Virtual Journal of Quantum Information

Coherent transfer of time-bin photons to electron spins in a semiconductor

Hideo Kosaka, Takahiro Inagaki, Ryuta Hitomi, Fumishige Izawa, Yoshiaki Rikitake et al.
We demonstrate that a coherent superposition state of two temporally separated optical pulses, called a time-bin state, can be transferred to that of up-down electron spins in a semiconductor by synchronizing the time separation to the precession period of either electrons or holes. The time-bin tra ... [Phys. Rev. A 85, 042304 ] published .

Cascaded single-photon emission from the Mollow triplet sidebands of a quantum dot

A. Ulhaq, S. Weiler, S. M. Ulrich, R. Robach, M. Jetter et al.
Emission from a resonantly excited quantum emitter is a fascinating research topic within the field of quantum optics and is a useful source for different types of quantum light fields. The resonance spectrum consists of a single spectral line that develops into a triplet above saturation of the qua ... [Nat. Photonics 6, 238 (2012)] published Tue Apr 3, 2012.

A state-insensitive, compensated nanofiber trap

C Lacrote, K Choi, A Goban, D Alton, D Ding et al.
Laser trapping and interfacing of laser-cooled atoms in an optical fiber network is an important tool for quantum information science. Following the pioneering work of Balykin et al (2004 Phys. Rev. A 70 011401) and Vetsch et al (2010 Phys. Rev. Lett. 104 203603), we propose a robust method for trap ... [New J. Phys. 14, 023056 (2012)] published Tue Apr 3, 2012.

Dissipative dynamics of qubits driven by a bichromatic field in the dispersive regime

A Saiko, R Fedaruk, A Kolasa, and S Markevich
We study the coherent dynamics of relaxing qubits driven by a bichromatic radiation in the dispersive regime in the case when the detuning of the frequency of a longitudinal radio-frequency field from the Rabi frequency in a transverse microwave field is comparable in magnitude to and . We analyt ... [Phys. Scr. 85, 045301 (2012)] published Thu Apr 5, 2012.

Engineering integrated pure narrow-band photon sources

E Pomarico, B Sanguinetti, C Osorio, H Herrmann, and R Thew
Engineering and controlling well-defined states of light for quantum information applications is of increasing importance as the complexity of quantum systems grows. For example, in quantum networks, high multi-photon interference visibility requires properly devised pure photon sources. In this pap ... [New J. Phys. 14, 033008 (2012)] published Tue Apr 3, 2012.

Superconducting qubit manipulated by fast pulses: experimental observation of distinct decoherence regimes

F Chiarello, E Paladino, M Castellano, C Cosmelli, A D'Arrigo et al.
A particular superconducting quantum interference device (SQUID) qubit, namely the double SQUID qubit, can be manipulated by rapidly modifying its potential with the application of fast flux pulses. In this system we observe coherent oscillations exhibiting non-exponential decay, indicating an uncon ... [New J. Phys. 14, 023031 (2012)] published Tue Apr 3, 2012.

Local probing of propagating acoustic waves in a gigahertz echo chamber

Martin V. Gustafsson, Paulo V. Santos, Gran Johansson, and Per Delsing
In the same way that micro-mechanical resonators resemble guitar strings and drums, surface acoustic waves resemble the sound these instruments produce, but moving over a solid surface rather than through air. In contrast with oscillations in suspended resonators, such propagating mechanical waves h ... [Nature Phys. 8, 338 (2012)] published Tue Apr 3, 2012.

Ultralong photon-echo-based quantum memories using optical locking

Byoung S. Ham
Unlike slow-light-based quantum memories, photon echoes offer the benefit of high speed and wide bandwidth. Over the last decade, the rephasing mechanism of photon echoes has been studied for quantum memories to overcome fundamental limitations in photon echoes, such as population inversion and low ... [J. Opt. Soc. Am. B 29, 463 (2012)] published Wed Apr 4, 2012.

Dynamics of encrypted information in superconducting qubits with the presence of imperfect operations

N. Metwally
The original dense coding protocol is achieved via a quantum channel generated between a single Cooper pair and a cavity. The dynamics of the coded and decoded information is investigated for different values of the channel's parameters. It is shown that these two types of information increase as th ... [J. Opt. Soc. Am. B 29, 389 (2012)] published Wed Apr 4, 2012.

Manipulating single photons from disparate quantum sources to be indistinguishable [Invited]

Glenn S. Solomon, Edward B. Flagg, Sergey V. Polyakov, Tim Thomay, and Andreas Muller
Quantum information networks will likely require different quantum systems for different functionality within the network. Indistinguishable photons can be used to interconnect these different subsystems. We discuss methods for coherently manipulating the single photons from different quantum system ... [J. Opt. Soc. Am. B 29, 319 (2012)] published Wed Apr 4, 2012.

One-step implementation of a multiqubit controlled-phase gate with superconducting quantum interference devices coupled to a resonator

Y. Q. Zhang, S. Zhang, K. H. Yeon, and S. C. Yu
A scheme for one-step implementation of an n-qubit controlled-phase gate is proposed in a superconducting quantum interference device (SQUID) system. The distinguishing feature of this scheme is the simultaneous and nonidentical off-resonant Raman coupling of the n SQUID qubits to a single-mode reso ... [J. Opt. Soc. Am. B 29, 300 (2012)] published Wed Apr 4, 2012.

1/f noise of Josephson-junction-embedded microwave resonators at single photon energies and millikelvin temperatures

K. W. Murch, S. J. Weber, E. M. Levenson-Falk, R. Vijay, and I. Siddiqi
We present measurements of 1/f frequency noise in both linear and Josephson-junction-embedded superconducting aluminum resonators in the low power, low temperature regimetypical operating conditions for superconducting qubits. The addition of the Josephson junction does not result in additional freq ... [Appl. Phys. Lett. 100, 142601 (2012)] published Wed Apr 4, 2012.

Implementing general measurements on linear optical and solid-state qubits

Yukihiro Ota, Sahel Ashhab, and Franco Nori
We show a systematic construction for implementing general measurements on a single qubit, including both strong (or projection) and weak measurements. We mainly focus on linear optical qubits. The present approach is composed of simple and feasible elements, i.e., beam splitters, wave plates, and p ... [Phys. Rev. A 85, 043808 ] published .

Fragility of flux qubits against quasiparticle tunneling

Juha Leppakangas and Michael Marthaler
We study decoherence in superconducting qubits due to quasiparticle tunneling which is enhanced by two known deviations from the equilibrium BCS theory. The first process corresponds to tunneling of an already existing quasiparticle across the junction. The quasiparticle density is increased, e.g., ... [Phys. Rev. B 85, 144503 ] published .

Polarization properties of excitonic qubits in single self-assembled quantum dots

Catherine Tonin, Richard Hostein, Valia Voliotis, Roger Grousson, Aristide Lemaitre et al.
We investigate polarization properties of neutral exciton emission in single self-assembled InAs/GaAs quantum dots. The in-plane anisotropy of the confinement potential and strain strongly couple the heavy- and light-hole states and lead to large optical anisotropy with nonorthogonal linearly polari ... [Phys. Rev. B 85, 155303 ] published .

Adiabatic State Conversion and Pulse Transmission in Optomechanical Systems

Lin Tian
Optomechanical systems with strong coupling can be a powerful medium for quantum state engineering of the cavity modes. Here, we show that quantum state conversion between cavity modes of distinctively different wavelengths can be realized with high fidelity by adiabatically varying the effective op ... [Phys. Rev. Lett. 108, 153604 ] published .

Dual-probe decoherence microscopy: probing pockets of coherence in a decohering environment

Jan Jeske, Jared Cole, Clemens Mller, Michael Marthaler, and Gerd Schn
We study the use of a pair of qubits as a decoherence probe of a nontrivial environment. This dual-probe configuration is modelled by three two-level systems (TLSs), which are coupled in a chain in which the middle system represents an environmental TLS. This TLS resides within the environment of th ... [New J. Phys. 14, 023013 (2012)] published Tue Apr 3, 2012.

Coherent control of collective atom phase for ultralong, inversion-free photon echoes

Byoung S. Ham
To overcome the fundamental limitations of the pi optical pulse-induced population inversion and optical decay-caused short storage time in conventional photon echoes, a coherent control of collective atoms is studied for inversion-free, optical decay-halted photon echoes, where the constraint of ph ... [Phys. Rev. A 85, 031402 ] published .

Spin manipulation and relaxation in spin-orbit qubits

Massoud Borhani and Xuedong Hu
We derive a generalized form of the electric dipole spin resonance (EDSR) Hamiltonian in the presence of the spin-orbit interaction for single spins in an elliptic quantum dot (QD) subject to an arbitrary (in both direction and magnitude) applied magnetic field. We predict a nonlinear behavior of th ... [Phys. Rev. B 85, 125132 ] published .

A cryogen-free dilution refrigerator based Josephson qubit measurement system

Ye Tian, H. F. Yu, H. Deng, G. M. Xue, D. T. Liu et al.
We develop a small-signal measurement system on cryogen-free dilution refrigerator which is suitable for superconducting qubit studies. Cryogen-free refrigerators have several advantages such as less manpower for system operation and large sample space for experiment, but concern remains about wheth ... [Rev. Sci. Instrum. 83, 033907 (2012)] published Fri Mar 30, 2012.

Using Interference for High Fidelity Quantum State Transfer in Optomechanics

Ying-Dan Wang and Aashish A. Clerk
We revisit the problem of using a mechanical resonator to perform the transfer of a quantum state between two electromagnetic cavities (e.g., optical and microwave). We show that this system possesses an effective mechanically dark mode which is immune to mechanical dissipation; utilizing this featu ... [Phys. Rev. Lett. 108, 153603 ] published .

Optical decoherence and energy level structure of 0.1%Tm:LiNbO

Y. Sun, C. W. Thiel, and R. L. Cone
We report the energy level structure of the H and H multiplets for Tm doped congruent LiNbO, as well as the decoherence properties and their temperature dependencies for the H(1)<-->H(1a) transition at 794 nm. It is shown that this material provides very significant improvements in bandwidth, time-b ... [Phys. Rev. B 85, 165106 ] published .

Coherent Sensing of a Mechanical Resonator with a Single-Spin Qubit

Shimon Kolkowitz, Ania C. Bleszynski Jayich, Quirin P. Unterreithmeier, Steven D. Bennett, Peter Rabl et al.
Mechanical systems can be influenced by a wide variety of small forces, ranging from gravitational to optical, electrical, and magnetic. When mechanical resonators are scaled down to nanometer-scale dimensions, these forces can be harnessed to enable coupling to individual quantum systems. We demons ... [Science 335, 1603 (2012)] published Wed Mar 28, 2012.

