carlo Ottaviani and David Vitali
The correction of errors is of fundamental importance for the development of contemporary computing devices and robust communication protocols. In this paper we propose a scheme for the implementation of the three-qubit quantum repetition code, exploiting the interaction of Rydberg atoms with the qu ... [Phys. Rev. A 82, 012319 ] published .

Designing Quantum Memories with Embedded Control: Photonic Circuits for Autonomous Quantum Error Correction

Joseph Kerckhoff, Hendra I. Nurdin, Dmitri S. Pavlichin, and Hideo Mabuchi
We propose an approach to quantum error correction based on coding and continuous syndrome readout via scattering of coherent probe fields, in which the usual steps of measurement and discrete restoration are replaced by direct physical processing of the probe beams and coherent feedback to the regi ... [Phys. Rev. Lett. 105, 040502 ] published .

Protection of center-spin coherence by a dynamically polarized nuclear spin core

Wenxian Zhang, Jian-Liang Hu, Jun Zhuang, J. Q. You, and Ren-Bao Liu
Understanding fully the dynamics of coupled electron-nuclear spin systems, which are important for the development of long-lived qubits based on solid-state systems, remains a challenge. We show that in a singly charged semiconductor quantum dot with inhomogeneous hyperfine coupling, the nuclear spi ... [Phys. Rev. B 82, 045314 ] published .

Correcting errors in a quantum gate with pushed ions via optimal control

Uffe V. Poulsen, Shlomo Sklarz, David Tannor, and Tommaso Calarco
We analyze in detail the so-called pushing gate for trapped ions, introducing a time-dependent harmonic approximation for the external motion. We show how to extract the average fidelity for the gate from the resulting semiclassical simulations. We characterize and quantify precisely all types of er ... [Phys. Rev. A 82, 012339 ] published .

Fault-Tolerant Greenberger-Horne-Zeilinger Paradox Based on Non-Abelian Anyons

Dong-Ling Deng, Chunfeng Wu, Jing-Ling Chen, and C. H. Oh
We propose a scheme to test the Greenberger-Horne-Zeilinger paradox based on braidings of non-Abelian anyons, which are exotic quasiparticle excitations of topological states of matter. Because topological ordered states are robust against local perturbations, this scheme is in some sense fault-tole ... [Phys. Rev. Lett. 105, 060402 ] published .

Crossover of the three-dimensional topological insulator BiSe to the two-dimensional limit

Ke He, Yi Zhang, Ke He, Cui-Zu Chang, and Can-Li Song Qi-Kun Xue et al.
A topological insulator is a new state of quantum matter that is characterized by a finite energy gap in the bulk and gapless modes flowing along the boundaries that are robust against disorder scattering. The topological protection of the surface state could be useful for both low-power electronic ... [Nature Phys. 6, 584 (2010)] published Wed Aug 4, 2010.

Intrinsic geometry of quantum adiabatic evolution and quantum phase transitions

A. T. Rezakhani, D. F. Abasto, D. A. Lidar, and P. Zanardi
We elucidate the geometry of quantum adiabatic evolution. By minimizing the deviation from adiabaticity, we find a Riemannian metric tensor underlying adiabatic evolution. Equipped with this tensor, we identify a unified geometric description of quantum adiabatic evolution and quantum phase transiti ... [Phys. Rev. A 82, 012321 ] published .

Hidden dimer order in the quantum compass model

Wojciech Brzezicki and Andrzej M. Oleś
We introduce an exact spin transformation that maps frustrated ZZ and XX spin interactions along the rows and columns of the quantum compass model (QCM) on an L x L square lattice to (L1) x (L1) quantum spin models with 2(L1) classical spins. Using the symmetry properties we unravel the hidden dimer ... [Phys. Rev. B 82, 060401 ] published .

Visualization of an entangled channel spin-1 system

Swarnamala Sirsi and Veena Adiga
Covariance matrix formalism gives powerful entanglement criteria for continuous as well as finite dimensional systems. We use this formalism to study a mixed channel spin-1 system which is well known in nuclear reactions. A spin-j state can be visualized as being made up of 2j spinors which are repr ... [Phys. Rev. C 82, 027601 ] published .

Topological phases and phase transitions on the square-octagon lattice

Mehdi Kargarian and Gregory A. Fiete
We theoretically investigate a tight-binding model of fermions hopping on the square-octagon lattice which consists of a square lattice with plaquette corners themselves decorated by squares. Upon the inclusion of second-neighbor spin-orbit coupling or non-Abelian gauge fields, time-reversal symmetr ... [Phys. Rev. B 82, 085106 ] published .

Relation of the entanglement entropy and uncertainty product in ground states of coupled anharmonic oscillators

N. N. Chung, C. H. Er, Y. S. Teo, and L. Y. Chew
We explore quantitatively the fundamental connection between the entropy and local uncertainty product as an entanglement measure within the lowest energy eigenstate of a class of two-coupled anharmonic oscillator systems. We find that these two quantum aspects approach a unique relation as the enta ... [Phys. Rev. A 82, 014101 ] published .

Entangled superfluids: Condensate dynamics of the entangled Bose-Einstein condensation

Yu Shi
We study the condensate dynamics of the so-called entangled Bose-Einstein condensation (EBEC), which is the ground state of a mixture of two species of pseudospin-(1/2) atoms with interspecies spin-exchange scattering in certain parameter regimes. EBEC leads to four interdependent superfluid compone ... [Phys. Rev. A 82, 013637 ] published .

