
Marvian, I., & Spekkens, R. W. (2014). Extending Noether's theorem by quantifying the asymmetry of quantum states. Nat. Commun., 5, 8 pp.
Abstract: Noether's theorem is a fundamental result in physics stating that every symmetry of the dynamics implies a conservation law. It is, however, deficient in several respects: for one, it is not applicable to dynamics wherein the system interacts with an environment; furthermore, even in the case where the system is isolated, if the quantum state is mixed then the Noether conservation laws do not capture all of the consequences of the symmetries. Here we address these deficiencies by introducing measures of the extent to which a quantum state breaks a symmetry. Such measures yield novel constraints on state transitions: for nonisolated systems they cannot increase, whereas for isolated systems they are conserved. We demonstrate that the problem of finding nontrivial asymmetry measures can be solved using the tools of quantum information theory. Applications include deriving modelindependent bounds on the quantum noise in amplifiers and assessing quantum schemes for achieving highprecision metrology.


Gacesa, M., & Cote, R. (2014). Photoassociation of ultracold molecules near a Feshbach resonance as a probe of the electronproton mass ratio variation. J. Mol. Spectrosc., 300, 124–130.
Abstract: We show that the photoassociation (PA) rate of ultracold diatomic molecules in the vicinity of a Feshbach resonance could be used to probe variations of the electrontoproton mass ratio beta = m(e)/m(p), a quantity related to other fundamental constants via the Lambda(QCD) scale. The PA rate exhibits two features near a Feshbach resonance, a minimum and a maximum, which are very sensitive to the details of the interactions and the exact mass of the system. The effect and detection threshold are illustrated in the formation rates of ultracold Li2 and LiNa molecules by performing coupledchannel calculations in an external magnetic field. We find that the PA rate is particularly sensitive near narrow Feshbach resonances in heavy molecules, where it might be possible to detect relative variability of beta on the order of 10(15)10(16). We also use a twochannel model to derive a proportionality relation between the variation of the PA rate and beta applicable to diatomic molecules. (C) 2014 Elsevier Inc. All rights reserved.
Keywords: Photoassociation; Ultracold diatomic molecules; Feshbach resonances; Electronproton mass ratio


Li, Z. K., Zhou, H., Ju, C. Y., Chen, H. W., Zheng, W. Q., Lu, D. W., et al. (2014). Experimental Realization of a Compressed Quantum Simulation of a 32Spin Ising Chain. Phys. Rev. Lett., 112(22), 5 pp.
Abstract: Certain nqubit quantum systems can be faithfully simulated by quantum circuits with only O(log(n)) qubits [B. Kraus, Phys. Rev. Lett. 107, 250503 (2011)]. Here we report an experimental realization of this compressed quantum simulation on a onedimensional Ising chain. By utilizing an nuclear magnetic resonance quantum simulator with only five qubits, the property of groundstate magnetization of an openboundary 32spin Ising model is experimentally simulated, prefacing the expected quantum phase transition in the thermodynamic limit. This experimental protocol can be straightforwardly extended to systems with hundreds of spins by compressing them into up to merely 10qubit systems. Our experiment paves the way for exploring physical phenomena in largescale quantum systems with quantum simulators under current technology.


Howard, M., Wallman, J., Veitch, V., & Emerson, J. (2014). Contextuality supplies the 'magic' for quantum computation. Nature, 510(7505), 351–355.
Abstract: Quantum computers promise dramatic advantages over their classical counterparts, but the source of the power in quantum computing has remained elusive. Here we prove a remarkable equivalence between the onset of contextuality and the possibility of universal quantum computation via 'magic state' distillation, which is the leading model for experimentally realizing a faulttolerant quantum computer. This is a conceptually satisfying link, because contextuality, which precludes a simple 'hidden variable' model of quantum mechanics, provides one of the fundamental characterizations of uniquely quantum phenomena. Furthermore, this connection suggests a unifying paradigm for the resources of quantum information: the nonlocality of quantum theory is a particular kind of contextuality, and nonlocality is already known to be a critical resource for achieving advantages with quantum communication. In addition to clarifying these fundamental issues, this work advances the resource framework for quantum computation, which has a number of practical applications, such as characterizing the efficiency and tradeoffs between distinct theoretical and experimental schemes for achieving robust quantum computation, and putting bounds on the overhead cost for the classical simulation of quantum algorithms.


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.