Electron temperature in electrically isolated Si double quantum dots

A. Rossi, T. Ferrus, and D. A. Williams
Charge-based quantum computation can be attained through reliable control of single electrons in lead-less quantum systems. Single-charge transitions in electrically isolated double quantum dots (DQDs) realised in phosphorus-doped silicon can be detected via capacitively coupled single-electron tunn ... [Appl. Phys. Lett. 100, 133503 (2012)] published Tue Mar 27, 2012.

Structure of Si-capped Ge/SiC/Si (001) epitaxial nanodots: Implications for quantum dot patterning

C. W. Petz, D. Yang, J. Levy, and J. A. Floro
Artificially ordered quantum dot (QD) arrays, where confined carriers can interact via direct exchange coupling, may create unique functionalities such as cluster qubits and spintronic bandgap systems. Development of such arrays for quantum computing requires fine control over QD size and spatial ar ... [Appl. Phys. Lett. 100, 141603 (2012)] published Mon Apr 2, 2012.

Probing a topological quantum critical point in semiconductor-superconductor heterostructures

Sumanta Tewari, J. D. Sau, V. W. Scarola, Chuanwei Zhang, and S. Das Sarma
Quantum ground states on the nontrivial side of a topological quantum critical point (TQCP) have unique properties that make them attractive candidates for quantum information applications. A recent example is provided by s-wave superconductivity on a semiconductor platform, which is tuned through a ... [Phys. Rev. B 85, 155302 ] published .

Communication: Engineered tunable decay rate and controllable dissipative dynamics

Zhiguo Lu and Hang Zheng
We investigate the steering dissipative dynamics of a two-level system (qubit) by means of the modulation of an assisted tunneling degree of freedom which is described by a quantum-oscillator spin-boson model. Our results reveal that the decoherence rate of the qubit can be significantly suppressed ... [J. Chem. Phys. 136, 121103 (2012)] published Thu Mar 29, 2012.

Spin Coherence during Optical Excitation of a Single Nitrogen-Vacancy Center in Diamond

G. D. Fuchs, A. L. Falk, V. V. Dobrovitski, and D. D. Awschalom
We examine the quantum spin state of a single nitrogen-vacancy (NV) center in diamond at room temperature as it makes a transition from the orbital ground state (GS) to the orbital excited state (ES) during nonresonant optical excitation. While the fluorescence readout of NV-center spins relies on c ... [Phys. Rev. Lett. 108, 157602 ] published .

Fast Hybrid Silicon Double-Quantum-Dot Qubit

Zhan Shi, C. B. Simmons, J. R. Prance, John King Gamble, Teck Seng Koh et al.
We propose a quantum dot qubit architecture that has an attractive combination of speed and fabrication simplicity. It consists of a double quantum dot with one electron in one dot and two electrons in the other. The qubit itself is a set of two states with total spin quantum numbers S=3/4 (S=1/2) a ... [Phys. Rev. Lett. 108, 140503 ] published .

Localized End States in Density Modulated Quantum Wires and Rings

Suhas Gangadharaiah, Luka Trifunovic, and Daniel Loss
We study finite quantum wires and rings in the presence of a charge-density wave gap induced by a periodic modulation of the chemical potential. We show that the Tamm-Shockley bound states emerging at the ends of the wire are stable against weak disorder and interactions, for discrete open chains an ... [Phys. Rev. Lett. 108, 136803 ] published .

Quantum Interface between an Electrical Circuit and a Single Atom

D. Kielpinski, D. Kafri, M. J. Woolley, G. J. Milburn, and J. M. Taylor
We show how to bridge the divide between atomic systems and electronic devices by engineering a coupling between the motion of a single ion and the quantized electric field of a resonant circuit. Our method can be used to couple the internal state of an ion to the quantized circuit with the same spe ... [Phys. Rev. Lett. 108, 130504 ] published .

Valley-Based Noise-Resistant Quantum Computation Using Si Quantum Dots

Dimitrie Culcer, A. L. Saraiva, Belita Koiller, Xuedong Hu, and S. Das Sarma
We devise a platform for noise-resistant quantum computing using the valley degree of freedom of Si quantum dots. The qubit is encoded in two polarized (1,1) spin-triplet states with different valley compositions in a double quantum dot, with a Zeeman field enabling unambiguous initialization. A top ... [Phys. Rev. Lett. 108, 126804 ] published .

Ultrafast Quantum Gates in Circuit QED

G. Romero, D. Ballester, Y. M. Wang, V. Scarani, and E. Solano
We present a method to implement ultrafast two-qubit gates valid for the ultrastrong coupling and deep strong coupling regimes of light-matter interaction, considering state-of-the-art circuit quantum electrodynamics technology. Our proposal includes a suitable qubit architecture and is based on a f ... [Phys. Rev. Lett. 108, 120501 ] published .

Effective mass theory of monolayer delta doping in the high-density limit

Daniel W. Drumm, Lloyd C. L. Hollenberg, Michelle Y. Simmons, and Mark Friesen
Monolayer delta-doped structures in silicon have attracted renewed interest with their recent incorporation into atomic-scale device fabrication strategies as source and drain electrodes and in-plane gates. Modeling the physics of delta doping at this scale proves challenging, however, due to the la ... [Phys. Rev. B 85, 155419 ] published .

Fast preparation of a single-hole spin in an InAs/GaAs quantum dot in a Voigt-geometry magnetic field

T. M. Godden, J. H. Quilter, A. J. Ramsay, Yanwen Wu, P. Brereton et al.
The preparation of a coherent heavy-hole spin via ionization of a spin-polarized electron-hole pair in an InAs/GaAs quantum dot in a Voigt geometry magnetic field is experimentally investigated. For a dot with a typical bright-exciton fine-structure splitting of 17 [mu]eV, the fidelity of the spin ... [Phys. Rev. B 85, 155310 ] published .

Quantum speeding-up of computation demonstrated in a superconducting two-qubit processor

Andreas Dewes, Romain Lauro, Florian R. Ong, Vivient Schmitt, Perola Milman et al.
We operate a superconducting quantum processor consisting of two tunable transmon qubits coupled by a swapping interaction, and equipped with nondestructive single-shot readout of the two qubits. With this processor, we run the Grover search algorithm among four objects and find that the correct ans ... [Phys. Rev. B 85, 140503 ] published .

Voltage controlled exchange energies of a two-electron silicon double quantum dot with and without charge defects in the dielectric

Rajib Rahman, Erik Nielsen, Richard P. Muller, and Malcolm S. Carroll
Quantum dots are artificial atoms used for a multitude of purposes. Charge defects are commonly present and can significantly perturb the designed energy spectrum and purpose of the dots. Voltage controlled exchange energy in silicon double quantum dots (DQDs) represents a system that is very sensit ... [Phys. Rev. B 85, 125423 ] published .

Optically controlled phase gate for two spin qubits in coupled quantum dots

Li-Bo Chen, L. J. Sham, and Edo Waks
We present a feasible scheme for performing an optically controlled phase gate between two conduction electron spin qubits in adjacent self-assembled quantum dots. Interaction between the dots is mediated by the tunneling of the valence hole state, which is activated only by applying a laser pulse o ... [Phys. Rev. B 85, 115319 ] published .

Adiabatic Mach-Zehnder interferometer via an array of trapped ions

Y. M. Hu, M. Feng, and C. Lee
We explore the possibility of implementing a Heisenberg-limited Mach-Zehnder interferometer via an array of trapped ions, which obey a quantum Ising model within a transverse field. Based upon adiabatic processes of increasing the Ising interaction and then decreasing the transverse field, we demons ... [Phys. Rev. A 85, 043604 ] published .

Environment-assisted metrology with spin qubits

P. Cappellaro, G. Goldstein, J. S. Hodges, L. Jiang, J. R. Maze et al.
We investigate the sensitivity of a recently proposed method for precision measurement [Phys. Rev. Lett. 106, 140502 (2011)], focusing on an implementation based on solid-state spin systems. The scheme amplifies a quantum sensor response to weak external fields by exploiting its coupling to spin imp ... [Phys. Rev. A 85, 032336 ] published .

Reconstructing the quantum state of oscillator networks with a single qubit

Tommaso Tufarelli, Alessandro Ferraro, M. S. Kim, and Sougato Bose
We introduce a scheme to reconstruct arbitrary states of networks composed of quantum oscillatorse.g., the motional state of trapped ions or the radiation state of coupled cavities. The scheme involves minimal resources and minimal access, in the sense that it (i) requires only the interaction betwe ... [Phys. Rev. A 85, 032334 ] published .

Time- and frequency-domain polariton interference

G Campbell, M Hosseini, B Sparkes, P Lam, and B Buchler
We present experimental observations of interference between an atomic spin coherence and an optical field in a -type gradient echo memory. The interference is mediated by a strong classical field that couples a weak probe field to the atomic coherence through a resonant Raman transition. Interferen ... [New J. Phys. 14, 033022 (2012)] published Tue Apr 3, 2012.

Effective Hamiltonian for the impurity-impurity interaction in micropillars in the presence of decoherence

J. A. Andrade, A. A. Aligia, and G. F. Quinteiro
We study the optically induced effective interaction between spins in micropillars. Our theoretical model describes a circularly polarized cavity mode strongly coupled to two quantum-dot excitons, each of which interacts with a localized spin. The decoherence effect inducing a finite lifetime to the ... [Phys. Rev. B 85, 165421 ] published .

A steady-state superradiant laser with less than one intracavity photon

Justin G. Bohnet, Zilong Chen, Joshua M. Weiner, Dominic Meiser, Murray J. Holland et al.
The spectral purity of an oscillator is central to many applications, such as detecting gravity waves, defining the second, ground-state cooling and quantum manipulation of nanomechanical objects, and quantum computation. Recent proposals suggest that laser oscillators which use very narrow optical ... [Nature 484, 78 (2012)] published Thu Apr 5, 2012.

Majorization Theory Approach to the Gaussian Channel Minimum Entropy Conjecture

Raul Garcia-Patron, Carlos Navarrete-Benlloch, Seth Lloyd, Jeffrey H. Shapiro, and Nicolas J. Cerf
A long-standing open problem in quantum information theory is to find the classical capacity of an optical communication link, modeled as a Gaussian bosonic channel. It has been conjectured that this capacity is achieved by a random coding of coherent states using an isotropic Gaussian distribution ... [Phys. Rev. Lett. 108, 110505 ] published .

