Virtual Journal of Quantum Information

Size dependent biexciton binding energies in GaN quantum dots

S. Amloy, K. H. Yu, K. F. Karlsson, R. Farivar, T. G. Andersson et al.
Single GaN/Al(Ga)N quantum dots (QDs) have been investigated by means of microphotoluminescence. Emission spectra related to excitons and biexcitons have been identified by excitation power dependence and polarization resolved spectroscopy. All investigated dots exhibit a strong degree of linear pol ... [Appl. Phys. Lett. 99, 251903 (2011)] published Tue Dec 20, 2011.

Strong coupling between distant photonic nanocavities and its dynamic control

Yoshiya Sato, Yoshinori Tanaka, Jeremy Upham, Yasushi Takahashi, Takashi Asano et al.
The formation of a coupled state among on-chip, photonic nanostructures at arbitrary positions as well as its dynamic control are important in the realization of next-generation photonic circuits with elevated functionality such as stopping/slowing of light and quantum information processing. Here, ... [Nat. Photonics 6, 56 (2011)] published Thu Dec 22, 2011.

Atomic quantum state transferring and swapping via quantum Zeno dynamics

Zhi-Cheng Shi, Yan Xia, Jie Song, and He-Shan Song
In this paper, we first demonstrate how to realize quantum state transferring (QST) from one atom to another based on quantum Zeno dynamics. Then, the QST protocol is generalized to realize the quantum state swapping (QSS) between two arbitrary atoms with the help of a third one. Furthermore, we als ... [J. Opt. Soc. Am. B 28, 2909 (2011)] published Fri Dec 23, 2011.

Orthogonality catastrophe as a consequence of qubit embedding in an ultracold Fermi gas

J. Goold, T. Fogarty, N. Lo Gullo, M. Paternostro, and Th. Busch
We investigate the behavior of a two-level atom coupled to a one-dimensional, ultracold Fermi gas. The sudden switching on of the scattering between the two entities leads to the loss of any coherence in the initial state of the impurity and we show that the exact dynamics of this process is strongl ... [Phys. Rev. A 84, 063632 ] published .

Perfect spin-filter and highly spin-polarized current in a quantum network device

Hua-Hua Fu and Kai-Lun Yao
We report a design to realize perfect spin-filter and highly spin-polarized current in a quantum network device composed of multiple quantum dots (QDs). By producing an energy difference between the site energy of the upper QDs and down ones, the linear conductance spectra present well-defined insul ... [Appl. Phys. Lett. 100, 013502 (2012)] published Tue Jan 3, 2012.

Experimental implementation of unambiguous quantum reading

Michele Dall'Arno, Alessandro Bisio, Giacomo Mauro D'Ariano, Martina Mikova, Miroslav Jezek et al.
We provide the optimal strategy for the unambiguous quantum reading of optical memories, namely when perfect retrieving of information is achieved probabilistically, for the case where noise and loss are negligible. We describe the experimental quantum optical implementations, and provide experiment ... [Phys. Rev. A 85, 012308 ] published .

Coherent control of three-spin states in a triple quantum dot

L. Gaudreau, G. Granger, A. Kam, G. C. Aers, S. A. Studenikin et al.
Spin qubits involving individual spins in single quantum dots or coupled spins in double quantum dots have emerged as potential building blocks for quantum information processing applications. It has been suggested that triple quantum dots may provide additional tools and functionalities. These inc ... [Nature Phys. 8, 54 (2011)] published Thu Dec 22, 2011.

Dicke Quantum Spin Glass of Atoms and Photons

Philipp Strack and Subir Sachdev
Recent studies of strongly interacting atoms and photons in optical cavities have rekindled interest in the Dicke model of atomic qubits coupled to discrete photon cavity modes. We study the multimode Dicke model with variable atom-photon couplings. We argue that a quantum spin-glass phase can appea ... [Phys. Rev. Lett. 107, 277202 ] published .

Efficient Clocked Electron Transfer on Superfluid Helium

F. R. Bradbury, Maika Takita, T. M. Gurrieri, K. J. Wilkel, Kevin Eng et al.
Unprecedented transport efficiency is demonstrated for electrons on the surface of micron-scale superfluid helium-filled channels by co-opting silicon processing technology to construct the equivalent of a charge-coupled device. Strong fringing fields lead to undetectably rare transfer failures afte ... [Phys. Rev. Lett. 107, 266803 ] published .

Coherence in a transmon qubit with epitaxial tunnel junctions

Martin P. Weides, Jeffrey S. Kline, Michael R. Vissers, Martin O. Sandberg, David S. Wisbey et al.
We developed transmon qubits based on epitaxial tunnel junctions and interdigitated capacitors. This multileveled qubit, patterned by use of all-optical lithography, is a step towards scalable qubits with a high integration density. The relaxation time T is 0.720.86 [mu]s and the ensemble dephasing ... [Appl. Phys. Lett. 99, 262502 (2011)] published Tue Dec 27, 2011.

Spin Relaxation near the Metal-Insulator Transition: Dominance of the Dresselhaus Spin-Orbit Coupling

Guido A. Intronati, Pablo I. Tamborenea, Dietmar Weinmann, and Rodolfo A. Jalabert
We identify the Dresselhaus spin-orbit coupling as the source of the dominant spin-relaxation mechanism in the impurity band of a wide class of n-doped zinc blende semiconductors. The Dresselhaus hopping terms are derived and incorporated into a tight-binding model of impurity sites, and they are sh ... [Phys. Rev. Lett. 108, 016601 ] published .

Two-qubit parametric amplifier: Large amplification of weak signals

S. Savel'ev, A. M. Zagoskin, A. L. Rakhmanov, A. N. Omelyanchouk, Z. Washington et al.
Using numerical simulations, we show that two coupled qubits can amplify a weak signal about a hundredfold. This can be achieved if the two qubits are biased simultaneously by this weak signal and a strong pump signal, both having frequencies close to the interlevel transitions in the system. The we ... [Phys. Rev. A 85, 013811 ] published .

Macroscopic non-classical states and terahertz quantum processing in room-temperature diamond

K. C. Lee, B. J. Sussman, M. R. Sprague, P. Michelberger, K. F. Reim et al.
The nature of the transition between the familiar classical, macroscopic world and the quantum, microscopic one continues to be poorly understood. Expanding the regime of observable quantum behaviour to large-scale objects is therefore an exciting open problem. In macroscopic systems of interacting ... [Nat. Photonics 6, 41 (2011)] published Thu Dec 22, 2011.

Preparation of subradiant states using local qubit control in circuit QED

S. Filipp, A. F. van Loo, M. Baur, L. Steffen, and A. Wallraff
Transitions between quantum states by photon absorption or emission are intimately related to the symmetries of the system which lead to selection rules and the formation of dark states. In a circuit quantum electrodynamics setup, in which two resonant superconducting qubits are coupled through an o ... [Phys. Rev. A 84, 061805 ] published .

