
Yu Chen, P. R., D. Sank, C. Neill, Erik Lucero, Matteo Mariantoni, R. Barends, Chiaro, J. Kelly, A. Megrant, J.Y. Mutus, P.J.J. O’Malley, A. Vainsencher, J. Wenner, T.C. White, YiYin, w, A.N. Cleland & John M. Martinis. (2014). Emulating weak localization using a solidstate quantum circuit. ncomms, 5(5184).


T. Jennewein, C. Grant, E. Choi, C. Pugh, C. Holloway, JP. Bourgoin, et al. (2014). The NanoQEY mission: ground to space quantum key and entanglement distribution using a nanosatellite. In Proc. SPIE (Vol. 9254).


Wood, C. J., Abutaleb, M. O., Huber, M. G., Arif, M., Cory, D. G., & Pushin, D. A. (2014). Quantum correlations in a noisy neutron interferometer. Phys. Rev. A, 90(3), 9 pp.
Abstract: We investigate quantum coherences in the presence of noise by entangling the spin and path degrees of freedom of the output neutron beam from a noisy threeblade perfect crystal neutron interferometer. We find that in the presence of dephasing noise on the path degree of freedom the entanglement of the output state reduces to 0, however the quantum discord remains nonzero for all noise values. Hence even in the presence of strong phase noise nonclassical correlations persist between the spin and the path of the neutron beam. This indicates that measurements performed on the spin of the neutron beam will induce a disturbance on the path state. We calculate the effect of the spin measurement by observing the changes in the observed contrast of the interferometer for an output beam postselected on a given spin state. In doing so we demonstrate that these measurements allow us to implement a quantum eraser and a whichway measurement of the path taken by the neutron through the interferometer. While strong phase noise removes the quantum eraser, the spinfiltered whichway measurement is robust to phase noise. We experimentally demonstrate this disturbance by comparing the contrasts of the output beam with and without spin measurements of three neutron interferometers with varying noise strengths. This demonstrates that even in the presence of noise that suppresses path coherence and spinpath entanglement, a neutron interferometer still exhibits uniquely quantum behavior.


Chen, J. X., Ji, Z. F., Kribs, D., Lutkenhaus, N., & Zeng, B. (2014). Symmetric extension of twoqubit states. Phys. Rev. A, 90(3), 10 pp.
Abstract: A bipartite state rho(AB) is symmetric extendible if there exists a tripartite state rho(ABB') whose AB and AB' marginal states are both identical to rho(AB). Symmetric extendibility of bipartite states is of vital importance in quantum information because of its central role in separability tests, oneway distillation of EinsteinPodolskyRosen pairs, oneway distillation of secure keys, quantum marginal problems, and antidegradable quantum channels. We establish a simple analytic characterization for symmetric extendibility of any twoqubit quantum state rho(AB); specifically, tr(rho(2)(B)) >= tr(rho(2)(AB)) – 4 root det rho(AB). As a special case we solve the bosonic threerepresentability problem for the twobody reduced density matrix.


Sawant, R., Samuel, J., Sinha, A., Sinha, S., & Sinha, U. (2014). Nonclassical Paths in Quantum Interference Experiments. Phys. Rev. Lett., 113(12), 5 pp.
Abstract: In a double slit interference experiment, the wave function at the screen with both slits open is not exactly equal to the sum of the wave functions with the slits individually open one at a time. The three scenarios represent three different boundary conditions and as such, the superposition principle should not be applicable. However, most wellknown text books in quantum mechanics implicitly and/or explicitly use this assumption that is only approximately true. In our present study, we have used the Feynman path integral formalism to quantify contributions from nonclassical paths in quantum interference experiments that provide a measurable deviation from a naive application of the superposition principle. A direct experimental demonstration for the existence of these nonclassical paths is difficult to present. We find that contributions from such paths can be significant and we propose simple threeslit interference experiments to directly confirm their existence.


Radu Ionicioiu, T. J., Robert B. Mann, Daniel R. Terno. (2014). Is wave–particle objectivity compatible with determinism and locality? NATURE COMMUNICATIONS, 5.
Abstract: Wave–particle duality, superposition and entanglement are among the most counterintuitive features of quantum theory. Their clash with our classical expectations motivated hiddenvariable (HV) theories. With the emergence of quantum technologies, we can test experimentally the predictions of quantum theory versus HV theories and put strong restrictions on their key assumptions. Here, we study an entanglementassisted version of the quantum delayedchoice experiment and show that the extension of HV to the controlling devices only exacerbates the contradiction. We compare HV theories that satisfy the conditions of objectivity (a property of photons being either particles or waves, but not both), determinism and local independence of hidden variables with quantum mechanics. Any two of the above conditions are compatible with it. The conflict becomes manifest when all three conditions are imposed and persists for any nonzero value of entanglement. We propose an experiment to test our conclusions.