Not all pure entangled states are useful for sub-shot-noise interferometry

Philipp Hyllus, Otfried Guhne, and Augusto Smerzi
We investigate the connection between the shot-noise limit in linear interferometers and particle entanglement. In particular, we ask whether sub-shot-noise sensitivity can be reached with all pure entangled input states of N particles if they can be optimized with local operations. Results on the o ... [Phys. Rev. A 82, 012337 ] published .

Suppressing decoherence and improving entanglement by quantum-jump-based feedback control in two-level systems

S. C. Hou, X. L. Huang, and X. X. Yi
We study the quantum-jump-based feedback control on the entanglement shared between two qubits with one of them subject to decoherence while the other qubit is under the control. This situation is very relevant to a quantum system consisting of nuclear and electron spins in solid states. The possibi ... [Phys. Rev. A 82, 012336 ] published .

Degree of separability of bipartite quantum states

Guo Chuan Thiang
We investigate the problem of finding the optimal convex decomposition of a bipartite quantum state into a separable part and a positive remainder, in which the weight of the separable part is maximal. This weight is naturally identified with the degree of separability of the state. In a recent work ... [Phys. Rev. A 82, 012332 ] published .

Global control methods for Greenberger-Horne-Zeilinger-state generation on a one-dimensional Ising chain

Xiaoting Wang, Abolfazl Bayat, Sougato Bose, and Sophie G. Schirmer
We discuss how to prepare an Ising chain in a GHZ state using a single global control field only. This model does not require the spins to be individually addressable and is applicable to quantum systems such as cold atoms in optical lattices, some liquid- or solid-state NMR experiments, and many na ... [Phys. Rev. A 82, 012330 ] published .

Entanglement generation in planar Penning traps

Ana M. Martins, A. Guerreiro, J. T. Mendonca, and V. R. Vieira
We investigate the generation of entanglement between the axial degrees of freedom of electrons confined in separated locations in planar Penning traps. We show that there are two different sources of entanglement: one is related with the mechanism of switching on and off the electrical coupling bet ... [Phys. Rev. A 82, 012324 ] published .

Three-mode entanglement via tunneling-induced interference in a coupled triple-semiconductor quantum-well structure

Xin-You Lu and Jing Wu
A simple scheme is proposed to achieve three-mode continuous-variable (CV) entanglement in a coupled triple-semiconductor quantum-well (TSQW) structure via tunneling-induced interference. In the present scheme, the TSQW structure is trapped into a triply resonant cavity, and the tunneling-induced in ... [Phys. Rev. A 82, 012323 ] published .

Optimal distillation of three-qubit W states

Ali Yildiz
Some of the asymmetric three-qubit W states are used for perfect teleportation, superdense coding, and quantum-information splitting. We present the protocols for the optimal distillation of the asymmetric as well as the symmetric W states from a single copy of any three-qubit W class pure state. ... [Phys. Rev. A 82, 012317 ] published .

Nonclassicality thresholds for multiqubit states: Numerical analysis

Jacek Gruca, Wieslaw Laskowski, Marek Zukowski, Nikolai Kiesel, Witlef Wieczorek et al.
States that strongly violate Bell's inequalities are required in many quantum-informational protocols as, for example, in cryptography, secret sharing, and the reduction of communication complexity. We investigate families of such states with a numerical method which allows us to reveal nonclassical ... [Phys. Rev. A 82, 012118 ] published .

Temporally multiplexed quantum repeaters with atomic gases

Christoph Simon, Hugues de Riedmatten, and Mikael Afzelius
We propose a temporally multiplexed version of the Duan-Lukin-Cirac-Zoller (DLCZ) quantum-repeater protocol using controlled inhomogeneous spin broadening in atomic gases. A first analysis suggests that the advantage of multiplexing is negated by noise due to spin-wave excitations corresponding to u ... [Phys. Rev. A 82, 010304 ] published .

The maximally entangled symmetric state in terms of the geometric measure

Martin Aulbach, Damian Markham, and Mio Murao
The geometric measure of entanglement is investigated for permutation symmetric pure states of multipartite qubit systems, in particular the question of maximum entanglement. This is done with the help of the Majorana representation, which maps an n qubit symmetric state to n points on the unit sphe ... [New J. Phys. 12, 073025 (2010)] published Wed Aug 4, 2010.

Entanglement versus quantum discord in two coupled double quantum dots

F Fanchini, L Castelano, and A Caldeira
We study the dynamics of quantum correlations of two coupled double quantum dots containing two excess electrons. The dissipation is included through the contact with an oscillator bath. We solve the Redfield master equation in order to determine the dynamics of the quantum discord (QD) and the enta ... [New J. Phys. 12, 073009 (2010)] published Wed Aug 4, 2010.

Direct generation of photon triplets using cascaded photon-pair sources

Hannes Hbel, Deny R. Hamel, Alessandro Fedrizzi, Sven Ramelow, Kevin J. Resch et al.
Non-classical states of light, such as entangled photon pairs and number states, are essential for fundamental tests of quantum mechanics and optical quantum technologies. The most widespread technique for creating these quantum resources is spontaneous parametric down-conversion of laser light int ... [Nature 466, 601 (2010)] published Thu Jul 29, 2010.

Programmable purification of type-I polarization-entanglement

Simone Cialdi, Davide Brivio, and Matteo G. A. Paris
We suggest and demonstrate a scheme to compensate spatial and spectral decoherence effects in the generation of polarization entangled states by type-I parametric downconversion. In our device, a programmable spatial light modulator imposes a polarization dependent phase-shift on different spatial s ... [Appl. Phys. Lett. 97, 041108 ] published .