Possible use of a Cooper-pair box for low-dose electron microscopy

Hiroshi Okamoto
A transmission electron microscope that takes advantage of superconducting quantum circuitry is proposed. The microscope is designed to improve the image contrast of radiation-sensitive weak-phase objects, in particular, biological specimens. The objective in this setting is to measure the phase shi ... [Phys. Rev. A 85, 043810 ] published .

Implementation of a three-qubit Toffoli gate in a single step

Ai Min Chen, Sam Young Cho, and Mun Dae Kim
Single-step implementations of multiqubit gates are generally believed to provide a simpler design, a faster operation, and a lower decoherence. For three coupled qubits interacting with a photon field, a realizable scheme for a single-step Toffoli gate is investigated. We find that the three-qubit ... [Phys. Rev. A 85, 032326 ] published .

Physical interpretation of the Wigner rotations and its implications for relativistic quantum information

Pablo Saldanha and Vlatko Vedral
We present a new treatment for the spin of a massive relativistic particle in the context of quantum information based on a physical interpretation of the Wigner rotations, obtaining different results in relation to previous works. We are led to the conclusion that it is not possible to define a red ... [New J. Phys. 14, 023041 (2012)] published Tue Apr 3, 2012.

Measuring Trrho on Single Copies of rho Using Random Measurements

S. J. van Enk and C. W. J. Beenakker
While it is known that Trrho can be measured directly (i.e., without first reconstructing the density matrix) by performing joint measurements on n copies of the same state rho, it is shown here that random measurements on single copies suffice, too. Averaging over the random measurements directly y ... [Phys. Rev. Lett. 108, 110503 ] published .

Critical assessment of two-qubit post-Markovian master equations

S. Campbell, A. Smirne, L. Mazzola, N. Lo Gullo, B. Vacchini et al.
A post-Markovian master equation has been recently proposed as a tool to describe the evolution of a system coupled to a memory-keeping environment [A. Shabani and D. A. Lidar, Phys. Rev. A 71, 020101(R) (2005)]. For a single qubit affected by appropriately chosen environmental conditions, the corre ... [Phys. Rev. A 85, 032120 ] published .

Stochastic wave-function unravelling of the generalized Lindblad equation using correlated states

Mervlyn Moodley, S Paul, and T Nsio Nzundu
We perform a stochastic wave-function unravelling of the generalized Lindblad master equation using correlated states, a combination of the system state vectors and the environment population. The time-convolutionless projection operator method using correlated projection superoperators is applied t ... [Phys. Scr. 85, 045002 (2012)] published Thu Apr 5, 2012.

Generalized toric codes coupled to thermal baths

O Viyuela, A Rivas, and M Martin-Delgado
We have studied the dynamics of a generalized toric code based on qudits at finite temperature by finding the master equation coupling the code's degrees of freedom to a thermal bath. We find that in the case of qutrits, new types of anyons and thermal processes appear that are forbidden for qubits. ... [New J. Phys. 14, 033044 (2012)] published Tue Apr 3, 2012.

Efficient two-step entanglement concentration for arbitrary W states

Yu-Bo Sheng, Lan Zhou, and Sheng-Mei Zhao
We present two two-step practical entanglement concentration protocols for concentrating an arbitrary three-particle less-entangled W state into a maximally entangled W state assisted with single photons. The first protocol uses the linear optics and the second protocol adopts the crossKerr nonlinea ... [Phys. Rev. A 85, 042302 ] published .

Short-time quantum detection: Probing quantum fluctuations

Marco del Rey, Carlos Sabin, and Juan Leon
We study the information provided by a detector click about the state of an initially excited two-level system. By computing the time evolution of the corresponding conditioned probability beyond the rotating-wave approximation, we show that a click in the detector is related to the decay of the sou ... [Phys. Rev. A 85, 045802 ] published .

Cavity approach to variational quantum mechanics

A. Ramezanpour
A local and distributive algorithm is proposed to find an optimal trial wave function minimizing the Hamiltonian expectation in a quantum system. To this end, the quantum state of the system is connected to the Gibbs state of a classical system with the set of couplings playing the role of variation ... [Phys. Rev. B 85, 125131 ] published .

Discord and Nonclassicality in Probabilistic Theories

Paolo Perinotti
Quantum discord quantifies nonclassical correlations in quantum states. We introduce discord for states in causal probabilistic theories, inspired by the original definition proposed by H. Ollivier and W. H. Zurek [Phys. Rev. Lett. 88, 017901 (2001)]. We show that the only probabilistic theory in wh ... [Phys. Rev. Lett. 108, 120502 ] published .

Macroscopic quantum tunneling in quartic and sextic potentials: Application to a phase qubit

N. Didier and F. W. J. Hekking
Macroscopic quantum tunneling of the phase is a fundamental phenomenon in the quantum dynamics of superconducting nanocircuits. The tunneling rate can be controlled in such circuits, where the potential landscape for the phase can be tuned with different external bias parameters. Precise theoretical ... [Phys. Rev. B 85, 104522 ] published .

Synthesis of arbitrary SU(3) transformations of atomic qutrits

Nikolay V. Vitanov
Several scenarios are proposed and analyzed for engineering of arbitrary preselected SU(3) transformations of laser-driven atomic qutrits. Two of the most natural implementations of qutrits are considered, in which the three qutrit states are coupled to each other via two-photon transitions through ... [Phys. Rev. A 85, 032331 ] published .

Measuring geometric quantum discord using one bit of quantum information

G. Passante, O. Moussa, and R. Laflamme
We describe an efficient DQC1 (deterministic quantum computation with one quantum qubit) algorithm to quantify the amount of geometric quantum discord present in the output state of a DQC1 computation. DQC1 is a model of computation that utilizes separable states to solve a problem with no known eff ... [Phys. Rev. A 85, 032325 ] published .

Structure of Reversible Computation Determines the Self-Duality of Quantum Theory

Markus P. Muller and Cozmin Ududec
Predictions for measurement outcomes in physical theories are usually computed by combining two distinct notions: a state, describing the physical system, and an observable, describing the measurement which is performed. In quantum theory, however, both notions are in some sense identical: outcome p ... [Phys. Rev. Lett. 108, 130401 ] published .

Heisenberg-style bounds for arbitrary estimates of shift parameters including prior information

Michael Hall and Howard Wiseman
A rigorous lower bound is obtained for the average resolution of any estimate of a shift parameter, such as an optical phase shift or a spatial translation. The bound has the asymptotic form k/2|G| where G is the generator of the shift (with an arbitrary discrete or continuous spectrum), and hence e ... [New J. Phys. 14, 033040 (2012)] published Tue Apr 3, 2012.

Comparison of dynamical decoupling protocols for a nitrogen-vacancy center in diamond

Zhi-Hui Wang, G. de Lange, D. Riste, R. Hanson, and V. V. Dobrovitski
We perform a detailed theoretical-experimental study of the dynamical decoupling (DD) of the nitrogen-vacancy (NV) center in diamond. We investigate the DD sequences applied to suppress the dephasing of the electron spin of the NV center induced by the coupling to a spin bath composed of the substit ... [Phys. Rev. B 85, 155204 ] published .

Fidelity of a Rydberg-blockade quantum gate from simulated quantum process tomography

X. L. Zhang, A. T. Gill, L. Isenhower, T. G. Walker, and M. Saffman
We present a detailed error analysis of a Rydberg blockade mediated controlled-not quantum gate between two neutral atoms as demonstrated recently in Isenhower et al. [Phys. Rev. Lett. 104, 010503 (2010)] and Zhang et al. [Phys. Rev. A 82, 030306 (2010)]. Numerical solutions of a master equation for ... [Phys. Rev. A 85, 042310 ] published .

Characterizing quantum gates via randomized benchmarking

Easwar Magesan, Jay M. Gambetta, and Joseph Emerson
We describe and expand upon the scalable randomized benchmarking protocol proposed in Phys. Rev. Lett. 106, 180504 (2011) which provides a method for benchmarking quantum gates and estimating the gate dependence of the noise. The protocol allows the noise to have weak time and gate dependence, and w ... [Phys. Rev. A 85, 042311 ] published .

Adiabatic manipulations of Majorana fermions in a three-dimensional network of quantum wires

Bertrand I. Halperin, Yuval Oreg, Ady Stern, Gil Refael, Jason Alicea et al.
It has been proposed that localized zero-energy Majorana states can be realized in a two-dimensional network of quasi-one-dimensional semiconductor wires that are proximity coupled to a bulk superconductor. The wires should have strong spin-orbit coupling with appropriate symmetry, and their electro ... [Phys. Rev. B 85, 144501 ] published .

Universality of the Heisenberg limit for estimates of random phase shifts

Michael J. W. Hall, Dominic W. Berry, Marcin Zwierz, and Howard M. Wiseman
The Heisenberg limit traditionally provides a lower bound on the phase uncertainty scaling as 1/, where is the mean number of photons in the probe. However, this limit has a number of loopholes which potentially might be exploited to achieve measurements with even greater accuracy. Here we close th ... [Phys. Rev. A 85, 041802 ] published .

Optimal control for non-Markovian open quantum systems

Bin Hwang and Hsi-Sheng Goan
An efficient optimal-control theory based on the Krotov method is introduced for a non-Markovian open quantum system with a time-nonlocal master equation in which the control parameter and the bath correlation function are correlated. This optimal-control method is developed via a quantum dissipatio ... [Phys. Rev. A 85, 032321 ] published .

Feshbach projection-operator partitioning for quantum open systems: Stochastic approach

Jun Jing, Lian-Ao Wu, J. Q. You, and Ting Yu
The dynamics of a state of interest coupled to a non-Markovian environment is studied by concatenating the non-Markovian quantum state diffusion equation and the Feshbach projection-operator partitioning technique. An exact one-dimensional stochastic master equation is derived as a general tool for ... [Phys. Rev. A 85, 032123 ] published .

Reducing decoherence in optical and spin transitions in rare-earth-metal-iondoped materials

D. L. McAuslan, J. G. Bartholomew, M. J. Sellars, and J. J. Longdell
In many important situations, the dominant dephasing mechanism in cryogenic rare-earth-metal-iondoped systems is due to magnetic field fluctuations from spins in the host crystal. Operating at a magnetic field where a transition has a zero first-order Zeeman (ZEFOZ) shift can greatly reduce this dep ... [Phys. Rev. A 85, 032339 ] published .

Robust trapped-ion quantum logic gates by continuous dynamical decoupling

A. Bermudez, P. O. Schmidt, M. B. Plenio, and A. Retzker
We introduce a scheme that combines phonon-mediated quantum logic gates in trapped ions with the benefits of continuous dynamical decoupling. We demonstrate theoretically that a strong driving of the qubit decouples it from external magnetic-field noise, enhancing the fidelity of two-qubit quantum g ... [Phys. Rev. A 85, 040302 ] published .