Spin Guides and Spin Splitters: Waveguide Analogies in One-Dimensional Spin Chains

Melissa I. Makin, Jared H. Cole, Charles D. Hill, and Andrew D. Greentree
Here we show a mapping between waveguide theory and spin-chain transport, opening an alternative approach to solid-state quantum information transport. By applying temporally varying control profiles to a spin chain, we design a virtual waveguide or spin guide to conduct spin excitations along defin ... [Phys. Rev. Lett. 108, 017207 ] published .

Coupling mechanism between microscopic two-level system and superconducting qubits

Zhen-Tao Zhang and Yang Yu
We propose a scheme to clarify the coupling nature between superconducting Josephson qubits and microscopic two-level systems. Although dominant interest in studying two-level systems was in phase qubits previously, we find that the sensitivity of the generally used spectral method in phase qubits i ... [Phys. Rev. A 84, 064301 ] published .

Parallel interaction-free measurement using spatial adiabatic passage

Charles Hill, Andrew D Greentree, and Lloyd C L Hollenberg
Interaction-free measurement (IFM) is a surprising consequence of quantum interference, where the presence of objects can be sensed without any disturbance of the object being measured. Here, we show an extension of IFM using techniques from spatial adiabatic passage, specifically multiple recipient ... [New J. Phys. 13, 125002 (2011)] published Thu Dec 29, 2011.

The influence of anisotropic gate potentials on the phonon induced spin-flip rate in GaAs quantum dots

Sanjay Prabhakar, Roderick V. N. Melnik, and Luis L. Bonilla
We study the anisotropic orbital effect in the electric field tunability of the phonon induced spin-flip rate in quantum dots (QDs). Our study shows that anisotropic gate potential enhances the spin-flip rate and reduces the level crossing point to a lower QDs radius due to the suppression of the La ... [Appl. Phys. Lett. 100, 023108 (2012)] published Tue Jan 10, 2012.

Reconfiguration of spectral absorption features using a frequency-chirped laser pulse

Mingzhen Tian, Tiejun Chang, Kristian D. Merkel, and W. Randall
A technique is proposed to manipulate atomic population in an inhomogeneously broadened medium, which can set an arbitrary absorption spectrum to a uniform transparency (erasure) or to a nearly complete inversion. These reconfigurations of atomic spectral distribution are achieved through excitation ... [Appl. Opt. 50, 6548 (2012)] published Tue Jan 3, 2012.

Manipulating Majorana fermions using supercurrents

Alessandro Romito, Jason Alicea, Gil Refael, and Felix von Oppen
Topological insulator edges and spin-orbit-coupled quantum wires in proximity to s-wave superconductors can be tuned through a topological quantum phase transition by a Zeeman field. Here we show that a supercurrent flowing in the s-wave superconductor also drives such a transition. We propose to us ... [Phys. Rev. B 85, 020502 ] published .

Parametric correlations versus fidelity decay: The symmetry breaking case

H. Kohler, T. Nagao, and H.-J. Stockmann
We provide formulas for fidelity decay and parametric energy correlations for random matrix ensembles where time-reversal invariance of the original Hamiltonian is broken by the perturbation. Like in the case of a symmetry conserving perturbation a simple relation between both quantities exists. Fid ... [Phys. Rev. E 84, 061133 ] published .

Electrical Tuning of Single Nitrogen-Vacancy Center Optical Transitions Enhanced by Photoinduced Fields

L. C. Bassett, F. J. Heremans, C. G. Yale, B. B. Buckley, and D. D. Awschalom
We demonstrate precise control over the zero-phonon optical transition energies of individual nitrogen-vacancy (NV) centers in diamond by applying multiaxis electric fields, via the dc Stark effect. The Stark shifts display surprising asymmetries that we attribute to an enhancement and rectification ... [Phys. Rev. Lett. 107, 266403 ] published .

Ohm's Law Survives to the Atomic Scale

B. Weber, S. Mahapatra, H. Ryu, S. Lee, A. Fuhrer et al.
As silicon electronics approaches the atomic scale, interconnects and circuitry become comparable in size to the active device components. Maintaining low electrical resistivity at this scale is challenging because of the presence of conning surfaces and interfaces. We report on the fabrication of w ... [Science 335, 64 (2012)] published Fri Jan 6, 2012.

Interface trap density metrology from sub-threshold transport in highly scaled undoped Si n-FinFETs

Abhijeet Paul, Giuseppe C. Tettamanzi, Sunhee Lee, Saumitra R. Mehrotra, Nadine Collaert et al.
Channel conductance measurements can be used as a tool to study thermally activated electron transport in the sub-threshold region of state-of-art FinFETs. Together with theoretical tight-binding (TB) calculations, this technique can be used to understand the dependence of the source-to-channel barr ... [J. Appl. Phys. 110, 124507 (2011)] published Tue Dec 20, 2011.

Coherent Optical Control of the Spin of a Single Hole in an InAs/GaAs Quantum Dot

T. M. Godden, J. H. Quilter, A. J. Ramsay, Yanwen Wu, P. Brereton et al.
We demonstrate coherent optical control of a single hole spin confined to an InAs/GaAs quantum dot. A superposition of hole-spin states is created by fast (10100 ps) dissociation of a spin-polarized electron-hole pair. Full control of the hole spin is achieved by combining coherent rotations about t ... [Phys. Rev. Lett. 108, 017402 ] published .

Orbital excitation blockade and algorithmic cooling in quantum gases

Waseem S. Bakr, Philipp M. Preiss, M. Eric Tai, Ruichao Ma, Jonathan Simon et al.
Interaction blockade occurs when strong interactions in a confined, few-body system prevent a particle from occupying an otherwise accessible quantum state. Blockade phenomena reveal the underlying granular nature of quantum systems and allow for the detection and manipulation of the constituent pa ... [Nature 480, 500 (2011)] published Thu Dec 22, 2011.

Theory of control of optomechanical transducers for quantum networks

Fang-Yu Hong, Yang Xiang, W. H. Tang, Zhi-Yan Zhu, Li-zhen Jiang et al.
We present a scheme of control for the arbitrary optical interface mediated by a nanoscale mechanical oscillator between flying qubits and optical nonactive solid-state qubits. This quantum interface lays the foundation for many key functions of a quantum network, such as transferring, swapping, and ... [Phys. Rev. A 85, 012309 ] published .

Spin-1 atoms in optical superlattices: Single-atom tunneling and entanglement

Andreas Wagner, Christoph Bruder, and Eugene Demler
We examine spinor Bose-Einstein condensates in optical superlattices theoretically using a Bose-Hubbard Hamiltonian that takes spin effects into account. Assuming that a small number of spin-1 bosons is loaded in an optical potential, we study single-particle tunneling that occurs when one lattice s ... [Phys. Rev. A 84, 063636 ] published .