Newton's Cradle and Entanglement Transport in a Flexible Rydberg Chain

S. Wuster, C. Ates, A. Eisfeld, and J. M. Rost
In a regular, flexible chain of Rydberg atoms, a single electronic excitation localizes on two atoms that are in closer mutual proximity than all others. We show how the interplay between excitonic and atomic motion causes electronic excitation and diatomic proximity to propagate through the Rydberg ... [Phys. Rev. Lett. 105, 053004 ] published .

Entanglement Entropy and the Fermi Surface

Brian Swingle
Free fermions with a finite Fermi surface are known to exhibit an anomalously large entanglement entropy. The leading contribution to the entanglement entropy of a region of linear size L in d spatial dimensions is S~LlogL, a result that should be contrasted with the usual boundary law S~L. This ter ... [Phys. Rev. Lett. 105, 050502 ] published .

Arbitrary Control of Entanglement between two Superconducting Resonators

Frederick W. Strauch, Kurt Jacobs, and Raymond W. Simmonds
We present a method to synthesize an arbitrary quantum state of two superconducting resonators. This state-synthesis algorithm utilizes a coherent interaction of each resonator with a tunable artificial atom to create entangled quantum superpositions of photon number (Fock) states in the resonators. ... [Phys. Rev. Lett. 105, 050501 ] published .

Tolman Mass, Generalized Surface Gravity, and Entropy Bounds

Gabriel Abreu and Matt Visser
In any static spacetime the quasilocal Tolman mass contained within a volume can be reduced to a Gauss-like surface integral involving the flux of a suitably defined generalized surface gravity. By introducing some basic thermodynamics and invoking the Unruh effect one can then develop elementary bo ... [Phys. Rev. Lett. 105, 041302 ] published .

Remote Preparation of Single-Photon Hybrid Entangled and Vector-Polarization States

Julio T. Barreiro, Tzu-Chieh Wei, and Paul G. Kwiat
Quantum teleportation faces increasingly demanding requirements for transmitting large or even entangled systems. However, knowledge of the state to be transmitted eases its reconstruction, resulting in a protocol known as remote state preparation. A number of experimental demonstrations to date hav ... [Phys. Rev. Lett. 105, 030407 ] published .

Dynamics of disentanglement, density matrix, and coherence in neutrino oscillations

Jun Wu, Jimmy A. Hutasoit, Daniel Boyanovsky, and Richard Holman
In charged current weak interaction processes, neutrinos are produced in an entangled state with the charged lepton. This correlated state is disentangled by the measurement of the charged lepton in a detector at the production site. We study the dynamical aspects of disentanglement, propagation, an ... [Phys. Rev. D 82, 013006 ] published .

Ground states of a mixture of two species of spinor Bose gases with interspecies spin exchange

Yu Shi
We consider a mixture of two species of spin-1 atoms with interspecies spin exchange, which may cooperate or compete with intraspecies spin exchanges and thus dramatically affect the ground state. It represents a new class of bosonic gases differing from single-species spinor gases. We determine the ... [Phys. Rev. A 82, 023603 ] published .

Nonequilibrium forces between neutral atoms mediated by a quantum field

Ryan O. Behunin and Bei-Lok Hu
We study forces between two neutral atoms, modeled as three-dimensional harmonic oscillators, arising from mutual influences mediated by an electromagnetic field but not from their direct interactions. We allow as dynamical variables the center-of-mass motion of the atom, its internal degrees of fre ... [Phys. Rev. A 82, 022507 ] published .

Entanglement-distribution maximization over one-sided Gaussian noisy channels

Xiang-Bin Wang, Zong-Wen Yu, and Jia-Zhong Hu
We present an upper bound of the entanglement evolution for two-mode Gaussian pure states under a one-sided Gaussian map. Based on this, the optimization of entanglement evolution is studied. Even if complete information about the one-sided Gaussian noisy channel does not exist, one can still maximi ... [Phys. Rev. A 82, 022316 ] published .

Genuine multipartite nonlocality of entangled thermal states

G. McKeown, F. L. Semiao, H. Jeong, and M. Paternostro
We assess quantum nonlocality of multiparty entangled thermal states by studying, quantitatively, both tripartite and quadripartite states belonging to the Greenberger-Horne-Zeilinger, W, and linear cluster-state classes and showing violation of relevant Bell-like inequalities. We discuss the condit ... [Phys. Rev. A 82, 022315 ] published .

Single-experiment-detectable nonclassical correlation witness

Robabeh Rahimi and Akira SaiToh
Recent progress in theories of quantum information has determined nonclassical correlation defined differently from the widely used entanglement as an important property to evaluate computation and communication with mixed quantum states. We introduce an operational method to detect nonclassical cor ... [Phys. Rev. A 82, 022314 ] published .

Local cloning of entangled states

Vlad Gheorghiu, Li Yu, and Scott M. Cohen
We investigate the conditions under which a set [script S] of pure bipartite quantum states on a D x D system can be locally cloned deterministically by separable operations, when at least one of the states is full Schmidt rank. We allow for the possibility of cloning using a resource state that is ... [Phys. Rev. A 82, 022313 ] published .