Decoherence-protected quantum gates for a hybrid solid-state spin register

T. van der Sar, Z. H. Wang, M. S. Blok, H. Bernien, T. H. Taminiau et al.
Protecting the dynamics of coupled quantum systems from decoherence by the environment is a key challenge for solid-state quantum information processing. An idle quantum bit (qubit) can be efficiently insulated from the outside world by dynamical decoupling, as has recently been demonstrated for in ... [Nature 484, 82 (2012)] published Thu Apr 5, 2012.

Dynamical Recurrence and the Quantum Control of Coupled Oscillators

Marco G. Genoni, Alessio Serafini, M. S. Kim, and Daniel Burgarth
Controllabilitythe possibility of performing any target dynamics by applying a set of available operationsis a fundamental requirement for the practical use of any physical system. For finite-dimensional systems, such as spin systems, precise criteria to establish controllability, such as the so-cal ... [Phys. Rev. Lett. 108, 150501 ] published .

Spectral properties of high-gain parametric down-conversion

K. Yu. Spasibko, T. Sh. Iskhakov, and M. V. Chekhova
High-gain parametric down-conversion (PDC) is a source of bright squeezed vacuum, which is a macroscopic nonclassical state of light and a promising candidate for quantum information applications. Here we study its properties, such as the intensity spectral width and the spectral width of pairwise c ... [Opt. Express 20, 7507 (2012)] published Thu Mar 29, 2012.

Spin-nematic squeezed vacuum in a quantum gas

C. D. Hamley, C. S. Gerving, T. M. Hoang, E. M. Bookjans, and M. S. Chapman
The standard quantum limit of measurement uncertainty can be surpassed using squeezed states, which minimize the uncertainty product in Heisenbergs relation by reducing the uncertainty of one property at the expense of another. Collisions in ultracold atomic gases have been used to induce quadratur ... [Nature Phys. 8, 305 (2012)] published Tue Apr 3, 2012.

Information gap for classical and quantum communication in a Schwarzschild spacetime

Dominic Hosler, Carsten van de Bruck, and Pieter Kok
Communication between a free-falling observer and an observer hovering above the Schwarzschild horizon of a black hole suffers from Unruh-Hawking noise, which degrades communication channels. Ignoring time dilation, which affects all channels equally, we show that for bosonic communication using sin ... [Phys. Rev. A 85, 042312 ] published .

Resonant energy transfer assisted by off-diagonal coupling

Ning Wu, Ke-Wei Sun, Zhe Chang, and Yang Zhao
Dynamics of resonant energy transfer of a single excitation in a molecular dimer system are studied in the simultaneous presence of diagonal and off-diagonal exciton-phonon coupling. It is found that, at given temperatures, the off-diagonal coupling can enhance both the coherence of the resonant ene ... [J. Chem. Phys. 136, 124513 (2012)] published Fri Mar 30, 2012.

Monolithic Source of Photon Pairs

Rolf Horn, Payam Abolghasem, Bhavin J. Bijlani, Dongpeng Kang, A. S. Helmy et al.
The creation of monolithically integratable sources of single and entangled photons is a top research priority with formidable challenges: The production, manipulation, and measurement of the photons should all occur in the same material platform, thereby fostering stability and scalability. Here we ... [Phys. Rev. Lett. 108, 153605 ] published .

Two-Mode Correlation of Microwave Quantum Noise Generated by Parametric Down-Conversion

N. Bergeal, F. Schackert, L. Frunzio, and M. H. Devoret
In this Letter, we report the observation of the correlation between two modes of microwave radiation resulting from the amplification of quantum noise by the Josephson parametric converter. This process, seen from the pump, can be viewed as parametric down-conversion. The correlation is measured by ... [Phys. Rev. Lett. 108, 123902 ] published .

Decomposition of entanglement entropy in lattice gauge theory

William Donnelly
We consider entanglement entropy between regions of space in lattice gauge theory. The Hilbert space corresponding to a region of space includes edge states that transform nontrivially under gauge transformations. By decomposing the edge states in irreducible representations of the gauge group, the ... [Phys. Rev. D 85, 085004 ] published .

Critical properties and Renyi entropies of the spin-(3/2) XXZ chain

M. Dalmonte, E. Ercolessi, and L. Taddia
We discuss entanglement and critical properties of the spin-3/2 XXZ chain in its entire gapless region. Employing density-matrix renormalization-group calculations combined with different methods based on level spectroscopy, correlation functions, and entanglement entropies, we determine the sound v ... [Phys. Rev. B 85, 165112 ] published .

Deterministic preparation of Dicke states of donor nuclear spins in silicon by cooperative pumping

Yu Luo, Hongyi Yu, and Wang Yao
For donor nuclear spins in silicon, we show how to deterministically prepare various symmetric and asymmetric Dicke states which span a complete basis of the many-body Hilbert space. The state preparation is realized by cooperative pumping of nuclear spins by coupled donor electrons, and the require ... [Phys. Rev. B 85, 155304 ] published .

Real-space entanglement spectrum of quantum Hall states

A. Sterdyniak, A. Chandran, N. Regnault, B. A. Bernevig, and Parsa Bonderson
We investigate the entanglement spectra arising from sharp real-space partitions of the system for quantum Hall states. These partitions differ from the previously utilized orbital and particle partitions and reveal complementary aspects of the physics of these topologically ordered systems. We show ... [Phys. Rev. B 85, 125308 ] published .

Three-sublattice order in the SU(3) Heisenberg model on the square and triangular lattice

Bela Bauer, Philippe Corboz, Andreas M. Lauchli, Laura Messio, Karlo Penc et al.
We present a numerical study of the SU(3) Heisenberg model of three-flavor fermions on the triangular and square lattice by means of the density-matrix renormalization group and infinite projected entangled-pair states. For the triangular lattice we confirm that the ground state has a three-sublatti ... [Phys. Rev. B 85, 125116 ] published .

First-principles investigation of hyperfine interactions for nuclear spin entanglement in photoexcited fullerenes

Vasileia Filidou, Davide Ceresoli, John J. L. Morton, and Feliciano Giustino
The study of hyperfine interactions in optically excited fullerenes has recently acquired importance within the context of nuclear spin entanglement for quantum-information technology. We here report a first-principles pseudopotential study of the hyperfine coupling parameters of optically excited f ... [Phys. Rev. B 85, 115430 ] published .

Vibrational spectra and tripartite entanglement in hydrogen sulfide

Xi-Wen Hou, Jun Huang, and Ming-Fang Wan
In this Brief Report an improved u(2) algebraic model is proposed to study both stretching and bending vibrational spectra of a bent triatomic molecule. The model with fewer parameters is used to reproduce the observed spectra of hydrogen sulfide (HS) with better precision than the u(4) algebraic mo ... [Phys. Rev. A 85, 044501 ] published .

Production of entanglement with highly mixed states

Minsu Kang, M. S. Kim, and Hyunseok Jeong
We study production of entanglement with highly mixed states. We find that entanglement between highly mixed states can be generated via a direct unitary interaction even when both states have purities arbitrarily close to zero. This indicates that purity of a subsystem is not required for entanglem ... [Phys. Rev. A 85, 044301 ] published .

Second-order superposition operations via Hong-Ou-Mandel interference

Su-Yong Lee and Hyunchul Nha
We propose an experimental scheme to implement a second-order nonlocal superposition operation a+eb-hat and its variants by way of Hong-Ou-Mandel interference. The second-order coherent operations enable us to generate a NOON state with high particle number in a heralded fashion and also can be used ... [Phys. Rev. A 85, 043816 ] published .

Collapse-revival dynamics and atom-field entanglement in the nonresonant Dicke model

A. Alvermann, L. Bakemeier, and H. Fehske
We consider the dynamics of atomic and field coherent states in the nonresonant Dicke model. At weak coupling an initial product state evolves into a superposition of multiple field coherent states that are correlated with the atomic configuration. This process is accompanied by the buildup and deca ... [Phys. Rev. A 85, 043803 ] published .

Quantum correlations in continuous-time quantum walks of two indistinguishable particles

Claudia Benedetti, Fabrizio Buscemi, and Paolo Bordone
We evaluate the degree of quantum correlation between two fermions (bosons) subject to continuous-time quantum walks in a one-dimensional (1D) ring lattice with periodic boundary conditions. In our approach, no particle-particle interaction is considered. We show that the interference effects due to ... [Phys. Rev. A 85, 042314 ] published .

Scalable one-way quantum computer using on-chip resonator qubits

Chun-Wang Wu, Ming Gao, Hong-Yi Li, Zhi-Jiao Deng, Hong-Yi Dai et al.
We propose a scalable and robust architecture for one-way quantum computation using coupled networks of superconducting transmission line resonators. In our protocol, quantum information is encoded into the long-lived photon states of the resonators, which have a much longer coherence time than the ... [Phys. Rev. A 85, 042301 ] published .

General hidden-variable model for a two-qubit system

GianCarlo Ghirardi and Raffaele Romano
We generalize Bell's hidden-variable model describing the singlet state of a two-qubit system by extending it to arbitrary states and observables. As in the original work, we assume a uniform, state-independent probability distribution for the hidden variables which are identified with the unit vect ... [Phys. Rev. A 85, 042102 ] published .

Resonant cascaded down-conversion

Christian Weedbrook, Ben Perrett, Karen V. Kheruntsyan, Peter D. Drummond, Raphael C. Pooser et al.
We analyze an optical parametric oscillator (OPO) in which cascaded down-conversion occurs inside a cavity resonant for all modes but the initial pump. Due to the resonant cascade design, the OPO presents two chi-level oscillation thresholds that are therefore much lower than for a chi OPO. This is ... [Phys. Rev. A 85, 033821 ] published .

Quantum teleportation using non-orthogonal entangled channels

Satyabrata Adhikari, A Majumdar, Dipankar Home, A Pan, and P Joshi
We study quantum teleportation with the resource of non-orthogonal qubit states. We first extend the standard teleportation protocol to the case of such states. We investigate how the loss of teleportation fidelity resulting from the use of non-orthogonal states compares to a similar loss of fidelit ... [Phys. Scr. 85, 045001 (2012)] published Thu Apr 5, 2012.

Experimental studies in generation of high-purity photon-pairs using cascaded chi processes in a periodically poled LiNbO ridge-waveguide device

Shin Arahira, Naoto Namekata, Tadashi Kishimoto, and Shuichiro Inoue
We report detailed experimental studies in a single wavelength-band system of correlated photon-pair generation in a 1.5 [mu]m telecommunication wavelength-band using cascaded chi:chi processes, second-harmonic generation, and the following spontaneous parametric down conversion (c-SHG/SPDC), in a p ... [J. Opt. Soc. Am. B 29, 434 (2012)] published Wed Apr 4, 2012.

