
Amy, M., Maslov, D., & Mosca, M. (2014). PolynomialTime TDepth Optimization of Clifford plus T Circuits Via Matroid Partitioning. IEEE Trans. ComputAided Des. Integr. Circuits Syst., 33(10), 1476–1489.
Abstract: Most work in quantum circuit optimization has been performed in isolation from the results of quantum faulttolerance. Here we present a polynomialtime algorithm for optimizing quantum circuits that takes the actual implementation of faulttolerant logical gates into consideration. Our algorithm resynthesizes quantum circuits composed of Clifford group and T gates, the latter being typically the most costly gate in faulttolerant models, e. g., those based on the Steane or surface codes, with the purpose of minimizing both Tcount and Tdepth. A major feature of the algorithm is the ability to resynthesize circuits with ancillae at effectively no additional cost, allowing spacetime tradeoffs to be easily explored. The tested benchmarks show up to 65.7% reduction in Tcount and up to 87.6% reduction in Tdepth without ancillae, or 99.7% reduction in Tdepth using ancillae.
Keywords: Circuit optimization; circuit synthesis; Clifford plus T; matroid partitioning; quantum circuits; sum over paths


Serbyn, M., Knap, M., Gopalakrishnan, S., Papic, Z., Yao, N. Y., Laumann, C. R., et al. (2014). Interferometric Probes of ManyBody Localization. Phys. Rev. Lett., 113(14), 5 pp.
Abstract: We propose a method for detecting manybody localization (MBL) in disordered spin systems. The method involves pulsed coherent spin manipulations that probe the dephasing of a given spin due to its entanglement with a set of distant spins. It allows one to distinguish the MBL phase from a noninteracting localized phase and a delocalized phase. In particular, we show that for a properly chosen pulse sequence the MBL phase exhibits a characteristic powerlawdecay reflecting its slow growth of entanglement. We find that this powerlawdecay is robust with respect to thermal and disorder averaging, provide numerical simulations supporting our results, and discuss possible experimental realizations in solidstate and coldatom systems.


Dot, A., MeyerScott, E., Ahmad, R., Rochette, M., & Jennewein, T. (2014). Converting one photon into two via fourwave mixing in optical fibers. Phys. Rev. A, 90(4), 12 pp.
Abstract: Observing nonlinear optical quantum effects or implementing quantum information protocols using nonlinear optics requires moving to eversmaller input light intensities. However, low light intensities generally mean weak optical nonlinearities, which are inadequate for many applications. Here we calculate the performance of fourwave mixing in various optical fibers for the case where one of the input beams is a single photon. We show that in tapered chalcogenide glass fibers (microwires) a single photon plus strong pump beam can produce a pair of photons with a probability of 0.1%, much higher than in previous work on bulk and waveguided crystal sources. Such a photon converter could be useful for creating large entangled photon states, for performing a loopholefree test of Bell's inequalities, and for quantum communication.


Johnsen, K. D., Kolenderski, P., Scarcella, C., Thibault, M., Tosi, A., & Jennewein, T. (2014). Time and spectrumresolving multiphoton correlator for 300900 nm. J. Appl. Phys., 116(14), 4 pp.
Abstract: We demonstrate a singlephoton sensitive spectrometer in the visible range, which allows us to perform timeresolved and multiphoton spectral correlation measurements at room temperature. It is based on a monochromator composed of two gratings, collimation optics, and an array of single photon avalanche diodes. The time resolution can reach 110 ps and the spectral resolution is 2nm/pixel, limited by the design of the monochromator. This technique can easily be combined with commercial monochromators and can be useful for joint spectrum measurements of two photons emitted in the process of parametric down conversion, as well as timeresolved spectrum measurements in optical coherence tomography or medical physics applications. (C) 2014 AIP Publishing LLC.


Wallman, J. J., & Flammia, S. T. (2014). Randomized benchmarking with confidence. New J. Phys., 16, 34 pp.
Abstract: Randomized benchmarking is a promising tool for characterizing the noise in experimental implementations of quantum systems. In this paper, we prove that the estimates produced by randomized benchmarking (both standard and interleaved) for arbitrary Markovian noise sources are remarkably precise by showing that the variance due to sampling random gate sequences is small. We discuss how to choose experimental parameters, in particular the number and lengths of random sequences, in order to characterize average gate errors with rigorous confidence bounds. We also show that randomized benchmarking can be used to reliably characterize timedependent Markovian noise (e.g., when noise is due to a magnetic field with fluctuating strength). Moreover, we identify a necessary property for timedependent noise that is violated by some sources of nonMarkovian noise, which provides a test for nonMarkovianity.
Keywords: quantum information; randomized benchmarking; quantum noise


Leung, D., & Wang, B. J. (2014). Characteristics of universal embezzling families. Phys. Rev. A, 90(4), 8 pp.
Abstract: Quantum state embezzlement is the transformation mu > bar right arrow mu > phi > using only local operations, where phi > and mu > are multipartite quantum states. Exact embezzlement is an impossible task since it implies the increase of entanglement without communication. Surprisingly, van Dam and Hayden [Phys. Rev. A 67, 060302 (2003)] find a universal embezzling family of states mu > that enables embezzlement in the bipartite setting with arbitrary precision as the dimension of mu > increases. Furthermore, the family is independent of the state phi > to be embezzled. We study embezzlement in the bipartite setting. We present various requirements and consequences, and infinitely many universal embezzling families inequivalent to that proposed by van Dam and Hayden. We include numerical studies of up to 33qubit large local systems.