Delayed birth of distillable entanglement in the evolution of bound entangled states

Lukasz Derkacz and Lech Jakobczyk
The dynamical creation of entanglement between three-level atoms coupled to the common vacuum is investigated. For the class of bound entangled initial states, we show that the dynamics of closely separated atoms generates stationary distillable entanglement of asymptotic states. We also find that t ... [Phys. Rev. A 82, 022312 ] published .

Universal behavior of the geometric entanglement measure of many-qubit W states

Levon Tamaryan, Zaruhi Ohanyan, and Sayatnova Tamaryan
We show that when N>>1 the geometric entanglement measure of general N-qubit W states, except maximally entangled W states, is a one-variable function and depends only on the Bloch vector with the minimal z component. Hence one can prepare a W state with the required maximal product overlap by alter ... [Phys. Rev. A 82, 022309 ] published .

Physical realization of quantum teleportation for a nonmaximal entangled state

Yoshiharu Tanaka, Masanari Asano, and Masanori Ohya
Recently, Kossakowski and Ohya (K-O) proposed a new teleportation scheme which enables perfect teleportation even for a nonmaximal entangled state [A. Kossakowski and M. Ohya, Infinite Dimensional Analysis Quantum Probability and Related Topics 10, 411 (2007)]. To discuss a physical realization of t ... [Phys. Rev. A 82, 022308 ] published .

Decoherence-free subspace and disentanglement dynamics for two qubits in a common non-Markovian squeezed reservoir

Md. Manirul Ali, Po-Wen Chen, and Hsi-Sheng Goan
We study the non-Markovian entanglement dynamics of two qubits in a common squeezed bath. We see a remarkable difference between the non-Markovian entanglement dynamics and its Markovian counterpart. We show that a non-Markovian decoherence-free state is also decoherence free in the Markovian regime ... [Phys. Rev. A 82, 022103 ] published .

Atomic spatial coherence with spontaneous emission in a strong-coupling cavity

Zhen Fang, Rui Guo, Xiaoji Zhou, and Xuzong Chen
The role of spontaneous emission in the interaction between a two-level atom and a pumped microcavity in the strong-coupling regime is discussed. In particular, using a quantum Monte Carlo simulation, we investigate atomic spatial coherence. It is found that atomic spontaneous emission destroys the ... [Phys. Rev. A 82, 015601 ] published .

Spatiotemporal structure of biphoton entanglement in type-II parametric down-conversion

E. Brambilla, L. Caspani, L. A. Lugiato, and A. Gatti
We investigate the spatiotemporal structure of the biphoton correlation in type-II parametric down-conversion (PDC). As in type-I PDC [Phys. Rev. Lett. 102, 223601 (2009)], we find that the correlation is nonfactorizable in space and time. Differently from type I, the type-II correlation in the spon ... [Phys. Rev. A 82, 013835 ] published .

Testing nonclassicality in multimode fields: A unified derivation of classical inequalities

Adam Miranowicz, Monika Bartkowiak, Xiaoguang Wang, Yu-xi Liu, and Franco Nori
We consider a way to generate operational inequalities to test nonclassicality (or quantumness) of multimode bosonic fields (or multiparty bosonic systems) that unifies the derivation of many known inequalities and allows to propose new ones. The nonclassicality criteria are based on Vogel's criteri ... [Phys. Rev. A 82, 013824 ] published .

Evaluation of two different entanglement measures on a bound entangled state

Cyril Branciard, Huangjun Zhu, Lin Chen, and Valerio Scarani
We consider the mixed three-qubit bound entangled state defined as the normalized projector on the subspace that is complementary to an unextendible product basis [C. H. Bennett et al., Phys. Rev. Lett. 82, 5385 (1999)]. Using the fact that no product state lies in the support of that state, we comp ... [Phys. Rev. A 82, 012327 ] published .

Continuous-variable teleportation of a negative Wigner function

Ladislav Mista, Jr., Radim Filip, and Akira Furusawa
Teleportation is a basic primitive for quantum communication and quantum computing. We address the problem of continuous-variable (unconditional and conditional) teleportation of a pure single-photon state and a mixed attenuated single-photon state generally in a nonunity-gain regime. Our figure of ... [Phys. Rev. A 82, 012322 ] published .

Adiabatic creation of atomic squeezing in dark states versus decoherences

Z. R. Gong (龚志瑞), Xiaoguang Wang (王晓光), and C. P. Sun (孙昌璞)
We study the multipartite correlations of the multiatom dark states, which are characterized by the atomic squeezing beyond the pairwise entanglement. It is shown that, in the photon storage process with atomic ensemble via the electromagnetically induced transparency (EIT) mechanism, the atomic squ ... [Phys. Rev. A 82, 012112 ] published .

Quantum and classical thermal correlations in the XY spin-(1/2) chain

J. Maziero, H. C. Guzman, L. C. Celeri, M. S. Sarandy, and R. M. Serra
We investigate pairwise quantum correlation as measured by the quantum discord as well as its classical counterpart in the thermodynamic limit of anisotropic XY spin-1/2 chains in a transverse magnetic field for both zero and finite temperatures. Analytical expressions for both classical and quantum ... [Phys. Rev. A 82, 012106 ] published .

Entanglement, intractability and no-signaling

R Srikanth
We consider the problem of deriving the no-signaling condition from the assumption that, as seen from a complexity theoretic perspective, the universe is not an exponential place. A fact that disallows such a derivation is the existence of polynomial superluminal gates, hypothetical primitive operat ... [Phys. Scr. 81, 065002 (2010)] published Mon Jul 26, 2010.