Photon-subtracted squeezed coherent state: nonclassicality and decoherence in thermal environment

Zhen Wang, Xiang-guo Meng, and Hong-yi Fan
We introduce the photon-subtracted squeezed coherent state (PSSCS), which is theoretically constructed by repeatedly subtracting photons from the squeezed coherent state (SCS) with squeezing parameter r and displacement amplitude alpha=|alpha|e. Employing the normal ordering form of density operator ... [J. Opt. Soc. Am. B 29, 397 (2012)] published Wed Apr 4, 2012.

Realization of a general quantum cloning machine via cavity-assisted interaction

B.-L. Fang, T. Wu, and L. Ye
We present a scheme to realize a general quantum cloning machine via cavity-assisted interaction. First, this scheme can prepare entangled states of three remote trapped atoms including the W state and the GHZ state. Based on this approach, the quantum information carried by a photon pulse or a trap ... [EPL 97, 60002 (2012)] published Wed Mar 28, 2012.

Unification of multiqubit polygamy inequalities

Jeong San Kim
I establish a unified view of polygamy of multiqubit entanglement. I first introduce a two-parameter generalization of the entanglement of assistance, namely, the unified entanglement of assistance for bipartite quantum states, and provide an analytic lower bound in two-qubit systems. I show a broad ... [Phys. Rev. A 85, 032335 ] published .

Localized quadripartite entanglement

Shao-xiong Wu and Chang-shui Yu
In this paper, we show that the average three-tangle of the reduced tripartite density matrix for some quadripartite pure states can be increased by some potential measurements on the fourth subsystem, which means this type of quadripartite entanglement can be localized. In particular, we prove that ... [Phys. Rev. A 85, 032332 ] published .

Entanglement scaling in two-dimensional gapless systems

Hyejin Ju, Ann B. Kallin, Paul Fendley, Matthew B. Hastings, and Roger G. Melko
We numerically determine subleading scaling terms in the ground-state entanglement entropy of several two-dimensional (2D) gapless systems, including a Heisenberg model with Neel order, a free Dirac fermion in the pi-flux phase, and the nearest-neighbor resonating-valence-bond wave function. For the ... [Phys. Rev. B 85, 165121 ] published .

Controlled-X gate with cache function for one-way quantum computation

Heng Shen, Kenan Qu, Weigang Zhang, and Jing Jin
One-way quantum computation by utilizing multipartite cluster states paves the way for the realization of universal quantum computation. However, quantum memory should be implemented in the practical systems to ensure the operations could be timed appropriately. A scheme of the controlled-X gate wit ... [Phys. Rev. A 85, 032317 ] published .

Demonstration of Entanglement of Electrostatically Coupled Singlet-Triplet Qubits

M. D. Shulman, O. E. Dial, S. P. Harvey, H. Bluhm, V. Umansky et al.
Quantum computers have the potential to solve certain problems faster than classical computers. To exploit their power, it is necessary to perform interqubit operations and generate entangled states. Spin qubits are a promising candidate for implementing a quantum processor because of their potentia ... [Science 336, 202 (2012)] published Wed Apr 11, 2012.

Two-Photon Path-Entangled States in Multimode Waveguides

Eilon Poem, Yehonatan Gilead, and Yaron Silberberg
We experimentally show that two-photon path-entangled states can be coherently manipulated by multimode interference in multimode waveguides. By measuring the output two-photon spatial correlation function versus the phase of the input state, we show that multimode waveguides perform as nearly ideal ... [Phys. Rev. Lett. 108, 153602 ] published .

Observable Measure of Bipartite Quantum Correlations

Davide Girolami and Gerardo Adesso
We introduce a measure Q of bipartite quantum correlations for arbitrary two-qubit states, expressed as a state-independent function of the density matrix elements. The amount of quantum correlations can be quantified experimentally by measuring the expectation value of a small set of observables on ... [Phys. Rev. Lett. 108, 150403 ] published .

Is a System's Wave Function in One-to-One Correspondence with Its Elements of Reality?

Roger Colbeck and Renato Renner
Although quantum mechanics is one of our most successful physical theories, there has been a long-standing debate about the interpretation of the wave functionthe central object of the theory. Two prominent views are that (i) it corresponds to an element of reality, i.e., an objective attribute that ... [Phys. Rev. Lett. 108, 150402 ] published .

Quantum Mutual Information Capacity for High-Dimensional Entangled States

P. Ben Dixon, Gregory A. Howland, James Schneeloch, and John C. Howell
High-dimensional Hilbert spaces used for quantum communication channels offer the possibility of large data transmission capabilities. We propose a method of characterizing the channel capacity of an entangled photonic state in high-dimensional position and momentum bases. We use this method to meas ... [Phys. Rev. Lett. 108, 143603 ] published .

Quantum Interference of Single Photons from Remote Nitrogen-Vacancy Centers in Diamond

A. Sipahigil, M. L. Goldman, E. Togan, Y. Chu, M. Markham et al.
We demonstrate quantum interference between indistinguishable photons emitted by two nitrogen-vacancy centers in distinct diamond samples separated by two meters. Macroscopic solid immersion lenses are used to enhance photon collection efficiency. Quantum interference is verified by measuring a valu ... [Phys. Rev. Lett. 108, 143601 ] published .

Information Trade-Offs for Optical Quantum Communication

Mark M. Wilde, Patrick Hayden, and Saikat Guha
Recent work has precisely characterized the achievable trade-offs between three key information processing tasksclassical communication (generation or consumption), quantum communication (generation or consumption), and shared entanglement (distribution or consumption), measured in bits, qubits, and ... [Phys. Rev. Lett. 108, 140501 ] published .

Nonlinear Plasmon-Photon Interaction Resolved by k-Space Spectroscopy

Nicolai B. Grosse, Jan Heckmann, and Ulrike Woggon
Metallic nanostructures support extreme localization and enhancement of optical fields via surface-plasmon (SP) resonances. Although SP are associated with giant enhancements of nonlinear phenomena such as second-harmonic generation (SHG), the role of SP in the process, whether as a field-enhancing ... [Phys. Rev. Lett. 108, 136802 ] published .

Generation of Mesoscopic Entangled States in a Cavity Coupled to an Atomic Ensemble

G. Nikoghosyan, M. J. Hartmann, and M. B. Plenio
We propose a novel scheme for the efficient production of entangled states for N photons of the form |N>|0>+|0>|N> (NOON states) based on the resonant interaction of a pair of quantized cavity modes with an ensemble of atoms. We show that, in the strong-coupling regime, the adiabatic evolution of th ... [Phys. Rev. Lett. 108, 123603 ] published .

Proposal for Inverting the Quantum Cloning of Photons

Sadegh Raeisi, Wolfgang Tittel, and Christoph Simon
We propose an experiment where a photon is first cloned by stimulated parametric down-conversion, making many (imperfect) copies, and then the cloning transformation is inverted, regenerating the original photon while destroying the copies. Focusing on the case where the initial photon is entangled ... [Phys. Rev. Lett. 108, 120404 ] published .

Entangled and Sequential Quantum Protocols with Dephasing

Sergio Boixo and Chris Heunen
Sequences of commuting quantum operators can be parallelized using entanglement. This transformation is behind some optimal quantum metrology protocols and recent results on quantum circuit complexity. We show that dephasing quantum maps in arbitrary dimension can also be parallelized. This implies ... [Phys. Rev. Lett. 108, 120402 ] published .

Entanglement Entropy of Highly Degenerate States and Fractal Dimensions

Olalla A. Castro-Alvaredo and Benjamin Doyon
We consider the bipartite entanglement entropy of ground states of extended quantum systems with a large degeneracy. Often, as when there is a spontaneously broken global Lie group symmetry, basis elements of the lowest-energy space form a natural geometrical structure. For instance, the spins of a ... [Phys. Rev. Lett. 108, 120401 ] published .

Classical dynamics of quantum entanglement

Giulio Casati, Italo Guarneri, and Jose Reslen
We analyze numerically the dynamical generation of quantum entanglement in a system of two interacting particles, started in a coherent separable state, for decreasing values of [h-bar]. As [h-bar]-->0 the entanglement entropy, computed at any finite time, converges to a finite nonzero value. The li ... [Phys. Rev. E 85, 036208 ] published .

Shape evolution of a core-shell spherical particle under hydrostatic pressure

Jerome Colin
The morphological evolution by surface diffusion of a core-shell spherical particle has been investigated theoretically under hydrostatic pressure when the shear modulii of the core and shell are different. A linear stability analysis has demonstrated that depending on the pressure, shear modulii, a ... [Phys. Rev. E 85, 032601 ] published .

Scattering in an environment

Janos Polonyi and Karima Zazoua
The cross section of elastic electron-proton scattering taking place in an electron gas is calculated within the closed time path method by means of the resummation of the one-loop self-energy in the photon propagator. Backreaction of the colliding particles on the gas has been taken into account an ... [Phys. Rev. D 85, 065013 ] published .

Theta vacuum and entanglement interaction in the three-flavor Polyakov-loop extended Nambu-Jona-Lasinio model

Takahiro Sasaki, Junichi Takahashi, Yuji Sakai, Hiroaki Kouno, and Masanobu Yahiro
We investigate theta-vacuum effects on the QCD phase diagram for the realistic 2+1 flavor system, using the three-flavor Polyakov-extended Nambu-Jona-Lasinio (PNJL) model and the entanglement PNJL model as an extension of the PNJL model. The theta-vacuum effects make the chiral transition sharper. F ... [Phys. Rev. D 85, 056009 ] published .

Quantized response and topology of magnetic insulators with inversion symmetry

Ari M. Turner, Yi Zhang, Roger S. K. Mong, and Ashvin Vishwanath
We study three-dimensional insulators with inversion symmetry in which other point group symmetries, such as time reversal, are generically absent. We find that certain information about such materials' behavior is determined by just the eigenvalues under inversion symmetry of occupied states at tim ... [Phys. Rev. B 85, 165120 ] published .

Kondo resonance of a microwave photon

Karyn Le Hur
We emulate renormalization group models, such as the spin-boson Hamiltonian or the anisotropic Kondo model, from a quantum optics perspective by considering a superconducting device. The infrared confinement involves photon excitations of two tunable transmission lines entangled to an artificial spi ... [Phys. Rev. B 85, 140506 ] published .