Optical-force-induced artifacts in scanning probe microscopy

Dana C. Kohlgraf-Owens, Sergey Sukhov, and Aristide Dogariu
In the practice of near-field scanning probe microscopy, it is typically assumed that the distance regulation is independent of the optical signal. However, we demonstrate that these two signals are entangled due to the inherent action of optically induced force. This coupling leads to artifacts in ... [Opt. Lett. 36, 4758 (2011)] published Tue Dec 20, 2011.

Efficient fluorescence collection and ion imaging with the tack ion trap

Gang Shu, Chen-Kuan Chou, Nathan Kurz, Matthew R. Dietrich, and Boris B. Blinov
Trapped, laser-cooled atoms and ions produce intense fluorescence of the order 10~10 photons per second. Detection of this fluorescence enables efficient measurement of the quantum state of qubits based on trapped atoms. It is desirable to collect a large fraction of the photons to make the detectio ... [J. Opt. Soc. Am. B 28, 2865 (2011)] published Fri Dec 23, 2011.

Implementation of a Toffoli gate with superconducting circuits

A. Fedorov, L. Steffen, M. Baur, M. P. da Silva, and A. Wallraff
The Toffoli gate is a three-quantum-bit (three-qubit) operation that inverts the state of a target qubit conditioned on the state of two control qubits. It makes universal reversible classical computation possible and, together with a Hadamard gate, forms a universal set of gates in quantum computa ... [Nature 481, 170 (2012)] published Thu Jan 12, 2012.

Topological quantum information, virtual Jones polynomials and Khovanov homology

Louis Kauffman
In this paper, we give a quantum statistical interpretation of the bracket polynomial state sum K, the Jones polynomial V (t) and virtual knot theory versions of the Jones polynomial, including the arrow polynomial. We use these quantum mechanical interpretations to give new quantum algorithms for t ... [New J. Phys. 13, 125007 (2011)] published Thu Dec 29, 2011.

Commutator formalism for pairs correlated through Schmidt decomposition as used in Quantum Information

M. Combescot
To easily calculate statistical properties of pairs correlated through Schmidt decomposition, as commonly used in Quantum Information, we propose a commutator formalism for these single-index pairs, somewhat simpler than the one we previously developed for double-index Wannier excitons. We use it he ... [EPL 96, 60002 (2011)] published Thu Dec 22, 2011.

Maximum-confidence discrimination among symmetric qudit states

O. Jimenez, M. A. Solis-Prosser, A. Delgado, and L. Neves
We study the maximum-confidence (MC) measurement strategy for discriminating among nonorthogonal symmetric qudit states. Restricting to linearly dependent and equally likely pure states, we find the optimal positive operator valued measure (POVM) that maximizes our confidence in identifying each sta ... [Phys. Rev. A 84, 062315 ] published .

Measuring non-Markovianity of processes with controllable system-environment interaction

Jian-Shun Tang, Chuan-Feng Li, Yu-Long Li, Xu-Bo Zou, Guang-Can Guo et al.
Non-Markovian processes have recently become a central topic in the study of open quantum systems. We realize experimentally non-Markovian decoherence processes of single photons by combining time delay and evolution in a polarization-maintaining optical fiber. The experiment allows the identificati ... [EPL 97, 10002 (2012)] published Tue Jan 10, 2012.

Achievable qubit rates for quantum-information wires

Hulya Yadsan-Appleby and Tobias J. Osborne
Suppose Alice and Bob have access to two separated regions, respectively, of a system of electrons moving in the presence of a regular one-dimensional lattice of binding atoms. We consider the problem of communicating as much quantum information, as measured by the qubit rate, through this quantum-i ... [Phys. Rev. A 85, 012310 ] published .

Communication cost of classically simulating a quantum channel with subsequent rank-1 projective measurement

Alberto Montina
A process of preparation, transmission, and subsequent projective measurement of a qubit can be simulated by a classical model with only two bits of communication and some amount of shared randomness. However, to the best of our knowledge, no model for n qubits with a finite amount of classical comm ... [Phys. Rev. A 84, 060303 ] published .

Quantum Filtering One Bit at a Time

Jason F. Ralph and Neil P. Oxtoby
In this Letter we consider the purification of a quantum state using the information obtained from a continuous measurement record, where the classical measurement record is digitized to a single bit per measurement after the measurements have been made. Analysis indicates that efficient and reliabl ... [Phys. Rev. Lett. 107, 260503 ] published .

Achieving the Holevo bound via sequential measurements

Vittorio Giovannetti, Seth Lloyd, and Lorenzo Maccone
We present a decoding procedure to transmit classical information in a quantum channel which, saturating asymptotically the Holevo bound, achieves the optimal rate of the communication line. In contrast to previous proposals, it is based on performing a sequence of projective yes-no measurements whi ... [Phys. Rev. A 85, 012302 ] published .

LOCC distinguishability of unilaterally transformable quantum states

Somshubhro Bandyopadhyay, Sibasish Ghosh, and Guruprasad Kar
We consider the question of perfect local distinguishability of mutually orthogonal bipartite quantum states, with the property that every state can be specified by a unitary operator acting on the local Hilbert space of Bob. We show that if the states can be exactly discriminated by one-way local o ... [New J. Phys. 13, 123013 (2011)] published Thu Dec 29, 2011.

Experimental Realization of Optimal Noise Estimation for a General Pauli Channel

A. Chiuri, V. Rosati, G. Vallone, S. Padua, H. Imai et al.
We present the experimental realization of the optimal estimation protocol for a Pauli noisy channel. The method is based on the generation of 2-qubit Bell states and the introduction of quantum noise in a controlled way on one of the state subsystems. The efficiency of the optimal estimation, achie ... [Phys. Rev. Lett. 107, 253602 ] published .

Lower bound on concurrence and distillation for arbitrary-dimensional bipartite quantum states

Ming-Jing Zhao, Xue-Na Zhu, Shao-Ming Fei, and Xianqing Li-Jost
We present a lower bound of concurrence for arbitrary-dimensional bipartite quantum states. This lower bound may be used to improve all the known lower bounds of concurrence. Moreover, the lower bound gives rise to an operational sufficient criterion of distillability of quantum entanglement. The si ... [Phys. Rev. A 84, 062322 ] published .

M-ary-state phase-shift-keying discrimination below the homodyne limit

F. E. Becerra, J. Fan, G. Baumgartner, S. V. Polyakov, J. Goldhar et al.
We investigate a strategy for M-ary discrimination of nonorthogonal phase states with error rates below the homodyne limit. This strategy uses feed forward to update a reference field and signal nulling for the state discrimination. We experimentally analyze the receiver performance using postproces ... [Phys. Rev. A 84, 062324 ] published .

Environment-assisted error correction of single-qubit phase damping

Benjamin Trendelkamp-Schroer, Julius Helm, and Walter T. Strunz
Open quantum system dynamics of random unitary type may in principle be fully undone. Closely following the scheme of environment-assisted error correction proposed by Gregoratti and Werner [J. Mod. Opt. 50, 915 (2003)], we explicitly carry out all steps needed to invert a phase-damping error on a s ... [Phys. Rev. A 84, 062314 ] published .