HongOuMandel dip measurements of polarization-entangled photon pairs at 1550 nm

Yinghong Xue, Akio Yoshizawa, and Hidemi Tsuchida
We performed a quantum interference experiment using two polarization-entangled photon pairs at 1550 nm, created in periodically poled lithium niobate waveguides. Using four-fold coincidences, a HongOuMandel dip at diagonal polarization was observed with a visibility of 74.5% before subtracting acci ... [Opt. Express 18, 8182 (2010)] published Thu Aug 5, 2010.

Two-photon interference with a semiconductor integrated source at room temperature

X. Caillet, A. Orieux, A. Lemaitre, P. Filloux, I. Favero et al.
We experimentally demonstrate an integrated semiconductor ridge microcavity source of counterpropagating twin photons at room temperature in the telecom range. Based on type II parametric down conversion with a counterpropagating phase-matching, pump photons generate photon pairs with an efficiency ... [Opt. Express 18, 9967 (2010)] published Thu Aug 5, 2010.

Experimental Optimal Cloning of Four-Dimensional Quantum States of Photons

E. Nagali, D. Giovannini, L. Marrucci, S. Slussarenko, E. Santamato et al.
Optimal quantum cloning is the process of making one or more copies of an arbitrary unknown input quantum state with the highest possible fidelity. All reported demonstrations of quantum cloning have so far been limited to copying two-dimensional quantum states, or qubits. We report the experimental ... [Phys. Rev. Lett. 105, 073602 ] published .

Preparing a Mechanical Oscillator in Non-Gaussian Quantum States

Farid Khalili, Stefan Danilishin, Haixing Miao, Helge Muller-Ebhardt, Huan Yang et al.
We propose a protocol for coherently transferring non-Gaussian quantum states from an optical field to a mechanical oscillator. We demonstrate its experimental feasibility in future gravitational-wave detectors and tabletop optomechanical devices. This work not only outlines a feasible way to invest ... [Phys. Rev. Lett. 105, 070403 ] published .

Ultrafast double-quantum-coherence spectroscopy of excitons with entangled photons

Marten Richter and Shaul Mukamel
We calculate the four-wave-mixing signal of excitons generated at k=k+k-k by two pulsed entangled photon pairs (k, k) and (k, k), where all four modes are chronologically ordered. Entangled photons offer an unusual combination of bandwidths and temporal resolution not possible by classical beams. Co ... [Phys. Rev. A 82, 013820 ] published .

Decomposition of bipartite states with applications to quantum no-broadcasting theorems

Shunlong Luo and Wei Sun
Correlations in bipartite quantum states are fundamental objects in quantum information theory. A canonical framework for studying correlations is the entangled versus separable dichotomy in which the decompositions of separable states as convex combinations of product states play an instrumental ro ... [Phys. Rev. A 82, 012338 ] published .

Entanglement of mechanical oscillators coupled to a nonequilibrium environment

Max Ludwig, K. Hammerer, and Florian Marquardt
Recent experiments aim at cooling nanomechanical resonators to the ground state by coupling them to nonequilibrium environments in order to observe quantum effects such as entanglement. This raises the general question of how such environments affect entanglement. Here we show that there is an optim ... [Phys. Rev. A 82, 012333 ] published .

Optical spin-1 chain and its use as a quantum-computational wire

Andrew S. Darmawan and Stephen D. Bartlett
Measurement-based quantum computing, a powerful alternative to the standard circuit model, proceeds using only local adaptive measurements on a highly entangled resource state of many spins on a graph or lattice. Along with the canonical cluster state, the valence-bond solid ground state on a chain ... [Phys. Rev. A 82, 012328 ] published .

The invariant-comb approach and its relation to the balancedness of multipartite entangled states

Andreas Osterloh and Jens Siewert
The invariant-comb approach is a method to construct entanglement measures for multipartite systems of qubits. The essential step is the construction of an antilinear operator that we call comb in reference to the hairy-ball theorem. An appealing feature of this approach is that, for qubits (or spin ... [New J. Phys. 12, 075025 (2010)] published Wed Aug 4, 2010.

Homogeneous multiscale entanglement renormalization ansatz tensor networks for quantum critical systems

M Rizzi, S Montangero, P Silvi, V Giovannetti, and Rosario Fazio
In this paper, we review the properties of homogeneous multiscale entanglement renormalization ansatz (MERA) to describe quantum critical systems. We discuss in more detail our results for one-dimensional (1D) systems (the Ising and Heisenberg models) and present new data for the 2D Ising model. Tog ... [New J. Phys. 12, 075018 (2010)] published Wed Aug 4, 2010.

Entanglement scaling of fractional quantum Hall states through geometric deformations

Andreas Lauchli, Emil Bergholtz, and Masudul Haque
We present a new approach for obtaining the scaling behavior of the entanglement entropy in fractional quantum Hall (FQH) states from finite-size wavefunctions. By employing the torus geometry and the fact that the torus aspect ratio can be readily varied, we can extract the entanglement entropy of ... [New J. Phys. 12, 075004 (2010)] published Wed Aug 4, 2010.

Two-photon subwavelength lithography with thermal light

De-Zhong Cao, Gui-Ju Ge, and Kaige Wang
We propose an incoherent interferometer which can partly modify the spatial correlation property of thermal light. Applying the interferometer to the two-photon double-slit interference with thermal light, we find that the subwavelength interference patterns appear in the intensity correlation not o ... [Appl. Phys. Lett. 97, 051105 (2010)] published Tue Aug 3, 2010.