Correlation length and unusual corrections to entanglement entropy

Elisa Ercolessi, Stefano Evangelisti, Fabio Franchini, and Francesco Ravanini
We study analytically the corrections to the leading terms in the Renyi entropy of a massive lattice theory, showing significant deviations from naive expectations. In particular, we show that finite size and finite mass effects give rise to different contributions (with different exponents) and thu ... [Phys. Rev. B 85, 115428 ] published .

Real-space entanglement spectrum of quantum Hall systems

J. Dubail, N. Read, and E. H. Rezayi
We study the real-space entanglement spectrum for fractional quantum Hall systems, which maintains locality along the spatial cut, and provide evidence that it possesses a scaling property. We also consider the closely related particle entanglement spectrum and carry out the Schmidt decomposition of ... [Phys. Rev. B 85, 115321 ] published .

Quantum phase transition in the Dicke model with critical and noncritical entanglement

L. Bakemeier, A. Alvermann, and H. Fehske
We study the quantum phase transition of the Dicke model in the classical oscillator limit, where it occurs already for finite spin length. In contrast to the classical spin limit, for which spin-oscillator entanglement diverges at the transition, entanglement in the classical oscillator limit remai ... [Phys. Rev. A 85, 043821 ] published .

Tavis-Cummings model beyond the rotating wave approximation: Quasidegenerate qubits

S. Agarwal, S. M. Hashemi Rafsanjani, and J. H. Eberly
The Tavis-Cummings model for more than one qubit interacting with a common oscillator mode is extended beyond the rotating wave approximation (RWA). We explore the parameter regime in which the frequencies of the qubits are much smaller than the oscillator frequency and the coupling strength is allo ... [Phys. Rev. A 85, 043815 ] published .

Classification of multipartite entanglement via negativity fonts

S. Shelly Sharma and N. K. Sharma
Partial transposition of state operator is a well-known tool to detect quantum correlations between two parts of a composite system. In this paper, the global partial transpose (GPT) is linked to conceptually multipartite underlying structures in a statethe negativity fonts. If K-way negativity font ... [Phys. Rev. A 85, 042315 ] published .

Tuning entanglement and ergodicity in two-dimensional spin systems using impurities and anisotropy

Gehad Sadiek, Qing Xu, and Sabre Kais
We consider the entanglement in a two-dimensional XY model in an external magnetic field h. The model consists of a set of seven localized spin-(1/2) particles in a two-dimensional triangular lattice coupled through nearest-neighbor exchange interaction J. We examine the effect of single and double ... [Phys. Rev. A 85, 042313 ] published .

Engineering steady three-atom singlet states via quantum-jump-based feedback

Xiao-Qiang Shao, Tai-Yu Zheng, and Shou Zhang
A scheme is presented for generating steady three-atom singlet states via three V-type atoms interacting with a strongly dissipative two-mode cavity. The local quantum feedback control is applied based on quantum-jump detection to make the target state fidelity as high as possible. This scheme is in ... [Phys. Rev. A 85, 042308 ] published .

Dissipative preparation of entangled states between two spatially separated nitrogen-vacancy centers

Peng-Bo Li, Shao-Yan Gao, Hong-Rong Li, Sheng-Li Ma, and Fu-Li Li
We present an efficient scheme for the generation of entangled states of two spatially separated nitrogen-vacancy (NV) centers with two whispering-gallery-mode (WGM) microresonators, which are coupled either by an optical fiber-taper waveguide or by the evanescent fields of the WGM. We show that the ... [Phys. Rev. A 85, 042306 ] published .

Entanglement and the interplay between staggered fields and couplings

Jenny Hide, Yoshifumi Nakata, and Mio Murao
We investigate how the interplay between a staggered magnetic field and staggered coupling strength affects both ground-state and thermal entanglement. Upon analytically calculating thermodynamic quantities and the correlation functions for such a system, we consider both the global Meyer-Wallach me ... [Phys. Rev. A 85, 042303 ] published .

Unification of different views of decoherence and discord

Patrick J. Coles
Macroscopic behavior such as the lack of interference patterns has been attributed to decoherence, a word with several possible definitions such as (1) the loss of off-diagonal density matrix elements, (2) the flow of information to the environment, (3) the loss of complementary information, and (4) ... [Phys. Rev. A 85, 042103 ] published .

Correlations in excited states of local Hamiltonians

Jianxin Chen, Zhengfeng Ji, Zhaohui Wei, and Bei Zeng
Physical properties of the ground and excited states of a k-local Hamiltonian are largely determined by the k-particle reduced density matrices (k-RDMs), or simply the k-matrix for fermionic systemsthey are at least enough for the calculation of the ground-state and excited-state energies. Moreover, ... [Phys. Rev. A 85, 040303 ] published .

Quantumness of correlations revealed in local measurements exceeds entanglement

Marco Piani and Gerardo Adesso
We analyze a family of measures of general quantum correlations for composite systems, defined in terms of the bipartite entanglement necessarily created between systems and apparatuses during local measurements. For every entanglement monotone E, this operational correspondence provides a different ... [Phys. Rev. A 85, 040301 ] published .

Fast generation of many-atom maximal entanglement in spin-1 Bose-Einstein condensates

GuangSi Meng, HuiMing Xu, Song Yang, ShengLi Zhang, GuangCan Guo et al.
We propose a scheme for generating many-atom maximal entanglement states in spin-1 Bose-Einstein condensates. By applying a far-off-resonate blue-detuned linear-polarized laser to the condensates, the hyperfine level F=1,m=0 ground state and the hyperfine F=0 excited state are conveniently coupled. ... [Phys. Rev. A 85, 035601 ] published .

Bosonic, fermionic, and anyonic symmetry in two-photon random scattering

M. P. van Exter, J. Woudenberg, H. Di Lorenzo Pires, and W. H. Peeters
We experimentally demonstrate the importance of two-photon symmetry for the propagation of spatial quantum correlations in a multiple-scattering disordered system. Two distinguishable entangled photons with tunable spatial exchange symmetry are sent through the scattering medium. The two photons are ... [Phys. Rev. A 85, 033823 ] published .

Continuous-mode effects and photon-photon phase gate performance

Bing He and Artur Scherer
The effects arising from the inherent continuous-mode nature of photonic pulses were poorly understood but significantly influence the performance of quantum devices employing photonic pulse interaction in nonlinear media. Such effects include the entanglement between the continuous wave-vector mode ... [Phys. Rev. A 85, 033814 ] published .

Creation of two vortex-entangled beams in a vortex-beam collision with a plane wave

Igor P. Ivanov
The physics of photons and electrons carrying orbital angular momentum (OAM) is an exciting field of research in quantum optics and electron microscopy. Usually, one considers propagation of these vortex beams in a medium or an external field and their absorption or scattering on fixed targets. Here ... [Phys. Rev. A 85, 033813 ] published .

Discrete symmetry breaking transitions between paired superfluids

M. J. Bhaseen, S. Ejima, F. H. L. Essler, H. Fehske, M. Hohenadler et al.
We explore the zero-temperature phase diagram of bosons interacting via Feshbach resonant pairing interactions in one dimension. Using DMRG (density matrix renormalization group) and field theory techniques we characterize the phases and quantum phase transitions in this low-dimensional setting. We ... [Phys. Rev. A 85, 033636 ] published .

Nonentangling channels for multiple collisions of quantum wave packets

Walter Hahn and Boris V. Fine
We consider multiple collisions of quantum wave packets in one dimension. The system under investigation consists of an impenetrable wall and two hard-core particles with very different masses. The lighter particle bounces between the heavier one and the wall. Both particles are initially represente ... [Phys. Rev. A 85, 032713 ] published .

Polarization-based entanglement swapping at the telecommunication wavelength using spontaneous parametric down-conversion photon-pair sources

Yinghong Xue, Akio Yoshizawa, and Hidemi Tsuchida
We experimentally demonstrated polarization-based entanglement swapping using spatially separated periodically poled lithium niobate waveguides that emit polarization-entangled photon pairs by spontaneous parametric down-conversion (SPDC). Entanglement swapping using such SPDC sources was theoretica ... [Phys. Rev. A 85, 032337 ] published .

Creating multimode squeezed states and Greenberger-Horne-Zeilinger entangled states using atomic coherent effects

Xiao Liang, Xiangming Hu, and Chang He
We propose a scalable scheme for a unitary multimode operator using an optical cavity with an atomic ensemble. We exemplify three-mode and four-mode cases and engineer the squeeze operators that are decoupled from the atomic degrees of freedom. The squeeze parameters can be large since they are prop ... [Phys. Rev. A 85, 032329 ] published .

Directed percolation effects emerging from superadditivity of quantum networks

L. Czekaj, R. W. Chhajlany, and P. Horodecki
Entanglement-induced nonadditivity of classical communication capacity in networks consisting of quantum channels is considered. Communication lattices consisting of butterfly-type entanglement-breaking channels augmented, with some probability, by identity channels are analyzed. The capacity supera ... [Phys. Rev. A 85, 032328 ] published .

Measurement-induced two-qubit entanglement in a bad cavity: Fundamental and practical considerations

Brian Julsgaard and Klaus Molmer
An entanglement-generating protocol is described for two qubits coupled to a cavity field in the bad-cavity limit. By measuring the amplitude of a field transmitted through the cavity, an entangled spin-singlet state can be established probabilistically. Both fundamental limitations and practical me ... [Phys. Rev. A 85, 032327 ] published .

Superoperator analysis of entanglement in a four-qubit cluster state

Yaakov S. Weinstein, Jay Feldman, Jacob Robins, Jason Zukus, and Gerald Gilbert
In this paper we utilize superoperator formalism to explore the entanglement evolution of four-qubit cluster states in a number of decohering environments. A four-qubit cluster state is a resource for the performance of an arbitrary single-logical-qubit rotation via measurement-based cluster-state q ... [Phys. Rev. A 85, 032324 ] published .

Scalable continuous-variable entanglement of optical fields via concurrent interactions with separated atomic ensembles

Hong Sun and Xiangming Hu
It has been known that one reservoir of driven two-level atoms can establish two-mode interactions and generate bipartite continuous-variable entangled light. Here we show that three-mode interactions can be created by combining two such reservoirs, each of which interacts with two adjacent fields i ... [Phys. Rev. A 85, 032320 ] published .

Multipartite entanglement generation assisted by inhomogeneous coupling

C. E. Lopez, F. Lastra, G. Romero, E. Solano, and J. C. Retamal
We show that controllable inhomogeneous coupling between two-level systems and a common data bus provides a fast mechanism to produce multipartite entanglement. Our proposal combines resonant interactions and engineering of coupling strengthsbetween the qubits and the single modeleading to well-defi ... [Phys. Rev. A 85, 032319 ] published .