Papic, Z. (2014). Solvable models for unitary and nonunitary topological phases. Phys. Rev. B, 90(7), 17 pp.
Abstract: We introduce a broad class of simple models for quantum Hall states based on the expansion of their parent Hamiltonians near the onedimensional limit of “thin cylinders,” i.e., when one dimension Ly of the Hall surface becomes comparable to the magnetic length l(B). Formally, the models can be viewed as topological generalizations of the 1D Hubbard model with centerofmasspreserving hopping of multiparticle clusters. In some cases, we show that the models can be exactly solved using elementary techniques, and yield simple wave functions for the ground states as well as the entire neutral excitation spectrum. We study a large class of Abelian and nonAbelian states in this limit, including the ReadRezayi Z(k) series, as well as states deriving from nonunitary or irrational conformal field theories: the “Gaffnian,” “ Haffnian,” HaldaneRezayi, and the “permanent” state. We find that the thincylinder limit of unitary (rational) states is “classical”: their effective Hamiltonians reduce to only Hartreetype terms, the ground states are trivial insulators, and excitation gaps result from simple electrostatic repulsion. In contrast, for states deriving from nonunitary or irrational conformal field theories, the thincylinder limit is found to be intrinsically quantum; it contains hopping terms that play an important role in the structure of the ground states and in the energetics of the lowlying neutral excitations.


Rundquist, A., Bajcsy, M., Majumdar, A., Sarmiento, T., Fischer, K., Lagoudakis, K. G., et al. (2014). Nonclassical higherorder photon correlations with a quantum dot strongly coupled to a photoniccrystal nanocavity. Phys. Rev. A, 90(2), 9 pp.
Abstract: We use the third and fourthorder autocorrelation functions g((3))(tau(1), tau(2)) and g((4))(tau(1), tau(2), tau(3)) to detect the nonclassical character of the light transmitted through a photoniccrystal nanocavity containing a strongly coupled quantum dot probed with a train of coherent light pulses. We contrast the value of g((3))(0,0) with the conventionally used g((2))(0) and demonstrate that, in addition to being necessary for detecting twophoton states emitted by a lowintensity source, g((3)) provides a more clear indication of the nonclassical character of a light source. We also present preliminary data that demonstrates bunching in the fourthorder autocorrelation function g((4))( tau(1), tau(2), tau(3)) as the first step toward detecting threephoton states.


Bruschi, D. E., Ralph, T. C., Fuentes, I., Jennewein, T., & Razavi, M. (2014). Spacetime effects on satellitebased quantum communications. Phys. Rev. D, 90(4), 13 pp.
Abstract: We investigate the consequences of spacetime being curved on spacebased quantum communication protocols. We analyze tasks that require either the exchange of single photons in a certain entanglement distribution protocol or beams of light in a continuousvariable quantum key distribution scheme. We find that gravity affects the propagation of photons, therefore adding additional noise to the channel for the transmission of information. The effects could be measured with current technology.


Parameswaran, S. A., Grover, T., Abanin, D. A., Pesin, D. A., & Vishwanath, A. (2014). Probing the Chiral Anomaly with Nonlocal Transport in ThreeDimensional Topological Semimetals. Phys. Rev. X, 4(3), 12 pp.
Abstract: Weyl semimetals are threedimensional crystalline systems where pairs of bands touch at points in momentum space, termed Weyl nodes, that are characterized by a definite topological charge: the chirality. Consequently, they exhibit the AdlerBellJackiw anomaly, which in this condensedmatter realization implies that the application of parallel electric (E) and magnetic (B) fields pumps electrons between nodes of opposite chirality at a rate proportional to E . B. We argue that this pumping is measurable via nonlocal transport experiments, in the limit of weak internode scattering. Specifically, we show that as a consequence of the anomaly, applying a local magnetic field parallel to an injected current induces a valley imbalance that diffuses over long distances. A probe magnetic field can then convert this imbalance into a measurable voltage drop far from source and drain. Such nonlocal transport vanishes when the injected current and magnetic field are orthogonal and therefore serves as a test of the chiral anomaly. We further demonstrate that a similar effect should also characterize Dirac semimetalsrecently reported to have been observed in experimentswhere the coexistence of a pair of Weyl nodes at a single point in the Brillouin zone is protected by a crystal symmetry. Since the nodes are analogous to valley degrees of freedom in semiconductors, the existence of the anomaly suggests that valley currents in threedimensional topological semimetals can be controlled using electric fields, which has potential practical “valleytronic” applications.