Universal dynamical decoupling of multiqubit states from environment

Liang Jiang and Adilet Imambekov
We study the dynamical decoupling of multiqubit states from environment. For a system of m qubits, the nested Uhrig dynamical decoupling (NUDD) sequence can efficiently suppress generic decoherence induced by the system-environment interaction to order N using (N+1) pulses. We prove that the NUDD se ... [Phys. Rev. A 84, 060302 ] published .

Capabilities of a Perturbed Toric Code as a Quantum Memory

Alastair Kay
We analyze the effect of typical, unknown perturbations on the 2D toric code when acting as a quantum memory, incorporating the effects of error correction on readout. By transforming the system into a 1D transverse Ising model undergoing an instantaneous quench, and making extensive use of Lieb-Rob ... [Phys. Rev. Lett. 107, 270502 ] published .

Design of additive quantum codes via the code-word-stabilized framework

Alexey A. Kovalev, Ilya Dumer, and Leonid P. Pryadko
We consider design of the quantum stabilizer codes via a two-step, low-complexity approach based on the framework of codeword-stabilized (CWS) codes. In this framework, each quantum CWS code can be specified by a graph and a binary code. For codes that can be obtained from a given graph, we give sev ... [Phys. Rev. A 84, 062319 ] published .

Reducing sequencing complexity in dynamical quantum error suppression by Walsh modulation

David Hayes, Kaveh Khodjasteh, Lorenza Viola, and Michael J. Biercuk
We study dynamical error suppression from the perspective of reducing sequencing complexity, with an eye toward facilitating the development of efficient semiautonomous quantum-coherent systems. To this end, we focus on digital sequences where all interpulse time periods are integer multiples of a m ... [Phys. Rev. A 84, 062323 ] published .

Rigorous performance bounds for quadratic and nested dynamical decoupling

Yuhou Xia, Gotz S. Uhrig, and Daniel A. Lidar
We present rigorous performance bounds for the quadratic dynamical decoupling pulse sequence which protects a qubit from general decoherence, and for its nested generalization to an arbitrary number of qubits. Our bounds apply under the assumptions of instantaneous pulses and of bounded perturbing e ... [Phys. Rev. A 84, 062332 ] published .

Entanglement-assisted codeword stabilized quantum codes

Jeonghwan Shin, Jun Heo, and Todd A. Brun
Entangled qubits can increase the capacity of quantum error-correcting codes based on stabilizer codes. In addition, by using entanglement quantum stabilizer codes can be construct from classical linear codes that do not satisfy the dual-containing constraint. We show that it is possible to construc ... [Phys. Rev. A 84, 062321 ] published .

Quantum fields on closed timelike curves

J. L. Pienaar, C. R. Myers, and T. C. Ralph
Recently, there has been much interest in the evolution of quantum particles on closed timelike curves (CTCs). However, such models typically assume pointlike particles with only two degrees of freedom; a very questionable assumption given the relativistic setting of the problem. We show that it is ... [Phys. Rev. A 84, 062316 ] published .

Non-local quantum superpositions of topologicaldefects

Jacek Dziarmaga, Wojciech H. Zurek, and Michael Zwolak
Topological defects, such as monopoles, vortex lines or domain walls, mark locations where disparate choices of a broken-symmetry vacuum elsewhere in the system lead to irreconcilable differences. They are energetically costly (the energy density in their core reaches that of the prior symmetric va ... [Nature Phys. 8, 49 (2011)] published Thu Dec 22, 2011.

Publisher's Note: Efficient multipartite entanglement purification with the entanglement link from a subspace [Phys. Rev. A 84, 052312 (2011)]

Fu-Guo Deng
Abstract not available. [Phys. Rev. A 84, 069907 ] published .

Erratum: Feasibility of Bell tests with the W state [Phys. Rev. A 81, 033823 (2010)]

Amine Laghaout and Gunnar Bjork
Abstract not available. [Phys. Rev. A 85, 019904 ] published .

Comblike entangled spectrum for composite spin-orbit modes from hyperconcentration

Lixiang Chen
I propose a practical scheme of hyperconcentration, by which two-qubit photon spin and two-qudit orbital angular momentum (OAM) maximally entangled states can be distilled simultaneously. The original nonmaximally hyperentangled photon pairs are generated by parametric down conversion and the symmet ... [Phys. Rev. A 85, 012311 ] published .

Monogamy of quantum correlations in three-qubit pure states

Sudha, A. R. Usha Devi, and A. K. Rajagopal
The limitation on the shareability of quantum entanglement over several parties, the so-called monogamy of entanglement, is an issue that has received considerable attention from the quantum information community over the last decade. A natural question of interest in this connection is whether mono ... [Phys. Rev. A 85, 012103 ] published .

Adaptive schemes for incomplete quantum process tomography

Yong Siah Teo, Berthold-Georg Englert, Jaroslav RehaCek, and Zdenek Hradil
We propose an iterative algorithm for incomplete quantum process tomography with the help of quantum state estimation. The algorithm, which is based on the combined principles of maximum likelihood and maximum entropy, yields a unique estimator for an unknown quantum process when one has less than a ... [Phys. Rev. A 84, 062125 ] published .

Direct measure of quantum correlation

Chang-shui Yu and Haiqing Zhao
The quantumness of the correlation known as quantum correlation is usually measured by quantum discord. So far various quantum discords can be roughly understood as indirect measure by some special discrepancy of two quantities. We present a direct measure of quantum correlation by revealing the dif ... [Phys. Rev. A 84, 062123 ] published .

Non-local geometric phase in two-photon interferometry

A. Martin, O. Alibart, J.-C. Flesch, J. Samuel, Supurna Sinha et al.
We report the experimental observation of the non-local geometric phase in Hanbury Brown-Twiss polarized intensity interferometry. The experiment involves two independent, polarized, incoherent sources, illuminating two polarized detectors. Varying the relative polarization angle between the detecto ... [EPL 97, 10003 (2012)] published Tue Jan 10, 2012.

Quantum entanglement between electronic and vibrational degrees of freedom in molecules

Laura K. McKemmish, Ross H. McKenzie, Noel S. Hush, and Jeffrey R. Reimers
We consider the quantum entanglement of the electronic and vibrational degrees of freedom in molecules with tendencies towards double welled potentials. In these bipartite systems, the von Neumann entropy of the reduced density matrix is used to quantify the electron-vibration entanglement for the l ... [J. Chem. Phys. 135, 244110 (2011)] published Fri Dec 30, 2011.

Collective Quantum Jumps of Rydberg Atoms

Tony E. Lee, H. Haffner, and M. C. Cross
We study an open quantum system of atoms with a long-range Rydberg interaction, laser driving, and spontaneous emission. Over time, the system occasionally jumps between a state of low Rydberg population and a state of high Rydberg population. The jumps are inherently collective, and in fact, exist ... [Phys. Rev. Lett. 108, 023602 ] published .