Revival of quantum correlations without system-environment back-action

R. Lo Franco, B. Bellomo, E. Andersson, and G. Compagno
Revivals of quantum correlations have often been explained in terms of back-action on quantum systems by their quantum environment(s). Here we consider a system of two independently evolving qubits, each locally interacting with a classical random external field. The environments of the qubits are a ... [Phys. Rev. A 85, 032318 ] published .

Experimental analysis of the quantum complementarity principle

R. Auccaise, R. M. Serra, J. G. Filgueiras, R. S. Sarthour, I. S. Oliveira et al.
One of the milestones of quantum mechanics is Bohr's complementarity principle. It states that a single quantum can exhibit a particlelike or a wavelike behavior, but never both at the same time. These are mutually exclusive and complementary aspects of the quantum system. This means that we need di ... [Phys. Rev. A 85, 032121 ] published .

Bilocal versus nonbilocal correlations in entanglement-swapping experiments

Cyril Branciard, Denis Rosset, Nicolas Gisin, and Stefano Pironio
Entanglement swapping is a process by which two initially independent quantum systems can become entangled and generate nonlocal correlations. To characterize such correlations, we compare them to those predicted by bilocal models, where systems that are initially independent are described by uncorr ... [Phys. Rev. A 85, 032119 ] published .

Dynamics of Gaussian discord between two oscillators interacting with a common environment

Jose Nahuel Freitas and Juan Pablo Paz
We analyze the evolution of the Gaussian discord between two resonant harmonic oscillators coupled to a common environment. For this, we use the same tools we applied before to fully characterize the evolution of the entanglement in this system [Paz and Roncaglia, Phys. Rev. Lett. 100, 220401 (2008) ... [Phys. Rev. A 85, 032118 ] published .

Passive optical switching of photon pairs using a spontaneous parametric fiber loop

Ningbo Zhao, Lei Yang, and Xiaoying Li
We study a novel scheme named the spontaneous parametric fiber loop (SPFL), configured by deliberately introducing dispersive elements into the nonlinear Sagnac loop, and show it can function as a passive switch of photon pairs. The two-photon state coming out of SPFL highly depends on the dispersio ... [Opt. Lett. 37, 1220 (2012)] published Thu Apr 5, 2012.

Experimental realization of a four-photon seven-qubit graph state for one-way quantum computation

Sang Min Lee, Hee Su Park, Jaeyoon Cho, Yoonshik Kang, Jae Yong Lee et al.
We propose and demonstrate the scaling up of photonic graph states through path qubit fusion. Two path qubits from separate two-photon four-qubit states are fused to generate a two-dimensional seven-qubit graph state composed of polarization and path qubits. Genuine seven-qubit entanglement is verif ... [Opt. Express 20, 6915 (2012)] published Thu Mar 29, 2012.

Beyond pure state entanglement for atomic ensembles

Julia Stasiska, Simone Paganelli, and Anna Sanpera
We analyze multipartite entanglement between atomic ensembles within quantum matterlight interfaces. In our proposal, a polarized light beam crosses sequentially several polarized atomic ensembles impinging on each of them at a given angle . These angles are crucial parameters for shaping the entang ... [New J. Phys. 14, 033034 (2012)] published Tue Apr 3, 2012.

Optimal linear optical implementation of a single-qubit damping channel

Kent Fisher, Robert Prevedel, Rainer Kaltenbaek, and Kevin Resch
We experimentally demonstrate a single-qubit decohering quantum channel using linear optics. We implement the channel, whose special cases include the familiar amplitude-damping channel and the bit-flip channel, using a single, static optical setup. Following a recent theoretical result (Piani et al ... [New J. Phys. 14, 033016 (2012)] published Tue Apr 3, 2012.

Quantum entanglement distribution using a magnetic field sensor

M Schaffry, S Benjamin, and Y Matsuzaki
Sensors based on crystal defects, especially nitrogen vacancy (NV) centres in nanodiamond, can achieve detection of single magnetic moments. Here, we show that this exquisite control can be utilized to entangle remote electronic spins for applications in quantum computing; the mobile sensor provides ... [New J. Phys. 14, 023046 (2012)] published Tue Apr 3, 2012.

Falsification of Leggett's model using neutron matter waves

Yuji Hasegawa, Claus Schmitzer, Hannes Bartosik, Jrgen Klepp, Stephan Sponar et al.
According to Bell's theorem, no theory based on the joint assumption of realism and locality can reproduce certain predictions of quantum mechanics. Another class of realistic models, proposed by Leggett, that demands realism but abandons reliance on locality, is predicted to be in conflict with qua ... [New J. Phys. 14, 023039 (2012)] published Tue Apr 3, 2012.

Experimental characterization of photonic fusion using fiber sources

B Bell, A Clark, M Tame, M Halder, J Fulconis et al.
We report on the fusion of photons from two independent photonic crystal fiber sources into polarization entangled states using a fiber-based polarizing beamsplitter. We achieve fidelities of up to F=0.740.01 with respect to the maximally entangled Bell state [Formula: see text] using a low pump pow ... [New J. Phys. 14, 023021 (2012)] published Tue Apr 3, 2012.

An elementary quantum network of single atoms in optical cavities

Stephan Ritter, Christian Nlleke, Carolin Hahn, Andreas Reiserer, Andreas Neuzner et al.
Quantum networks are distributed quantum many-body systems with tailored topology and controlled information exchange. They are the backbone of distributed quantum computing architectures and quantum communication. Here we present a prototype of such a quantum network based on single atoms embedded ... [Nature 484, 195 (2012)] published Thu Apr 12, 2012.

Heralded quantum entanglement between two crystals

Imam Usmani, Christoph Clausen, Flix Bussires, Nicolas Sangouard, Mikael Afzelius et al.
Quantum networks must have the crucial ability to entangle quantum nodes. A prominent example is the quantum repeater, which allows the distance barrier of direct transmission of single photons to be overcome, provided remote quantum memories can be entangled in a heralded fashion. Here, we report t ... [Nat. Photonics 6, 234 (2012)] published Tue Apr 3, 2012.

Observation of eight-photon entanglement

Xing-Can Yao, Tian-Xiong Wang, Ping Xu, He Lu, Ge-Sheng Pan et al.
The creation of increasingly large multipartite entangled states is not only a fundamental scientific endeavour in itself, but is also the enabling technology for quantum information. Tremendous experimental effort has been devoted to generating multiparticle entanglement with a growing number of qu ... [Nat. Photonics 6, 225 (2012)] published Tue Apr 3, 2012.

Macroscopic thermal entanglement in a spin chain caused by the magnetic field: Inhomogeneity effect

A. A. Zvyagin
The influence of the inhomogeneity on the macroscopic thermal pairwise entanglement for the system of coupled spins 1/2 (qubits) has been studied. The most important effect of the inhomogeneity on the thermal entanglement is in the new role of the external potential (magnetic field), which can produ ... [Low Temp. Phys. 38, 210 (2012)] published Tue Mar 27, 2012.

Noise-induced quantum correlations via quantum feedback control

Jie Song, Yan Xia, and Xiu-Dong Sun
We study a physical system for creation of quantum correlations by quantum feedback control. The system is composed of two three-level atoms, each of which interacts with one of two coupled cavities. By appropriately choosing the feedback strength, we show that an arbitrary initial state can be dete ... [J. Opt. Soc. Am. B 29, 268 (2012)] published Wed Apr 4, 2012.

Towards a state minimizing the output entropy of a tensor product of random quantum channels

Benoit Collins, Motohisa Fukuda, and Ion Nechita
We consider the image of some classes of bipartite quantum states under a tensor product of random quantum channels. Depending on natural assumptions that we make on the states, the eigenvalues of their outputs have new properties which we describe. Our motivation is provided by the additivity quest ... [J. Math. Phys. 53, 032203 (2012)] published Wed Mar 21, 2012.

Universal logarithmic terms in the entanglement entropy of 2d, 3d and 4d random transverse-field Ising models

I. Kovcs and F. Igli
The entanglement entropy of the random transverse-field Ising model is calculated by a numerical implementation of the asymptotically exact strong disorder renormalization group method in 2d, 3d and 4d hypercubic lattices for different shapes of the subregion. We find that the area law is always sat ... [EPL 97, 67009 (2012)] published Wed Mar 28, 2012.

Direct naked-eye detection of chiral and Faraday effects in white light

G. Ropars, A. Le Floch, J. Enoch, and V. Lakshminarayanan
We demonstrate that the human eye is able to detect the optical activity of chiral molecules and the Faraday effect, even under white-light viewing conditions, without the help of any polarizer. Indeed, we show that our eye acts as a differential analyzer and isolates the response in the blue part o ... [EPL 97, 64002 (2012)] published Wed Mar 28, 2012.

Mixed-state quantum transport in correlated spin networks

Ashok Ajoy and Paola Cappellaro
Quantum spin networks can be used to transport information between separated registers in a quantum-information processor. To find a practical implementation, the strict requirements of ideal models for perfect state transfer need to be relaxed, allowing for complex coupling topologies and general i ... [Phys. Rev. A 85, 042305 ] published .

Topological superconductivity and Majorana fermions in metallic surface states

Andrew C. Potter and Patrick A. Lee
Heavy metals, such as Au, Ag, and Pb, often have sharp surface states that are split by strong Rashba spin-orbit coupling. The strong spin-orbit coupling and two-dimensional nature of these surface states make them ideal platforms for realizing topological superconductivity and Majorana fermions. In ... [Phys. Rev. B 85, 094516 ] published .

Bose-Hubbard model in a strong effective magnetic field: Emergence of a chiral Mott insulator ground state

Arya Dhar, Maheswar Maji, Tapan Mishra, R. V. Pai, Subroto Mukerjee et al.
Motivated by experiments on Josephson junction arrays, and cold atoms in an optical lattice in a synthetic magnetic field, we study the fully frustrated Bose-Hubbard model with half a magnetic flux quantum per plaquette. We obtain the phase diagram of this model on a two-leg ladder at integer fillin ... [Phys. Rev. A 85, 041602 ] published .

Topological superfluid in one-dimensional spin-orbit-coupled atomic Fermi gases

Xia-Ji Liu and Hui Hu
We investigate theoretically the prospect of realizing a topological superfluid in one-dimensional spin-orbit-coupled atomic Fermi gases under a Zeeman field in harmonic traps. In the absence of spin-orbit coupling, it is well known that the system is either a Bardeen-Cooper-Schrieffer superfluid or ... [Phys. Rev. A 85, 033622 ] published .

