
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


Paetznick, A., & Svore, K. M. (2014). REPEATUNTILSUCCESS: NONDETERMINISTIC DECOMPOSITION OF SINGLEQUBIT UNITARIES. Quantum Inform. Comput., 14(1516), 1277–1301.
Abstract: We present a decomposition technique that uses nondeterministic circuits to approximate an arbitrary singlequbit unitary to within distance E and requires significantly fewer nonClifford gates than existing techniques. We develop “RepeatUntilSuccess” (RUS) circuits and characterize unitaries that can be exactly represented as an RUS circuit. Our RUS circuits operate by conditioning on a given measurement outcome and using only a small number of nonClifford gates and ancilla qubits. We construct an algorithm based on RUS circuits that approximates an arbitrary singlequbit Zaxis rotation to within distance e, where the number of T gates scales as 1.26 log(2)(1/is an element of) – 3.53, an improvement of roughly threefold over stateoftheart techniques. We then extend our algorithm and show that a scaling of 2.4 log(2)(1/is an element of) – 3.28 can be achieved for arbitrary unitaries and a small range of is an element of, which is roughly twice as good as optimal deterministic decomposition methods.
Keywords: quantum circuits; unitary decomposition


Gosset, D., Kliuchnikov, V., Mosca, M., & Russo, V. (2014). AN ALGORITHM FOR THE TCOUNT. Quantum Inform. Comput., 14(1516), 1261–1276.
Abstract: We consider quantum circuits composed of Clifford and T gates. In this context the T gate has a special status since it confers universal computation when added to the (classically simulable) Clifford gates. However it can be very expensive to implement faulttolerantly. We therefore view this gate as a resource which should be used only when necessary. Given an nqubit unitary U we are interested in computing a circuit that implements it using the minimum possible number of T gates (called the Tcount of U). A related task is to decide if the Tcount of U is less than or equal to m; we consider this problem as a function of N = 2(n) and m. We provide a classical algorithm which solves it using time and space both upper bounded as O(N(m)poly(m, N)). We implemented our algorithm and used it to show that any Clifford+T circuit for the Toffoli or the Fredkin gate requires at least 7 T gates. This implies that the known 7 T gate circuits for these gates are Toptimal. We also provide a simple expression for the Tcount of singlequbit unitaries.


Cosentino, A., & Russo, V. (2014). SMALL SETS OF LOCALLY INDISTINGUISHABLE ORTHOGONAL MAXIMALLY ENTANGLED STATES. Quantum Inform. Comput., 14(1314), 1098–1106.
Abstract: We study the problem of distinguishing quantum states using local operations and classical communication (LOCC). A question of fundamental interest is whether there exist sets of k <= d orthogonal maximally entangled states in Cd circle times Cd that are not perfectly distinguishable by LOCC. A recent result by Yu, Duan, and Ying [Phys. Rev. Lett. 109 020506 (2012)] gives an affirmative answer for the case k = d. We give, for the first time, a proof that such sets of states indeed exist even in the case k < d. Our result is constructive and holds for an even wider class of operations known as positivepartialtranspose measurements (PPT). The proof uses the characterization of the PPTdistinguishability problem as a semidefinite program.
Keywords: LOCC; PPT; state distinguishability; semidefinite programming


Grusdt, F., Abanin, D., & Demler, E. (2014). Measuring Z(2) topological invariants in optical lattices using interferometry. Phys. Rev. A, 89(4), 21 pp.
Abstract: We propose an interferometric method to measure Z(2) topological invariants of timereversal invariant topological insulators realized with optical lattices in two and three dimensions. We suggest two schemes which both rely on a combination of Bloch oscillations with Ramsey interferometry and can be implemented using standard tools of atomic physics. In contrast to topological Zak phase and Chern number, defined for individual onedimensional and twodimensional Bloch bands, the formulation of the Z(2) invariant involves at least two Bloch bands related by timereversal symmetry which one must keep track of in measurements. In one of our schemes this can be achieved by the measurement of Wilson loops, which are nonAbelian generalizations of Zak phases. The winding of their eigenvalues is related to the Z(2) invariant. We thereby demonstrate that Wilson loops are not just theoretical concepts but can be measured experimentally. For the second scheme we introduce a generalization of timereversal polarization which is continuous throughout the Brillouin zone. We show that its winding over half the Brillouin zone yields the Z(2) invariant. To measure this winding, our protocol only requires Bloch oscillations within a single band, supplemented by coherent transitions to a second band which can be realized by lattice shaking.


Marvian, I., & Spekkens, R. W. (2014). Asymmetry properties of pure quantum states. Phys. Rev. A, 90(1), 4 pp.
Abstract: The asymmetry properties of a state relative to some symmetry group specify how and to what extent the given symmetry is broken by the state. Characterizing these is found to be surprisingly useful for addressing a very common problem: to determine what follows from a system's dynamics (possibly open) having that symmetry. We demonstrate and exploit the fact that the asymmetry properties of a state can be understood in terms of informationtheoretic concepts. We show that for a pure state psi and a symmetry group G, they are completely specified by the characteristic function of the state, defined as chi(psi)(g) <psi broken vertical bar U (g) broken vertical bar psi >, where g is an element of G and U is the unitary representation of interest. Based on this observation, we study several important problems about the interconversion of pure states under symmetric dynamics, such as determining the conditions for reversible transformations, deterministic irreversible transformations, and asymptotic transformations.