Greenberger-Horne-Zeilinger States and Few-Body Hamiltonians

Paolo Facchi, Giuseppe Florio, Saverio Pascazio, and Francesco V. Pepe
The generation of Greenberger-Horne-Zeilinger (GHZ) states is a crucial problem in quantum information. We derive general conditions for obtaining GHZ states as eigenstates of a Hamiltonian. We find that a necessary condition for an n-qubit GHZ state to be a nondegenerate eigenstate of a Hamiltonian ... [Phys. Rev. Lett. 107, 260502 ] published .

Coarse Graining Makes It Hard to See Micro-Macro Entanglement

Sadegh Raeisi, Pavel Sekatski, and Christoph Simon
Observing quantum effects such as superpositions and entanglement in macroscopic systems requires not only a system that is well protected against environmental decoherence, but also sufficient measurement precision. Motivated by recent experiments, we study the effects of coarse graining in photon ... [Phys. Rev. Lett. 107, 250401 ] published .

Evolution of entanglement entropy in the D1-D5 brane system

Curtis T. Asplund and Steven G. Avery
We calculate the evolution of the geometric entanglement entropy following a local quench in the D1-D5 conformal field theory, a two-dimensional theory that describes a particular bound state of D1 and D5 branes. The quench corresponds to a localized insertion of the exactly marginal operator that d ... [Phys. Rev. D 84, 124053 ] published .

Bipartite fluctuations as a probe of many-body entanglement

H. Francis Song, Stephan Rachel, Christian Flindt, Israel Klich, Nicolas Laflorencie et al.
We investigate in detail the behavior of the bipartite fluctuations of particle number N-hat and spin S in many-body quantum systems, focusing on systems where such U(1) charges are both conserved and fluctuate within subsystems due to exchange of charges between subsystems. We propose that the bipa ... [Phys. Rev. B 85, 035409 ] published .

Entanglement spectra of the two-dimensional Affleck-Kennedy-Lieb-Tasaki model: Correspondence between the valence-bond-solid state and conformal field theory

Jie Lou, Shu Tanaka, Hosho Katsura, and Naoki Kawashima
We investigate the entanglement properties of the valence-bond-solid (VBS) state defined on two-dimensional lattices, which is the exact ground state of the Affleck-Kennedy-Lieb-Tasaki model. It is shown that the entanglement entropy obeys an area law and the nonuniversal prefactor of the leading te ... [Phys. Rev. B 84, 245128 ] published .

Magnetic field effect on the macroscopic thermal entanglement of a dimerized chain of qubits

A. A. Zvyagin
The effect of dimerization on the macroscopic tangle and thermal pairwise entanglement for a system of coupled qubits with interactions is studied. The exactly solvable model permits us to obtain analytic formulas for characteristics of the macroscopic thermal entanglement. Two special cases of the ... [Phys. Rev. B 84, 214431 ] published .

Extraction of timelike entanglement from the quantum vacuum

S. Jay Olson and Timothy C. Ralph
An intriguing property of the massless quantum vacuum state is that it contains entanglement between both spacelike and timelike separated regions of space-time. The implications of timelike entanglement and its connection to standard entanglement, however, are unexplored. Here we show that timelike ... [Phys. Rev. A 85, 012306 ] published .

Steady-state entanglement for distant atoms by dissipation in coupled cavities

Li-Tuo Shen, Xin-Yu Chen, Zhen-Biao Yang, Huai-Zhi Wu, and Shi-Biao Zheng
We propose a scheme for the generation of entangled states for two atoms trapped in separate cavities coupled to each other. The scheme is based on the competition between the unitary dynamics induced by the classical fields and the collective decays induced by the dissipation of two delocalized fie ... [Phys. Rev. A 84, 064302 ] published .

Steady-state entanglement and normal-mode splitting in an atom-assisted optomechanical system with intensity-dependent coupling

Sh. Barzanjeh, M. H. Naderi, and M. Soltanolkotabi
In this paper, we study theoretically bipartite and tripartite continuous variable entanglement as well as normal-mode splitting in a single-atom cavity optomechanical system with intensity-dependent coupling. The system under consideration is formed by a Fabry-Perot cavity with a thin vibrating end ... [Phys. Rev. A 84, 063850 ] published .

Partial angular coherence and the angular Schmidt spectrum of entangled two-photon fields

Anand Kumar Jha, Girish S. Agarwal, and Robert W. Boyd
We study partially coherent fields that have a coherent-mode representation in the orbital-angular-momentum-mode basis. For such fields, we introduce the concepts of the angular coherence function and the coherence angle. Such fields are naturally produced by the process of parametric down-conversio ... [Phys. Rev. A 84, 063847 ] published .

Quantum complementarity of cavity photons coupled to a three-level system

R. Vilardi, A. Ridolfo, S. Portolan, S. Savasta, and O. Di Stefano
Recently a device enabling the ultrafast all-optical control of the wave-particle duality of light was proposed [Ridolfo et al., Phys. Rev. Lett. 106, 013601 (2011)]. It is constituted by a three-level quantum emitter strongly coupled to a microcavity and can be realized by exploiting a great variet ... [Phys. Rev. A 84, 063842 ] published .

Efficient collection of single photons emitted from a trapped ion into a single-mode fiber for scalable quantum-information processing

Taehyun Kim, Peter Maunz, and Jungsang Kim
Interference and coincidence detection of two photons emitted by two remote ions can lead to an entangled state, which is a critical resource for scalable quantum-information processing. Currently the success probabilities of experimental realizations of this protocol are mainly limited by low coupl ... [Phys. Rev. A 84, 063423 ] published .

Irreducible multiqutrit correlations in Greenberger-Horne-Zeilinger-type states

Fu-Lin Zhang (张福林) and Jing-Ling Chen (陈景灵)
Following the idea of the continuity approach by D. L. Zhou [Phys. Rev. Lett. 101, 180505 (2008)], we obtain the degrees of irreducible multiparty correlations in two families of n-qutrit Greenberger-Horne-Zeilinger-type states. For the pure states in one of the families, the irreducible 2-party, n- ... [Phys. Rev. A 84, 062328 ] published .

Entanglement creation in low-energy scattering

Ricardo Weder
We study the entanglement creation in the low-energy scattering of two particles in three dimensions, for a general class of interaction potentials that are not required to be spherically symmetric. The incoming asymptotic state, before the collision, is a product of two normalized Gaussian states. ... [Phys. Rev. A 84, 062320 ] published .

Weaving independently generated photons into an arbitrary graph state

Qing Lin and Bing He
The controlled-Z (cz) operations acting separately on pairs of qubits are commonly adopted in the schemes of generating graph states, the multipartite entangled states for the one-way quantum computing. For this purpose, we propose a setup of cascade cz operation on a whole group of qubits in sequen ... [Phys. Rev. A 84, 062312 ] published .