Ng, K. K., Hodgkinson, L., Louko, J., Mann, R. B., & MartinMartinez, E. (2014). UnruhDeWitt detector response along static and circulargeodesic trajectories for Schwarzschildantide Sitter black holes. Phys. Rev. D, 90(6), 13 pp.
Abstract: We present novel methods to numerically address the problem of characterizing the response of particle detectors in curved spacetimes. These methods allow for the integration of the Wightman function, at least in principle, in rather general backgrounds. In particular we will use this tool to further understand the nature of conformal massless scalar Hawking radiation from a Schwarzschild black hole in antide Sitter space. We do that by studying an UnruhDeWitt detector at rest above the horizon and in circulargeodesic orbit. The method allows us to see that the response rate shows peaks at certain characteristic frequencies, which correspond to the quasinormal modes of the spacetime. It is in principle possible to apply these techniques to more complicated and interesting physical scenarios, e.g., geodesic infall or multiple detector entanglement evolution, or the study of the behavior of quantum correlations in spacetimes with black hole horizons.


JochymO'Connor, T., Kribs, D. W., Laflamme, R., & Plosker, S. (2014). Quantum subsystems: Exploring the complementarity of quantum privacy and error correction. Phys. Rev. A, 90(3), 12 pp.
Abstract: This paper addresses and expands on the contents of the recent Letter [Phys. Rev. Lett. 111, 030502 (2013)] discussing private quantum subsystems. Here we prove several previously presented results, including a condition for a given random unitary channel to not have a private subspace (although this does not mean that private communication cannot occur, as was previously demonstrated via private subsystems) and algebraic conditions that characterize when a general quantum subsystem or subspace code is private for a quantum channel. These conditions can be regarded as the private analog of the KnillLaflamme conditions for quantum error correction, and we explore how the conditions simplify in some special cases. The bridge between quantum cryptography and quantum error correction provided by complementary quantum channels motivates the study of a new, more general definition of quantum errorcorrecting code, and we initiate this study here. We also consider the concept of complementarity for the general notion of a private quantum subsystem.


Jordan, S. P., Lee, K. S. M., & Preskill, J. (2014). QUANTUM COMPUTATION OF SCATTERING IN SCALAR QUANTUM FIELD THEORIES. Quantum Inform. Comput., 14(1112), 1014–1080.
Abstract: Quantum field theory provides the framework for the most fundamental physical theories to be confirmed experimentally and has enabled predictions of unprecedented precision. However, calculations of physical observables often require great computational complexity and can generally be performed only when the interaction strength is weak. A full understanding of the foundations and rich consequences of quantum field theory remains an outstanding challenge. We develop a quantum algorithm to compute relativistic scattering amplitudes in massive phi(4) theory in spacetime of four and fewer dimensions. The algorithm runs in a time that is polynomial in the number of particles, their energy, and the desired precision, and applies at both weak and strong coupling. Thus, it offers exponential speedup over existing classical methods at high precision or strong coupling.
Keywords: quantum algorithm; simulation; quantum field theory


Ouyang, Y. K. (2014). CHANNEL COVARIANCE, TWIRLING, CONTRACTION, AND SOME UPPER BOUNDS ON THE QUANTUM CAPACITY. Quantum Inform. Comput., 14(1112), 917–936.
Abstract: Evaluating the quantum capacity of quantum channels is an important but difficult problem, even for channels of low input and output dimension. Smith and Smolin showed that the quantum capacity of the Cliffordtwirl of a qubit amplitude damping channel (a qubit depolarizing channel) has a quantum capacity that is at most the coherent information of the qubit amplitude damping channel evaluated on the maximally mixed input state. We restrict our attention to obtaining upper bounds on the quantum capacity using a generalization of Smith and Smolin's degradable extension technique. Given a degradable channel N and a finite projective group of unitaries V, we show that the Vtwirl of N has a quantum capacity at most the coherent information of N maximized over a Vcontracted space of input states. As a consequence, degradable channels that are covariant with respect to diagonal Pauli matrices have quantum capacities that are their coherent information maximized over just the diagonal input states. As an application of our main result, we supply new upper bounds on the quantum capacity of some unital and nonunital channels – ddimensional depolarizing channels, twoqubit locally symmetric Pauli channels, and shifted qubit depolarizing channels.
Keywords: Quantum information theory; quantum capacity; upper bounds; degradable twirling; contraction; depolarizing channel


Brod, D. J., & Childs, A. M. (2014). THE COMPUTATIONAL POWER OF MATCHGATES AND THE XY INTERACTION ON ARBITRARY GRAPHS. Quantum Inform. Comput., 14(1112), 901–916.
Abstract: Matchgates are a restricted set of twoqubit gates known to be classically simulable when acting on nearestneighbor qubits on a path, but universal for quantum computation when the qubits are arranged on certain other graphs. Here we characterize the power of matchgates acting on arbitrary graphs. Specifically, we show that they are universal on any connected graph other than a path or a cycle, and that they are classically simulable on a cycle. We also prove the same dichotomy for the XY interaction, a proper subset of matchgates related to some implementations of quantum computing.
Keywords: Matchgates; XY interaction; Encoded universality