Four-tangle for pure states

S. Shelly Sharma and N. K. Sharma
A novel approach to construct invariants that detect entanglement is presented. The method is based on the Peres separability criterion and Sylvester's criterion. An expression for four-tangle is obtained by examining the negativity fonts present in a four-way partial transpose under local unitary o ... [Phys. Rev. A 82, 012340 ] published .

On the photonic implementation of universal quantum gates, Bell states preparation circuit and quantum LDPC encoders and decoders based on directional couplers and HNLF

Ivan B. Djordjevic
The Bell states preparation circuit is a basic circuit required in quantum teleportation. We describe how to implement it in all-fiber technology. The basic building blocks for its implementation are directional couplers and highly nonlinear optical fiber (HNLF). Because the quantum information proc ... [Opt. Express 18, 8115 (2010)] published Thu Aug 5, 2010.

Matrix pencils and entanglement classification

Eric Chitambar, Carl A. Miller, and Yaoyun Shi
Quantum entanglement plays a central role in quantum information processing. A main objective of the theory is to classify different types of entanglement according to their interconvertibility through manipulations that do not require additional entanglement to perform. While bipartite entanglement ... [J. Math. Phys. 51, 072205 (2010)] published Thu Jul 22, 2010.

Generation of two-color continuous variable quantum entanglement at 0.8 and 1.5 [mu]m

Yongmin Li, Xiaomin Guo, Zengliang Bai, and Chunchun Liu
Two-color continuous variable quantum entanglement at 0.8 and 1.5 [mu]m was experimentally demonstrated by using an above-threshold optical parametric oscillator based on a periodically poled KTiOPO crystal. The system presented here has potential applications in future quantum information networks ... [Appl. Phys. Lett. 97, 031107 (2010)] published Tue Jul 20, 2010.

Quantum oscillatory exciton migration in photosynthetic reaction centers

Darius Abramavicius and Shaul Mukamel
The harvesting of solar energy and its conversion to chemical energy is essential for all forms of life. The primary photon absorption, transport, and charge separation events, which trigger a chain of chemical reactions, take place in membrane-bound photosynthetic complexes. Whether quantum effects ... [J. Chem. Phys. 133, 064510 (2010)] published Fri Aug 13, 2010.

Quantum Correlations in Optical AngleOrbital Angular Momentum Variables

Jonathan Leach, Barry Jack, Jacqui Romero, Anand K. Jha, Alison M. Yao et al.
Entanglement of the properties of two separated particles constitutes a fundamental signature of quantum mechanics and is a key resource for quantum information science. We demonstrate strong Einstein, Podolsky, and Rosen correlations between the angular position and orbital angular momentum of two ... [Science 329, 662 (2010)] published Wed Aug 11, 2010.

Entanglement Spectra of Quantum Heisenberg Ladders

Didier Poilblanc
Bipartite entanglement measures are surprisingly useful tools to investigate quantum phases of correlated electrons. Here, I analyze the entanglement spectrum of gapped two-leg quantum Heisenberg ladders on a periodic ribbon partitioned into two identical periodic chains. The entanglement spectrum c ... [Phys. Rev. Lett. 105, 077202 ] published .

Entanglement and Non-Markovianity of Quantum Evolutions

Angel Rivas, Susana F. Huelga, and Martin B. Plenio
We address the problem of quantifying the non-Markovian character of quantum time evolutions of general systems in contact with an environment. We introduce two different measures of non-Markovianity that exploit the specific traits of quantum correlations and are suitable for opposite experimental ... [Phys. Rev. Lett. 105, 050403 ] published .

Entanglement Rules for Random Mixtures

J. Duplat, A. Jouary, and E. Villermaux
We discuss how two subparts of a randomly stirred scalar mixture interact to form the overall concentration distribution. We derive, in particular, the appropriate composition laws in the absence and presence of a strong correlation between the fields. The resulting concentration distributions compa ... [Phys. Rev. Lett. 105, 034504 ] published .

Reduced-density-matrix spectrum and block entropy of permutationally invariant many-body systems

Mario Salerno and Vladislav Popkov
Spectral properties of the reduced density matrix (RDM) of permutational invariant quantum many-body systems are investigated. The RDM block diagonalization which accounts for all symmetries of the Hamiltonian is achieved. The analytical expression of the RDM spectrum is provided for arbitrary param ... [Phys. Rev. E 82, 011142 ] published .

Measuring B width difference at the Upsilon(5s) using quantum entanglement

David Atwood and Amarjit Soni
About 90% of B[overline B] pairs produced at the Upsilon(5s) resonance are initially B[overline B] pairs which decay radiatively to B[overline B]. This implies that the B[overline B] pair will then be in an eigenstate of charge conjugation (i.e. C=-1) and therefore in an entangled state. This allows ... [Phys. Rev. D 82, 036003 ] published .

Quantum entanglement between an optical photon and a solid-state spin qubit

E. Togan, Y. Chu, A. S. Trifonov, L. Jiang, J. Maze et al.
Quantum entanglement is among the most fascinating aspects of quantum theory. Entangled optical photons are now widely used for fundamental tests of quantum mechanics and applications such as quantum cryptography. Several recent experiments demonstrated entanglement of optical photons with trapped ... [Nature 466, 730 (2010)] published Thu Aug 5, 2010.