Realistic limits on the nonlocality of an N-partite single-photon superposition

Amine Laghaout, Gunnar Bjork, and Ulrik L. Andersen
A recent paper [L. Heaney, A. Cabello, M. F. Santos, and V. Vedral, New J. Phys. 13, 053054 (2011)] revealed that a single quantum symmetrically delocalized over N modes, namely a W state, effectively allows for all-versus-nothing proofs of nonlocality in the limit of large N. Ideally, this finding ... [Phys. Rev. A 84, 062127 ] published .

Asymptotically optimal data analysis for rejecting local realism

Yanbao Zhang, Scott Glancy, and Emanuel Knill
Reliable experimental demonstrations of violations of local realism are highly desirable for fundamental tests of quantum mechanics. One can quantify the violation witnessed by an experiment in terms of a statistical p value, which can be defined as the maximum probability according to local realism ... [Phys. Rev. A 84, 062118 ] published .

Entanglement Witness Operator for Quantum Teleportation

Nirman Ganguly, Satyabrata Adhikari, A. S. Majumdar, and Jyotishman Chatterjee
The ability of entangled states to act as a resource for teleportation is linked to a property of the fully entangled fraction. We show that the set of states with their fully entangled fraction bounded by a threshold value required for performing teleportation is both convex and compact. This featu ... [Phys. Rev. Lett. 107, 270501 ] published .

Quantum Nonlocality in Weak-Thermal-Light Interferometry

Mankei Tsang
In astronomy, interferometry of light collected by separate telescopes is often performed by physically bringing the optical paths together in the form of Young's double-slit experiment. Optical loss severely limits the efficiency of this so-called direct detection method, motivating the fundamental ... [Phys. Rev. Lett. 107, 270402 ] published .

Characterizing and Quantifying Frustration in Quantum Many-Body Systems

S. M. Giampaolo, G. Gualdi, A. Monras, and F. Illuminati
We present a general scheme for the study of frustration in quantum systems. We introduce a universal measure of frustration for arbitrary quantum systems and we relate it to a class of entanglement monotones via an exact inequality. If all the (pure) ground states of a given Hamiltonian saturate th ... [Phys. Rev. Lett. 107, 260602 ] published .

Quantum discord and classical correlations in the bond-charge Hubbard model: Quantum phase transitions, off-diagonal long-range order, and violation of the monogamy property for discord

Michele Allegra, Paolo Giorda, and Arianna Montorsi
We study the quantum discord (QD) and the classical correlations (CC) in a reference model for strongly correlated electrons, the one-dimensional bond-charge extended Hubbard model. We show that the comparison of QD and CC and of their derivatives in the direct and reciprocal lattice allows one to e ... [Phys. Rev. B 84, 245133 ] published .

Complete classification of one-dimensional gapped quantum phases in interacting spin systems

Xie Chen, Zheng-Cheng Gu, and Xiao-Gang Wen
Quantum phases with different orders exist with or without breaking the symmetry of the system. Recently, a classification of gapped quantum phases which do not break time reversal, parity, or on-site unitary symmetry has been given for 1D spin systems by X. Chen, Z.-C. Gu, and X.-G. Wen [Phys. Rev ... [Phys. Rev. B 84, 235128 ] published .

Unitary cavity spin squeezing by quantum erasure

Ian D. Leroux, Monika H. Schleier-Smith, Hao Zhang, and Vladan Vuletic
Deterministic light-induced spin squeezing in an atomic gas is limited by photon shot noise or, equivalently, by atomic state information escaping with the light field mediating the effective atom-atom interaction. We show theoretically that the performance of cavity spin squeezing [M. H. Schleier-S ... [Phys. Rev. A 85, 013803 ] published .

Efficient single-photon-assisted entanglement concentration for partially entangled photon pairs

Yu-Bo Sheng, Lan Zhou, Sheng-Mei Zhao, and Bao-Yu Zheng
We present two realistic entanglement concentration protocols (ECPs) for pure partially entangled photons. A partially entangled photon pair can be concentrated to a maximally entangled pair with only an ancillary single photon with a certain probability, while the conventional ECPs require two copi ... [Phys. Rev. A 85, 012307 ] published .

Local two-qubit entanglement-annihilating channels

Sergey N. Filippov, Tomas Rybar, and Mario Ziman
We address the problem of the robustness of entanglement of bipartite systems (qubits) interacting with dynamically independent environments. In particular, we focus on the characterization of so-called local entanglement-annihilating (EA) two-qubit channels, which set the maximum permissible noise ... [Phys. Rev. A 85, 012303 ] published .

Classification of the entangled states L x N x N

Jun-Li Li, Shi-Yuan Li, and Cong-Feng Qiao
We presented a general classification scheme for the tripartite L x N x N entangled system under stochastic local operation and classical communication. The whole classification procedure consists of two correlated parts: the simultaneous similarity transformation of a commuting matrix pair into a c ... [Phys. Rev. A 85, 012301 ] published .

Optimal path for a quantum teleportation protocol in entangled networks

C. Di Franco and D. Ballester
Bellman's optimality principle has been of enormous importance in the development of whole branches of applied mathematics, computer science, optimal control theory, economics, decision making, and classical physics. Examples are numerous: dynamic programming, Markov chains, stochastic dynamics, cal ... [Phys. Rev. A 85, 010303 ] published .

Ultimate sensitivity of precision measurements with intense Gaussian quantum light: A multimodal approach

Olivier Pinel, Julien Fade, Daniel Braun, Pu Jian, Nicolas Treps et al.
Multimode Gaussian quantum light, which includes multimode squeezed and multipartite quadrature entangled light, is a very general and powerful quantum resource with promising applications in quantum information processing and metrology. In this paper, we determine the ultimate sensitivity in the es ... [Phys. Rev. A 85, 010101 ] published .

High-sensitivity three-mode optomechanical transducer

C. Zhao, Q. Fang, S. Susmithan, H. Miao, L. Ju et al.
Three-mode optomechanical interactions have been predicted to allow the creation of very high sensitivity transducers in which very strong optical self-cooling and strong optomechanical quantum entanglement are predicted. Strong coupling is achieved by engineering a transducer in which both the pump ... [Phys. Rev. A 84, 063836 ] published .

Quantum entangled ground states of two spinor Bose-Einstein condensates

Z. F. Xu, R. Lu, and L. You
We revisit in detail the non-mean-field ground-state phase diagram for a binary mixture of spin-1 Bose-Einstein condensates including quantum fluctuations. The noncommuting terms in the spin-dependent Hamiltonian under the single-spatial-mode approximation make it difficult to obtain exact eigenstat ... [Phys. Rev. A 84, 063634 ] published .