Johnston, N. (2014). The structure of qubit unextendible product bases. J. Phys. AMath. Theor., 47(42), 19 pp.
Abstract: Unextendible product bases (UPBs) have been shown to have many important uses in quantum information theory, particularly in the qubit case. However, very little is known about their mathematical structure beyond three qubits. We present several new results about qubit UPBs, including a complete characterization of all fourqubit UPBs, which we show there are exactly 1446 of. We also show that there exist pqubit UPBs of almost all sizes less than 2p.
Keywords: unextendible product basis; quantum entanglement; graph factorization


Geller, J., & Piani, M. (2014). Quantifying nonclassical and beyondquantum correlations in the unified operator formalism. J. Phys. AMath. Theor., 47(42), 18 pp.
Abstract: Acin et al (2010 Phys. Rev. Lett. 104 140404) introduced a unified framework for the study of nosignalling correlations. Such a framework is based on the notion of local quantum measurements, but, in order to account for beyondquantum correlations, global pseudostates that are not positive semidefinite are allowed. After a short review of the formalism, we consider its use in the quantification of both general nonlocal and beyondquantum correlations. We argue that the unified framework for correlations provides a simple approach to such a quantification, in particular when the quantification is meant to be operational and meaningful in a resourcetheory scenario, i.e., when considering the processing of resources by means of nonresources. We relate different notions of robustness of correlations, both at the level of (pseudo) states and abstract probability distributions, with particular focus on the beyondquantum robustness of correlations and pseudostates. We revisit known results and argue that, within the unified framework, the relation between the two levelsthat of operators and that of probability distributionsis very strict. We point out how the consideration of robustness at the two levels leads to a natural framework for the quantification of entanglement in a deviceindependent way. Finally, we show that the beyondquantum robustness of the nonpositive operators needed to achieve beyondquantum correlations coincides with their negativity and their distance from the set of quantum states. As an example, we calculate the beyondquantum robustness for the case of a noisy PopescuRohrlich box.
Keywords: quantum nonlocality; robustness of correlations; entanglement; PopescuRohrlich box; pseudostate; deviceindependent


Brown, E. G., Kempf, W. D. A., Kempf, A., Mann, R. B., MartinMartinez, E., & Menicucci, N. C. (2014). Quantum seismology. New J. Phys., 16, 18 pp.
Abstract: We propose a quantum mechanical method of detecting weak vibrational disturbances inspired by the protocol of entanglement farming. We consider a setup where pairs of atoms in their ground state are successively sent through an optical cavity. It is known that in this way it is possible to drive that cavity toward a stable fixedpoint state. Here we study how that fixedpoint state depends on the time interval between pairs of atoms and on the distance between the cavity's mirrors. Taking advantage of an extremely precise resonance effect, we find that there are special values of these parameters where the fixedpoint state is highly sensitive to perturbations, even harmonic vibrations with frequencies several orders of magnitude below the cavity's natural frequency. We propose that this sensitivity may be useful for high precision metrology.
Keywords: entanglement; metrology; quantum optics; field theory; nonperturbative methods


Arrazola, J. M., & Lutkenhaus, N. (2014). Quantum communication with coherent states and linear optics. Phys. Rev. A, 90(4), 10 pp.
Abstract: We introduce a general mapping for encoding quantum communication protocols involving pure states of multiple qubits, unitary transformations, and projective measurements into another set of protocols that employ a coherent state of light in a linear combination of optical modes, linearoptics transformations, and measurements with singlephoton threshold detectors. This provides a general framework for transforming protocols in quantum communication into a form in which they can be implemented with current technology. We explore the similarity between properties of the original qubit protocols and the coherentstate protocols obtained from the mapping and make use of the mapping to construct additional protocols in the context of quantum communication complexity and quantum digital signatures. Our results have the potential of bringing a wide class of quantum communication protocols closer to their experimental demonstration.