Communication at the quantum speed limit along a spin chain

Michael Murphy, Simone Montangero, Vittorio Giovannetti, and Tommaso Calarco
Spin chains have long been considered as candidates for quantum channels to facilitate quantum communication. We consider the transfer of a single excitation along a spin-1/2 chain governed by Heisenberg-type interactions. We build on the work of Balachandran and Gong [V. Balachandran and J. Gong, P ... [Phys. Rev. A 82, 022318 ] published .

Self-similar spectral structures and edge-locking hierarchy in open-boundary spin chains

Masudul Haque
For an anisotropic Heisenberg (XXZ) spin chain, we show that an open boundary induces a series of approximately self-similar features at different energy scales, high up in the eigenvalue spectrum. We present a nonequilibrium phenomenon related to this fractal structure, involving states in which a ... [Phys. Rev. A 82, 012108 ] published .

Majorana Fermions and a Topological Phase Transition in Semiconductor-Superconductor Heterostructures

Roman M. Lutchyn, Jay D. Sau, and S. Das Sarma
We propose and analyze theoretically an experimental setup for detecting the elusive Majorana particle in semiconductor-superconductor heterostructures. The experimental system consists of one-dimensional semiconductor wire with strong spin-orbit Rashba interaction embedded into a superconducting qu ... [Phys. Rev. Lett. 105, 077001 ] published .

Limitations for transmission of photonic qubits in optical fibres carrying telecom traffic

G.B. Xavier and J.P. von der Weid
Practical limitations are discussed for the exchange of photonic qubits in optical fibres carrying telecom traffic based on the noise generated by Raman spontaneous scattered photons from the classical signals. It is shown that these limitations are in the co- and counter-propagating directions with ... [Electron. Lett. 46, 1071 ] published .

Optimal super dense coding over noisy quantum channels

Z Shadman, H Kampermann, C Macchiavello, and D Bruss
We investigate super dense coding in the presence of noise, i.e. the subsystems of the entangled resource state have to pass a noisy unital quantum channel between the sender and the receiver. We discuss explicitly the case of Pauli channels in an arbitrary dimension and derive the super dense codin ... [New J. Phys. 12, 073042 (2010)] published Wed Aug 4, 2010.

Proposal for Implementing Device-Independent Quantum Key Distribution Based on a Heralded Qubit Amplifier

Nicolas Gisin, Stefano Pironio, and Nicolas Sangouard
In device-independent quantum key distribution (DIQKD), the violation of a Bell inequality is exploited to establish a shared key that is secure independently of the internal workings of the QKD devices. An experimental implementation of DIQKD, however, is still awaited, since hitherto all optical B ... [Phys. Rev. Lett. 105, 070501 ] published .

Experimental study on discretely modulated continuous-variable quantum key distribution

Yong Shen, Hongxin Zou, Liang Tian, Pingxing Chen, and Jianmin Yuan
We present a discretely modulated continuous-variable quantum key distribution system in free space by using strong coherent states. The amplitude noise in the laser source is suppressed to the shot-noise limit by using a mode cleaner combined with a frequency shift technique. Also, it is proven tha ... [Phys. Rev. A 82, 022317 ] published .

Testing quantum randomness in single-photon polarization measurements with the NIST test suite

David Branning and Matthew Bermudez
A binary sequence was constructed from 1.7 x 10 polarization measurements of single photons from a spontaneous parametric downconversion source, under pumping conditions similar to those used in optical quantum cryptography. To search for correlations in the polarization measurement outcomes, we sub ... [J. Opt. Soc. Am. B 27, 1594 (2010)] published Fri Aug 13, 2010.

Heralded single-photon partial coherence

P. Ben Dixon, Gregory Howland, Mehul Malik, David J. Starling, R. W. Boyd et al.
We study transverse spatial coherence of approximately localized single-photon states. We demonstrate nonlocal control over single-photon spatial coherence via projective measurements of an entangled twin and provide a theoretical interpretation from quantum coherence theory. Our results show that t ... [Phys. Rev. A 82, 023801 ] published .

Semiquantum secret sharing using entangled states

Qin Li, W. H. Chan, and Dong-Yang Long
Secret sharing is a procedure for sharing a secret among a number of participants such that only the qualified subsets of participants have the ability to reconstruct the secret. Even in the presence of eavesdropping, secret sharing can be achieved when all the members are quantum. So what happens i ... [Phys. Rev. A 82, 022303 ] published .

Reexamination of the decoy-state quantum key distribution with an unstable source

Jia-Zhong Hu and Xiang-Bin Wang
We present an improved formula for the lower bound of the fraction of single-photon counts in a decoy-state protocol with an unstable source. Based on the formula, we study two-intensity protocol and three-intensity protocol. The major formula in the passive two-intensity protocol proposed by Adachi ... [Phys. Rev. A 82, 012331 ] published .

Decoy-state quantum key distribution with polarized photons over 200 km

Yang Liu, Teng-Yun Chen, Jian Wang, Wen-Qi Cai, Xu Wan et al.
We report an implementation of decoy-state quantum key distribution (QKD) over 200 km optical fiber cable through photon polarization encoding. This is achieved by constructing the whole QKD system operating at 320 MHz repetition rate, and developing high-speed transmitter and receiver modules. A no ... [Opt. Express 18, 8587 (2010)] published Thu Aug 5, 2010.