Benincasa, D. M. T., Borsten, L., Buck, M., & Dowker, F. (2014). Quantum information processing and relativistic quantum fields. Class. Quantum Gravity, 31(7), 14 pp.
Abstract: It is shown that an ideal measurement of a oneparticle wave packet state of a relativistic quantum field in Minkowski spacetime enables superluminal signalling. The result holds for a measurement that takes place over an intervention region in spacetime whose extent in time in some frame is longer than the lightcrossing time of the packet in that frame. Moreover, these results are shown to apply not only to ideal measurements but also to unitary transformations that rotate two orthogonal oneparticle states into each other. In light of these observations, possible restrictions on the allowed types of intervention are considered. A more physical approach to such questions is to construct explicit models of the interventions as interactions between the field and other quantum systems such as detectors. The prototypical UnruhDeWitt detector couples to the field operator itself and so most likely respects relativistic causality. On the other hand, detector models which couple to a finite set of frequencies of field modes are shown to lead to superluminal signalling. Such detectors do, however, provide successful phenomenological models of atomqubits interacting with quantum fields in a cavity but are valid only on time scales many orders of magnitude larger than the lightcrossing time of the cavity.
Keywords: quantum fields; relativistic quantum information; measurement


Ferrie, C., & Granade, C. E. (2014). LikelihoodFree Methods for Quantum Parameter Estimation. Phys. Rev. Lett., 112(13), 5 pp.
Abstract: In this Letter, we strengthen and extend the connection between simulation and estimation to exploit simulation routines that do not exactly compute the probability of experimental data, known as the likelihood function. Rather, we provide an explicit algorithm for estimating parameters of physical models given access to a simulator which is only capable of producing sample outcomes. Since our algorithm does not require that a simulator be able to efficiently compute exact probabilities, it is able to exponentially outperform standard algorithms based on exact computation. In this way, our algorithm opens the door for the application of new insights and resources to the problem of characterizing large quantum systems, which is exponentially intractable using standard simulation resources.


Miao, G. X., Chang, J., Assaf, B. A., Heiman, D., & Moodera, J. S. (2014). Spin regulation in composite spinfilter barrier devices. Nat. Commun., 5, 6 pp.
Abstract: Magnetic insulators are known to provide large effective Zeeman fields that are confined at an interface, making them especially powerful in modifying adjacent oneor twodimensional electronic structures. Utilizing this phenomenon and the other important property of magnetic insulatorsspin filteringhere we report the generation and subsequent detection of a large interface field, as large as tens of tesla in EuS/Al/EuS heterostructures with metallic coulomb islands confined within a magnetic insulator barrier. The unique energy profile across this sandwich structure produces spinassisted charge transfer across the device, generating a spontaneous spin current and voltage. These unique properties can be practical for controlling spin flows in electronic devices and for energy harvesting.


Ki, D. K., Fal'ko, V. I., Abanin, D. A., & Morpurgo, A. F. (2014). Observation of Even Denominator Fractional Quantum Hall Effect in Suspended Bilayer Graphene. Nano Lett., 14(4), 2135–2139.
Abstract: We investigate lowtemperature magnetotransport in recently developed, highquality multiterminal suspended bilayer graphene devices, enabling the independent measurement of the longitudinal and transverse resistance. We observe clear signatures of the fractional quantum Hall effect with different states that are either fully developed, and exhibit a clear plateau in the transverse resistance with a concomitant dip in longitudinal resistance or incipient, and exhibit only a longitudinal resistance minimum. All observed states scale as a function of filling factor v, as expected. An unprecedented evendenominator fractional state is observed at v = 1/2 on the hole side, exhibiting a clear plateau in Rxy quantized at the expected value of 2h/e(2) with a precision of similar to 0.5%. Many of our observations, together with a recent electronic compressibility measurement performed in graphene bilayers on hexagonal boronnitride (hBN) substrates, are consistent with a recent theory that accounts for the effect of the degeneracy between the N = 0 and N = 1 Landau levels in the fractional quantum Hall effect and predicts the occurrence of a MooreRead type v = 1/2 state. Owing to the experimental flexibility of bilayer graphene, which has a gatedependent band structure, can be easily accessed by scanning probes, and can be contacted with materials such as superconductors, our findings offer new possibilities to explore the microscopic nature of evendenominator fractional quantum Hall effect.
Keywords: Evendenominator fractional quantum Hall effect; multiterminal suspended bilayer graphene; NonAbelian MooreRead state


Erven, C., MeyerScott, E., Fisher, K., Lavoie, J., Higgins, B. L., Yan, Z., et al. (2014). Experimental threephoton quantum nonlocality under strict locality conditions. Nat. Photonics, 8(4), 292–296.
Abstract: Quantum correlations, often observed as violations of Bell inequalities(15), are critical to our understanding of the quantum world, with farreaching technological(69) and fundamental impact. Many tests of Bell inequalities have studied pairs of correlated particles. However, interest in multiparticle quantum correlations is driving the experimental frontier to test larger systems. All violations to date require supplementary assumptions that open results to loopholes, the closing of which is one of the most important challenges in quantum science. Seminal experiments have closed some loopholes(1016), but no experiment has closed locality loopholes with three or more particles. Here, we close both the locality and freedomofchoice loopholes by distributing threephoton GreenbergerHorneZeilinger entangled states(17) to independent observers. We measured a violation of Mermin's inequality(18) with parameter 2.77 +/ 0.08, violating its classical bound by nine standard deviations. These results are a milestone in multiparty quantum communication(19) and a significant advancement of the foundations of quantum mechanics(20).