Cubitt, T., Mancinska, L., Roberson, D. E., Severini, S., Stahlke, D., & Winter, A. (2014). Bounds on EntanglementAssisted SourceChannel Coding via the Lovasz nu Number and Its Variants. IEEE Trans. Inf. Theory, 60(11), 7330–7344.
Abstract: We study zeroerror entanglementassisted sourcechannel coding (communication in the presence of side information). Adapting a technique of Beigi, we show that such coding requires existence of a set of vectors satisfying orthogonality conditions related to suitably defined graphs G and H. Such vectors exist if and only if nu((G) over bar) <= nu((H) over bar), where nu represents the Lovasz number. We also obtain similar inequalities for the related Schrijver nu() and Szegedy nu(+) numbers. These inequalities reproduce several known bounds and also lead to new results. We provide a lower bound on the entanglementassisted cost rate. We show that the entanglementassisted independence number is bounded by the Schrijver number: alpha*(G) <= nu()(G). Therefore, we are able to disprove the conjecture that the oneshot entanglementassisted zeroerror capacity is equal to the integer part of the Lovasz number. Beigi introduced a quantity beta as an upper bound on a* and posed the question of whether beta(G) = right perpendicular nu(G)left perpendicular. We answer this in the affirmative and show that a related quantity is equal to inverted right perpendicular nu(G)inverted left perpendicular. We show that a quantity chi(vect)(G) recently introduced in the context of Tsirelson's problem is equal to inverted right perpendicular nu+((G) over bar )inverted left perpendicular. In an appendix, we investigate multiplicativity properties of Schrijver's and Szegedy's numbers, as well as projective rank.
Keywords: Graph theory; quantum entanglement; quantum information; zeroerror information theory; linear programming


Matthews, W., & Wehner, S. (2014). Finite Blocklength Converse Bounds for Quantum Channels. IEEE Trans. Inf. Theory, 60(11), 7317–7329.
Abstract: We derive upper bounds on the rate of transmission of classical information over quantum channels by block codes with a given blocklength and error probability, for both entanglementassisted and unassisted codes, in terms of a unifying framework of quantum hypothesis testing with restricted measurements. Our bounds do not depend on any special property of the channel (such as memorylessness) and generalize both a classical converse of Polyanskiy, Poor, and Verdu as well as a quantum converse of Renner and Wang, and have a number of desirable properties. In particular, our bound on entanglementassisted codes is a semidefinite program and for memoryless channels, its large blocklength limit is the wellknown formula for entanglementassisted capacity due to Bennett, Shor, Smolin, and Thapliyal.
Keywords: Channel coding; quantum channels; block codes; finite blocklength; quantum entanglement


Chen, A. X. (2014). Coherent manipulation of spontaneous emission spectra in coupled semiconductor quantum well structures. Opt. Express, 22(22), 26991–27000.
Abstract: In triple coupled semiconductor quantum well structures (SQWs) interacting with a coherent driving filed, a coherent coupling field and a weak probe field, spontaneous emission spectra are investigated. Our studies show emission spectra can easily be manipulated through changing the intensity of the driving and coupling field, detuning of the driving field. Some interesting physical phenomena such as spectralline enhancement/suppression, spectralline narrowing and spontaneous emission quenching may be obtained in our system. The theoretical studies of spontaneous emission spectra in SQWS have potential application in highprecision spectroscopy. Our studies are based on the real physical system [Appl. Phys. Lett. 86(20), 201112 (2005)], and this scheme might be realizable with presently available techniques. (C) 2014 Optical Society of America


Serbyn, M., Papic, Z., & Abanin, D. A. (2014). Quantum quenches in the manybody localized phase. Phys. Rev. B, 90(17), 10 pp.
Abstract: Manybody localized (MBL) systems are characterized by the absence of transport and thermalization and, therefore, cannot be described by conventional statistical mechanics. In this paper, using analytic arguments and numerical simulations, we study the behavior of local observables in an isolated MBL system following a quantum quench. For the case of a global quench, we find that the local observables reach stationary, highly nonthermal values at long times as a result of slow dephasing characteristic of the MBL phase. These stationary values retain the local memory of the initial state due to the existence of local integrals of motion in the MBL phase. The temporal fluctuations around stationary values exhibit universal powerlaw decay in time, with an exponent set by the localization length and the diagonal entropy of the initial state. Such a powerlaw decay holds for any local observable and is related to the logarithmic in time growth of entanglement in the MBL phase. This behavior distinguishes the MBL phase from both the Anderson insulator (where no stationary state is reached) and from the ergodic phase (where relaxation is expected to be exponential). For the case of a local quench, we also find a powerlaw approach of local observables to their stationary values when the system is prepared in a mixed state. Quench protocols considered in this paper can be naturally implemented in systems of ultracold atoms in disordered optical lattices, and the behavior of local observables provides a direct experimental signature of manybody localization.


MartinMartinez, E., & Menicucci, N. C. (2014). Entanglement in curved spacetimes and cosmology. Class. Quantum Gravity, 31(21), 41 pp.
Abstract: We review recent results regarding entanglement in quantum fields in cosmological spacetimes and related phenomena in flat spacetime such as the Unruh effect. We begin with a summary of important results about field entanglement and the mathematics of Bogoliubov transformations that is very often used to describe it. We then discuss the UnruhDeWitt detector model, which is a useful model of a generic local particle detector. This detector model has been successfully used as a tool to obtain many important results. In this context we discuss two specific types of these detectors: a qubit and a harmonic oscillator. The latter has recently been shown to have important applications when one wants to probe nonperturbative physics of detectors interacting with quantum fields. We then detail several recent advances in the study and application of these ideas, including echoes of the early universe, entanglement harvesting, and a nascent proposal for quantum seismology.
Keywords: entanglement; cosmology; relativistic quantum information