Convex polytopes and quantum separability

F. Holik and A. Plastino
We advance a perspective of the entanglement issue that appeals to the Schlienz-Mahler measure [Phys. Rev. A 52, 4396 (1995)]. Related to it, we propose a criterium based on the consideration of convex subsets of quantum states. This criterium generalizes a property of product states to convex subse ... [Phys. Rev. A 84, 062327 ] published .

Purification of mixed states with closed timelike curve is not possible

Arun K. Pati, Indranil Chakrabarty, and Pankaj Agrawal
In ordinary quantum theory, any mixed state can be purified in an enlarged Hilbert space by adding an ancillary system. The purified state does not depend on the state of any extraneous system with which the mixed state is going to interact or on the physical interaction. Here, we prove that it is n ... [Phys. Rev. A 84, 062325 ] published .

Fidelity spectrum and phase transitions of quantum systems

P. D. Sacramento, N. Paunkovic, and V. R. Vieira
Quantum fidelity between two density matrices F(rho,rho) is usually defined as the trace of the operator [script F]=sqrt(sqrt( rho[sub 1])rho[sub 2]sqrt( rho[sub 1])). We study the logarithmic spectrum of this operator, which we denote by the fidelity spectrum, in the cases of the XX spin chain in a ... [Phys. Rev. A 84, 062318 ] published .

Decoherence and the nature of system-environment correlations

A. Pernice and W. T. Strunz
We investigate system-environment correlations based on the exact dynamics of a qubit and its environment in the framework of pure decoherence (phase damping). We focus on the relation of decoherence and the buildup of system-reservoir entanglement for an arbitrary (possibly mixed) initial qubit sta ... [Phys. Rev. A 84, 062121 ] published .

Pulse control of sudden transition for two qubits in XY spin baths and quantum phase transition

Da-Wei Luo, Hai-Qing Lin, Jing-Bo Xu, and Dao-Xin Yao
We study the dynamics of two initially correlated qubits coupled to their own separate spin baths modeled by an XY spin chain and find the explicit expression of the quantum discord for the system. A sudden transition is found to exist between classical and quantum decoherence by choosing certain in ... [Phys. Rev. A 84, 062112 ] published .

Residual entanglement of accelerated fermions is not nonlocal

Nicolai Friis, Philipp Kohler, Eduardo Martin-Martinez, and Reinhold A. Bertlmann
We analyze the operational meaning of the residual entanglement in noninertial fermionic systems in terms of the achievable violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality. We demonstrate that the quantum correlations of fermions, which were previously found to survive in the infinite ... [Phys. Rev. A 84, 062111 ] published .

Enhancing quantum discord in cavity QED by an added nonlinear Kerr-like medium

Qi-Liang He and Jing-Bo Xu
We investigate the quantum discord dynamics of two noninteracting two-level atoms, each trapped in a single-mode optical cavity which is filled with a nonlinear Kerr-like medium. It is found that quantum discord vanishes only asymptotically although entanglement disappears suddenly during the time e ... [J. Opt. Soc. Am. B 28, 2966 (2011)] published Fri Dec 23, 2011.

Stochastic resonance of quantum discord

Chee Kong Lee, Leong Chuan Kwek, and Jianshu Cao
We study the stochastic resonance of quantum discord (discord resonance) in coupled quantum systems and make a comparison with the stochastic resonance of entanglement (entanglement resonance). It is found that the discord resonance is much more robust against dephasing noise and thermal effects tha ... [Phys. Rev. A 84, 062113 ] published .

Feasibility of loophole-free nonlocality tests with a single photon

Rafael Chaves and Jonatan Bohr Brask
Recently, much interest has been directed toward designing setups that achieve realistic loss thresholds for decisive tests of local realismin particular in the optical regime. We analyze the feasibility of such Bell tests based on a W state shared between multiple parties, which can be realized, fo ... [Phys. Rev. A 84, 062110 ] published .

Entangled-photon compressive ghost imaging

Petros Zerom, Kam Wai Clifford Chan, John C. Howell, and Robert W. Boyd
We have experimentally demonstrated high-resolution compressive ghost imaging at the single-photon level using entangled photons produced by a spontaneous parametric down-conversion source and using single-pixel detectors. For a given mean-squared error, the number of photons needed to reconstruct a ... [Phys. Rev. A 84, 061804 ] published .

Long quantum channels for high-quality entanglement transfer

L Banchi, T Apollaro, A Cuccoli, R Vaia, and P Verrucchi
High-quality quantum-state and entanglement transfer can be achieved in an unmodulated spin bus operating in the ballistic regime, which occurs when the endpoint qubits A and B are nonperturbatively coupled to the chain by a suitable exchange interaction j. Indeed, the transition amplitude character ... [New J. Phys. 13, 123006 (2011)] published Thu Dec 29, 2011.

Generating, manipulating and measuring entanglement and mixture with a reconfigurable photonic circuit

P. J. Shadbolt, M. R. Verde, A. Peruzzo, A. Politi, A. Laing et al.
Entanglement is the quintessential quantum-mechanical phenomenon understood to lie at the heart of future quantum technologies and the subject of fundamental scientific investigations. Mixture, resulting from noise, is often an unwanted result of interaction with an environment, but is also of funda ... [Nat. Photonics 6, 45 (2011)] published Thu Dec 22, 2011.

Violation of a Bell inequality in two-dimensional spin-orbit hypoentangled subspaces

Lixiang Chen, Xiancong Lu, and Zhongqun Cheng
Based on spin-orbit coupling induced by q-plates, we present a feasible experimental proposal for preparing two-dimensional spatially inhomogeneous polarizations of light. We further investigate the quantum correlations between these inhomogeneous polarizations of photon pairs generated by spontaneo ... [J. Opt. Soc. Am. B 28, 2915 (2011)] published Fri Dec 23, 2011.

Long-term performance of the SwissQuantum quantum key distribution network in a field environment

D Stucki, M Legr, F Buntschu, B Clausen, N Felber et al.
In this paper, we report on the performance of the SwissQuantum quantum key distribution (QKD) network. The network was installed in the Geneva metropolitan area and ran for more than one-and-a-half years, from the end of March 2009 to the beginning of January 2011. The main goal of this experiment ... [New J. Phys. 13, 123001 (2011)] published Thu Dec 29, 2011.

Long-distance continuous-variable quantum key distribution with a Gaussian modulation

Paul Jouguet, Sebastien Kunz-Jacques, and Anthony Leverrier
We designed high-efficiency error correcting codes allowing us to extract an errorless secret key in a continuous-variable quantum key distribution (CVQKD) protocol using a Gaussian modulation of coherent states and a homodyne detection. These codes are available for a wide range of signal-to-noise ... [Phys. Rev. A 84, 062317 ] published .

How to implement decoy-state quantum key distribution for a satellite uplink with 50-dB channel loss

Evan Meyer-Scott, Zhizhong Yan, Allison MacDonald, Jean-Philippe Bourgoin, Hannes Hubel et al.
Quantum key distribution (QKD) takes advantage of fundamental properties of quantum physics to allow two distant parties to share a secret key; however, QKD is hampered by a distance limitation of a few hundred kilometers on Earth. The most immediate solution for global coverage is to use a satellit ... [Phys. Rev. A 84, 062326 ] published .