Known plaintext attack on double random phase encoding using fingerprint as key and a method for avoiding the attack

Hideaki Tashima, Masafumi Takeda, Hiroyuki Suzuki, Takashi Obi, Masahiro Yamaguchi et al.
We have shown that the application of double random phase encoding (DRPE) to biometrics enables the use of biometrics as cipher keys for binary data encryption. However, DRPE is reported to be vulnerable to known-plaintext attacks (KPAs) using a phase recovery algorithm. In this study, we investigat ... [Opt. Express 18, 13772 (2010)] published Thu Aug 5, 2010.

Quantum entanglement distribution with 810 nm photons through telecom fibers

E. Meyer-Scott, H. Hubel, A. Fedrizzi, C. Erven, G. Weihs et al.
We demonstrate the distribution of polarization entangled photons of wavelength 810 nm through standard telecom fibers. This technique allows quantum communication protocols to be performed over established fiber infrastructure, and makes use of the smaller and better performing setups available aro ... [Appl. Phys. Lett. 97, 031117 (2010)] published Thu Jul 22, 2010.

Topological quantum computation away from the ground state using Majorana fermions

A. R. Akhmerov
We relax one of the requirements for topological quantum computation with Majorana fermions. Topological quantum computation was discussed so far as manipulation of the wave function within degenerate many-body ground state. The simplest particles providing degenerate ground state, Majorana fermions ... [Phys. Rev. B 82, 020509 ] published .

Quantum counting algorithm and its application in mesoscopic physics

G. B. Lesovik, M. V. Suslov, and G. Blatter
We discuss a quantum counting algorithm which transforms a physical particle-number state (and superpositions thereof) into a binary number. The algorithm involves two quantum Fourier transformations. One transformation is in physical space, where a stream of n

Solving Frustration-Free Spin Systems

N. de Beaudrap, M. Ohliger, T. J. Osborne, and J. Eisert
We identify a large class of quantum many-body systems that can be solved exactly: natural frustration-free spin-1/2 nearest-neighbor Hamiltonians on arbitrary lattices. We show that the entire ground-state manifold of such models can be found exactly by a tensor network of isometries acting on a sp ... [Phys. Rev. Lett. 105, 060504 ] published .

Quantum Computation on the Edge of a Symmetry-Protected Topological Order

Akimasa Miyake
We elaborate the idea of quantum computation through measuring the correlation of a gapped ground state, while the bulk Hamiltonian is utilized to stabilize the resource. A simple computational primitive, by pulling out a single spin adiabatically from the bulk followed by its measurement, is shown ... [Phys. Rev. Lett. 105, 040501 ] published .

Unfrustrated qudit chains and their ground states

Ramis Movassagh, Edward Farhi, Jeffrey Goldstone, Daniel Nagaj, Tobias J. Osborne et al.
We investigate chains of d-dimensional quantum spins (qudits) on a line with generic nearest-neighbor interactions without translational invariance. We find the conditions under which these systems are not frustrated, that is, when the ground states are also the common ground states of all the local ... [Phys. Rev. A 82, 012318 ] published .

Distributed quantum computation with arbitrarily poor photon detection

Yuichiro Matsuzaki, Simon C. Benjamin, and Joseph Fitzsimons
In a distributed quantum computer, scalability is accomplished by networking together many elementary nodes. Typically the network is optical and internode entanglement involves photon detection. In complex networks the entanglement fidelity may be degraded by the twin problems of photon loss and da ... [Phys. Rev. A 82, 010302 ] published .

Room-Temperature Implementation of the Deutsch-Jozsa Algorithm with a Single Electronic Spin in Diamond

Fazhan Shi, Xing Rong, Nanyang Xu, Ya Wang, Jie Wu et al.
The nitrogen-vacancy defect center (N-V center) is a promising candidate for quantum information processing due to the possibility of coherent manipulation of individual spins in the absence of the cryogenic requirement. We report a room-temperature implementation of the Deutsch-Jozsa algorithm by e ... [Phys. Rev. Lett. 105, 040504 ] published .

Matrix product state and mean-field solutions for one-dimensional systems can be found efficiently

Norbert Schuch and J. Ignacio Cirac
We consider the problem of approximating ground states of one-dimensional quantum systems within the two most common variational ansatzes, namely, the mean-field ansatz and matrix product states. We show that both for mean field and for matrix product states of fixed bond dimension, the optimal solu ... [Phys. Rev. A 82, 012314 ] published .

Ancilla-driven universal quantum computation

Janet Anders, Daniel K. L. Oi, Elham Kashefi, Dan E. Browne, and Erika Andersson
We introduce a model of quantum computation intermediate between the gate-based and measurement-based models. A quantum register is manipulated remotely with the help of a single ancilla that drives the evolution of the register. The fully controlled ancilla qubit is coupled to the computational reg ... [Phys. Rev. A 82, 020301 ] published .

Quantum search algorithms on a regular lattice

Birgit Hein and Gregor Tanner
Quantum algorithms for searching for one or more marked items on a d-dimensional lattice provide an extension of Grover's search algorithm including a spatial component. We demonstrate that these lattice search algorithms can be viewed in terms of the level dynamics near an avoided crossing of a one ... [Phys. Rev. A 82, 012326 ] published .

Efficient algorithm for approximating one-dimensional ground states

Dorit Aharonov, Itai Arad, and Sandy Irani
The density-matrix renormalization-group method is very effective at finding ground states of one-dimensional (1D) quantum systems in practice, but it is a heuristic method, and there is no known proof for when it works. In this article we describe an efficient classical algorithm which provably fin ... [Phys. Rev. A 82, 012315 ] published .