A novel, robust quantum detection scheme

B Hemmerling, F Gebert, Y Wan, and P Schmidt
Protocols used in quantum information and precision spectroscopy rely on efficient internal quantum state discrimination. With a single ion in a linear Paul trap, we implement a novel detection method which utilizes correlations between two detection events with an intermediate spin-flip. The techni ... [New J. Phys. 14, 023043 (2012)] published Tue Apr 3, 2012.

Implementation of two-way free space quantum key distribution

Mohd Fared bin Abdul Khir, Mohd Norzaliman Mohd Zain, Suryadi Soekardjo, Suhairi Saharudin, and Sahbudin Shaari
We report an implementation of a two-way, four-state quantum key distribution (QKD) protocol (namely, the LM05 protocol) over a free space medium. A stable quantum bit error rate of 3.34% and maximum tolerable channel loss for secure key generation of 5.68 dB in the absence of a photon number splitt ... [Opt. Eng. 51, 045006 (2012)] published Fri Apr 6, 2012.

Phase encoding schemes for measurement-device-independent quantum key distribution with basis-dependent flaw

Kiyoshi Tamaki, Hoi-Kwong Lo, Chi-Hang Fred Fung, and Bing Qi
In this paper, we study the unconditional security of the so-called measurement-device-independent quantum key distribution (MDIQKD) with the basis-dependent flaw in the context of phase encoding schemes. We propose two schemes for the phase encoding: The first one employs a phase locking technique ... [Phys. Rev. A 85, 042307 ] published .

Quantum anonymous voting for continuous variables

Lang Jiang, Guangqiang He, Ding Nie, Jin Xiong, and Guihua Zeng
A series of quantum voting protocols of continuous variables is proposed. Three methods are employed to ensure that the quantum voting obeys some desirable rules. Entanglement is used to keep voters away from the voting results. We enable voters to operate identically to represent the same vote to p ... [Phys. Rev. A 85, 042309 ] published .

Measurement-Device-Independent Quantum Key Distribution

Hoi-Kwong Lo, Marcos Curty, and Bing Qi
How to remove detector side channel attacks has been a notoriously hard problem in quantum cryptography. Here, we propose a simple solution to this problemmeasurement-device-independent quantum key distribution (QKD). It not only removes all detector side channels, but also doubles the secure distan ... [Phys. Rev. Lett. 108, 130503 ] published .

Information communicated by entangled photon pairs

Thomas Brougham and Stephen M. Barnett
A key goal of quantum communication is to determine the maximum number of bits shared between two quantum systems. An important example of this is in entanglement-based quantum key distribution (QKD) schemes. A realistic treatment of this general communication problem must take account of the nonide ... [Phys. Rev. A 85, 032322 ] published .

2 GHz clock quantum key distribution over 260 km of standard telecom fiber

Shuang Wang, Wei Chen, Jun-Fu Guo, Zhen-Qiang Yin, Hong-Wei Li et al.
We report a demonstration of quantum key distribution (QKD) over a standard telecom fiber exceeding 50 dB in loss and 250 km in length. The differential phase shift QKD protocol was chosen and implemented with a 2 GHz system clock rate. By careful optimization of the 1 bit delayed FaradayMichelson i ... [Opt. Lett. 37, 1008 (2012)] published Thu Mar 29, 2012.

Side-Channel-Free Quantum Key Distribution

Samuel L. Braunstein and Stefano Pirandola
Quantum key distribution (QKD) offers the promise of absolutely secure communications. However, proofs of absolute security often assume perfect implementation from theory to experiment. Thus, existing systems may be prone to insidious side-channel attacks that rely on flaws in experimental implemen ... [Phys. Rev. Lett. 108, 130502 ] published .

A quantum relay chip based on telecommunication integrated optics technology

A Martin, O Alibart, M De Micheli, D Ostrowsky, and S Tanzilli
We investigate an integrated optical circuit on lithium niobate designed to implement a teleportation-based quantum relay scheme for one-way quantum communication at a telecom wavelength. Such an advanced quantum circuit merges for the first time both opticaloptical and electro-optical nonlinear fun ... [New J. Phys. 14, 025002 (2012)] published Tue Apr 3, 2012.

Quantum limits for the magnetic sensitivity of a chemical compass

Jianming Cai, Filippo Caruso, and Martin B. Plenio
The chemical compass model, based on radical pair reactions, is an idea to explain avian magnetoreception. At present, to the best of our knowledge, questions concerning the key ingredients responsible for the high sensitivity of a chemical compass and the possible role of quantum coherence and deco ... [Phys. Rev. A 85, 040304 ] published .

A 2D Quantum Walk Simulation of Two-Particle Dynamics

Andreas Schreiber, Aurl Gbris, Peter P. Rohde, Kaisa Laiho, Martin Stefank et al.
Multidimensional quantum walks can exhibit highly nontrivial topological structure, providing a powerful tool for simulating quantum information and transport systems. We present a flexible implementation of a two-dimensional (2D) optical quantum walk on a lattice, demonstrating a scalable quantum w ... [Science 336, 55 (2012)] published Tue Apr 3, 2012.

Extracting Dynamical Equations from Experimental Data is NP Hard

Toby S. Cubitt, Jens Eisert, and Michael M. Wolf
The behavior of any physical system is governed by its underlying dynamical equations. Much of physics is concerned with discovering these dynamical equations and understanding their consequences. In this Letter, we show that, remarkably, identifying the underlying dynamical equation from any amount ... [Phys. Rev. Lett. 108, 120503 ] published .

Experimental demonstration of quantum gain in a zero-sum game

C Zu, Y-X Wang, X-Y Chang, Z-H Wei, S-Y Zhang et al.
We propose and experimentally demonstrate a zero-sum game that is in a fair Nash equilibrium for classical players, but has the property that a quantum player can always win using an appropriate strategy. The gain of the quantum player is measured experimentally for different quantum strategies and ... [New J. Phys. 14, 033002 (2012)] published Tue Apr 3, 2012.

Using quantum state protection via dissipation in a quantum-dot molecule to solve the Deutsch problem

M. M. Santos, F. O. Prado, H. S. Borges, A. M. Alcalde, J. M. Villas-Boas et al.
The wide set of control parameters and reduced size scale make semiconductor quantum dots attractive candidates to implement solid-state quantum computation. Considering an asymmetric double quantum dot coupled by tunneling, we combine the action of a laser field and the spontaneous emission of the ... [Phys. Rev. A 85, 032323 ] published .

Memory cost of quantum protocols

Alessandro Bisio, Giacomo Mauro D'Ariano, Paolo Perinotti, and Michal Sedlak
We consider the problem of minimizing the ancillary systems required to realize an arbitrary strategy of a quantum protocol, with the assistance of classical memory. For this purpose we introduce the notion of memory cost of a strategy to measure the resources required in terms of ancillary dimensio ... [Phys. Rev. A 85, 032333 ] published .

Parallel Information Transfer in a Multinode Quantum Information Processor

T. W. Borneman, C. E. Granade, and D. G. Cory
We describe a method for coupling disjoint quantum bits (qubits) in different local processing nodes of a distributed node quantum information processor. An effective channel for information transfer between nodes is obtained by moving the system into an interaction frame where all pairs of cross-no ... [Phys. Rev. Lett. 108, 140502 ] published .

Quantum Factorization of 143 on a Dipolar-Coupling Nuclear Magnetic Resonance System

Nanyang Xu, Jing Zhu, Dawei Lu, Xianyi Zhou, Xinhua Peng et al.
Quantum algorithms could be much faster than classical ones in solving the factoring problem. Adiabatic quantum computation for this is an alternative approach other than Shor's algorithm. Here we report an improved adiabatic factoring algorithm and its experimental realization to factor the number ... [Phys. Rev. Lett. 108, 130501 ] published .

Implementing Quantum Gates by Optimal Control with Doubly Exponential Convergence

Pierre de Fouquieres
We introduce a novel algorithm for the task of coherently controlling a quantum mechanical system to implement any chosen unitary dynamics. It performs faster than existing state of the art methods by 1 to 3 orders of magnitude (depending on which one we compare to), particularly for quantum informa ... [Phys. Rev. Lett. 108, 110504 ] published .

Computational power and correlation in a quantum computational tensor network

Keisuke Fujii and Tomoyuki Morimae
We investigate relationships between computational power and correlation in resource states for quantum computational tensor network, which is a general framework for measurement-based quantum computation. We find that if the size of resource states is finite, not all resource states allow correct p ... [Phys. Rev. A 85, 032338 ] published .

Universal two-body-Hamiltonian quantum computing

Daniel Nagaj
We present a Hamiltonian quantum-computation scheme universal for quantum computation. Our Hamiltonian is a sum of a polynomial number (in the number of gates L in the quantum circuit) of constant-norm, time-independent, two-body interaction terms. Furthermore, each qubit in the system interacts onl ... [Phys. Rev. A 85, 032330 ] published .

Relativistic quantum chemistry on quantum computers

Libor Veis, Jakub Visnak, Timo Fleig, Stefan Knecht, Trond Saue et al.
The past few years have witnessed a remarkable interest in the application of quantum computing for solving problems in quantum chemistry more efficiently than classical computers allow. Very recently, proof-of-principle experimental realizations have been reported. However, so far only the nonrelat ... [Phys. Rev. A 85, 030304 ] published .

Experimental simulation and limitations of quantum walks with trapped ions

R Matjeschk, Ch Schneider, M Enderlein, T Huber, H Schmitz et al.
We examine the prospects of discrete quantum walks (QWs) with trapped ions. In particular, we analyze in detail the limitations of the protocol of Travaglione and Milburn (2002 Phys. Rev. A 65 032310) that has been implemented by several experimental groups in recent years. Based on the first realiz ... [New J. Phys. 14, 035012 (2012)] published Tue Apr 3, 2012.

Stability of the k=3 ReadRezayi state in chiral two-dimensional systems with tunable interactions

D Abanin, Z Papi, Y Barlas, and R Bhatt
The k=3 ReadRezayi (RR) parafermion quantum Hall state hosts non-Abelian excitations which provide a platform for universal topological quantum computation. Although the RR state may be realized at the filling factor =12/5 in GaAs-based two-dimensional electron systems, the corresponding quantum Hal ... [New J. Phys. 14, 025009 (2012)] published Tue Apr 3, 2012.

Coherent cavity networks with complete connectivity

Elica Kyoseva, Almut Beige, and Leong Kwek
When cavity photons couple to an optical fiber with a continuum of modes, they usually leak out within a finite amount of time. However, if the fiber is about 1m long and linked to a mirror, photons bounce back and forth within the fiber on a much faster time scale. As a result, dynamical decoupling ... [New J. Phys. 14, 023023 (2012)] published Tue Apr 3, 2012.