Yu, N. K., Duan, R. Y., & Ying, M. S. (2014). Distinguishability of Quantum States by Positive OperatorValued Measures With Positive Partial Transpose. IEEE Trans. Inf. Theory, 60(4), 2069–2079.
Abstract: We study the distinguishability of bipartite quantum states by positive operatorvalued measures with positive partial transpose (PPT POVMs). The contributions of this paper include: 1) we give a negative answer to an open problem of showing a limitation of a previous known method for detecting nondistinguishability; 2) we show that a maximally entangled state and its orthogonal complement, no matter how many copies are supplied, cannot be distinguished by the PPT POVMs, even unambiguously. This result is much stronger than the previous known ones; and 3) we study the entanglement cost of distinguishing quantum states. It is proved that root 2/3 vertical bar 00 > + root 1/3 vertical bar 11 > is sufficient and necessary for distinguishing three Bell states by the PPT POVMs. An upper bound of entanglement cost of distinguishing a d circle times d pure state and its orthogonal complement is obtained for separable operations. Based on this bound, we are able to construct two orthogonal quantum states, which cannot be distinguished unambiguously by separable POVMs, but finite copies would make them perfectly distinguishable by local operations and classical communication. We further observe that a twoqubit maximally entangled state is always enough for distinguishing a d circle times d pure state and its orthogonal complement by the PPT POVMs, no matter the value of d. In sharp contrast, an entangled state with Schmidt number at least d is always needed for distinguishing such two states by separable POVMs. As an application, we show that the entanglement cost of distinguishing a d circle times d maximally entangled state and its orthogonal complement must be a maximally entangled state for d = 2, which implies that teleportation is optimal, and in general, it could be chosen as O(log d/d).
Keywords: Quantum nonlocality; local distinguishability; PPT POVMs; entanglement cost


Li, B., Chen, L., & Fan, H. (2014). Nonzero total correlation means nonzero quantum correlation. Phys. Lett. A, 378(1819), 1249–1253.
Abstract: We investigated the super quantum discord based on weak measurements. The super quantum discord is an extension of the standard quantum discord defined by projective measurements and also describes the quantumness of correlations. We provide some equivalent conditions for zero super quantum discord by using quantum discord, classical correlation and mutual information. In particular, we find that the super quantum discord is zero only for product states, which have zero mutual information. This result suggests that nonzero correlations can always be detected using the quantum correlation with weak measurements. As an example, we present the assisted statediscrimination method. (C) 2014 Elsevier B.V. All rights reserved.
Keywords: Super discord; Weak measurement; Optimal state discrimination


Atikian, H. A., Eftekharian, A., Salim, A. J., Burek, M. J., Choy, J. T., Majedi, A. H., et al. (2014). Superconducting nanowire single photon detector on diamond. Appl. Phys. Lett., 104(12), 4 pp.
Abstract: Superconducting nanowire single photon detectors are fabricated directly on diamond substrates and their optical and electrical properties are characterized. Dark count performance and photon count rates are measured at varying temperatures for 1310 nm and 632 nm photons. A multistep diamond surface polishing procedure is reported, involving iterative reactive ion etching and mechanical polishing to create a suitable diamond surface for the deposition and patterning of thin film superconducting layers. Using this approach, diamond substrates with less than 300 pm Root Mean Square surface roughness are obtained. (C) 2014 AIP Publishing LLC.


Tanner, M. G., Makarov, V., & Hadfield, R. H. (2014). Optimised quantum hacking of superconducting nanowire singlephoton detectors. Opt. Express, 22(6), 6734–6748.
Abstract: We explore brightlight control of superconducting nanowire singlephoton detectors (SNSPDs) in the shunted configuration (a practical measure to avoid latching). In an experiment, we simulate an illumination pattern the SNSPD would receive in a typical quantum key distribution system under hacking attack. We show that it effectively blinds and controls the SNSPD. The transient blinding illumination lasts for a fraction of a microsecond and produces several deterministic fake clicks during this time. This attack does not lead to elevated timing jitter in the spoofed output pulse, and hence does not introduce significant errors. Five different SNSPD chip designs were tested. We consider possible countermeasures to this attack.


Johri, S., Papic, Z., Bhatt, R. N., & Schmitteckert, P. (2014). Quasiholes of 1/3 and 7/3 quantum Hall states: Size estimates via exact diagonalization and densitymatrix renormalization group. Phys. Rev. B, 89(11), 8 pp.
Abstract: We determine the size of the elementary quasihole in nu = 1/3 and nu = 7/3 quantum Hall states via exactdiagonalization and densitymatrix renormalization group calculations on the sphere and cylinder, using a variety of shortand longrange pinning potentials. The size of the quasihole at filling factor nu = 1/3 is estimated to be approximate to 4l(B), and that of nu = 7/3 is approximate to 7l(B), where l(B) is the magnetic length. In contrast, the size of the Laughlin quasihole, expected to capture the basic physics in these two states, is around approximate to 2.5l(B). Our work supports the earlier findings that the quasihole in the first excited Landau level is significantly larger than in the lowest Landau level.