Fair and optimistic quantum contract signing

N. Paunkovic, J. Bouda, and P. Mateus
We present a fair and optimistic quantum-contract-signing protocol between two clients that requires no communication with the third trusted party during the exchange phase. We discuss its fairness and show that it is possible to design such a protocol for which the probability of a dishonest client ... [Phys. Rev. A 84, 062331 ] published .

One-sided device-independent quantum key distribution: Security, feasibility, and the connection with steering

Cyril Branciard, Eric G. Cavalcanti, Stephen P. Walborn, Valerio Scarani, and Howard M. Wiseman
We analyze the security and feasibility of a protocol for quantum key distribution (QKD) in a context where only one of the two parties trusts his measurement apparatus. This scenario lies naturally between standard QKD, where both parties trust their measurement apparatuses, and device-independent ... [Phys. Rev. A 85, 010301 ] published .

Security problem on arbitrated quantum signature schemes

Jeong Woon Choi, Ku-Young Chang, and Dowon Hong
Many arbitrated quantum signature schemes implemented with the help of a trusted third party have been developed up to now. In order to guarantee unconditional security, most of them take advantage of the optimal quantum one-time encryption based on Pauli operators. However, in this paper we point o ... [Phys. Rev. A 84, 062330 ] published .

Spontaneous Parametric Down-Conversion and Quantum Walks in Arrays of Quadratic Nonlinear Waveguides

Alexander S. Solntsev, Andrey A. Sukhorukov, Dragomir N. Neshev, and Yuri S. Kivshar
We analyze the process of photon-pair generation with simultaneous quantum walks in a quadratic nonlinear waveguide array. We demonstrate that the spontaneous parametric down-conversion in the array allows for creating quantum states with strongly pronounced spatial correlations, which are qualitati ... [Phys. Rev. Lett. 108, 023601 ] published .

Two-Particle Bosonic-Fermionic Quantum Walk via Integrated Photonics

Linda Sansoni, Fabio Sciarrino, Giuseppe Vallone, Paolo Mataloni, Andrea Crespi et al.
Quantum walk represents one of the most promising resources for the simulation of physical quantum systems, and has also emerged as an alternative to the standard circuit model for quantum computing. Here we investigate how the particle statistics, either bosonic or fermionic, influences a two-parti ... [Phys. Rev. Lett. 108, 010502 ] published .

Fast protocols for local implementation of bipartite nonlocal unitaries

Li Yu, Robert B. Griffiths, and Scott M. Cohen
In certain cases the communication time required to deterministically implement a nonlocal bipartite unitary using prior entanglement and local operations and classical communication can be reduced by a factor of two. We introduce two such fast protocols and illustrate them with various examples. Fo ... [Phys. Rev. A 85, 012304 ] published .

Ramsey Numbers and Adiabatic Quantum Computing

Frank Gaitan and Lane Clark
The graph-theoretic Ramsey numbers are notoriously difficult to calculate. In fact, for the two-color Ramsey numbers R(m,n) with m, n>=3, only nine are currently known. We present a quantum algorithm for the computation of the Ramsey numbers R(m,n). We show how the computation of R(m,n) can be mappe ... [Phys. Rev. Lett. 108, 010501 ] published .

Quantum Simulation of Quantum Field Theories in Trapped Ions

J. Casanova, L. Lamata, I. L. Egusquiza, R. Gerritsma, C. F. Roos et al.
We propose the quantum simulation of fermion and antifermion field modes interacting via a bosonic field mode, and present a possible implementation with two trapped ions. This quantum platform allows for the scalable add up of bosonic and fermionic modes, and represents an avenue towards quantum si ... [Phys. Rev. Lett. 107, 260501 ] published .

Nonexistence of a universal quantum machine to examine the precision of unknown quantum states

Shengshi Pang, Shengjun Wu, and Zeng-Bing Chen
In this work, we reveal a type of impossibility discovered in our recent research which forbids comparing the closeness of multiple unknown quantum states with any nontrivial threshold in a perfect or unambiguous way. This impossibility is distinct from the existing impossibilities in that it is a c ... [Phys. Rev. A 84, 062313 ] published .

Nested topological order

H Bombin and M Martin-Delgado
We introduce the concept of nested topological order in a class of exact quantum lattice Hamiltonian models with non-Abelian discrete gauge symmetry. The topological order present in the models can be partially destroyed by introducing a gauge symmetry reduction mechanism. When symmetry is reduced i ... [New J. Phys. 13, 125001 (2011)] published Thu Dec 29, 2011.

Balanced ternary addition using a gated silicon nanowire

J. A. Mol, J. van der Heijden, J. Verduijn, M. Klein, F. Remacle et al.
Ternary logic has the lowest cost of complexity, here, we demonstrate a CMOS hardware implementation of a ternary adder using a silicon metal-on-insulator single electron transistor. Gate dependent rectifying behavior of a single electron transistor (SET) results in a robust three-valued output as a ... [Appl. Phys. Lett. 99, 263109 (2011)] published Thu Dec 29, 2011.

Increasing efficiency of a linear-optical quantum gate using electronic feed-forward

Martina Mikova, Helena Fikerova, Ivo Straka, Michal MiCuda, Jaromir Fiurasek et al.
We have successfully used fast electronic feed-forward to increase the probability of success of a linear optical implementation of a programmable phase gate from 25% to its theoretical limit of 50%. The feed-forward applies a conditional unitary operation, which changes the incorrect output states ... [Phys. Rev. A 85, 012305 ] published .

Exponential complexity of the quantum adiabatic algorithm for certain satisfiability problems

Itay Hen and A. P. Young
We determine the complexity of several constraint satisfaction problems using the quantum adiabatic algorithm in its simplest implementation. We do so by studying the size dependence of the gap to the first excited state of typical instances. We find that, at large sizes N, the complexity increases ... [Phys. Rev. E 84, 061152 ] published .

Quantum tests for the linearity and permutation invariance of Boolean functions

Mark Hillery and Erika Andersson
The goal in function property testing is to determine whether a black-box Boolean function has a certain property or is epsilon-far from having that property. The performance of the algorithm is judged by how many calls need to be made to the black box in order to determine, with high probability, w ... [Phys. Rev. A 84, 062329 ] published .

Perfect quantum state transfer in two- and three-dimensional structures

V. Karimipour, M. Sarmadi Rad, and M. Asoudeh
We introduce a scheme for perfect state transfer in regular two- and three-dimensional structures. The interactions on the lattices are of the XX spin type with uniform couplings. In two dimensions, the structure is a hexagonal lattice, and in three dimensions, it consists of hexagonal planes joined ... [Phys. Rev. A 85, 010302 ] published .