
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.


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.


Shashi, A., Grusdt, F., Abanin, D. A., & Demler, E. (2014). Radiofrequency spectroscopy of polarons in ultracold Bose gases. Phys. Rev. A, 89(5), 17 pp.
Abstract: Recent experimental advances enabled the realization of mobile impurities immersed in a BoseEinstein condensate (BEC) of ultracold atoms. Here, we consider impurities with two or more internal hyperfine states, and study their radiofrequency (rf) absorption spectra, which correspond to transitions between two different hyperfine states. We calculate rf spectra for the case when one of the hyperfine states involved interacts with the BEC, while the other state is noninteracting, by performing a nonperturbative resummation of the probabilities of exciting different numbers of phonon modes. In the presence of interactions, the impurity gets dressed by Bogoliubov excitations of the BEC, and forms a polaron. The rf signal contains a deltafunction peak centered at the energy of the polaron measured relative to the bare impurity transition frequency with a weight equal to the amount of bare impurity character in the polaron state. The rf spectrum also has a broad incoherent part arising from the background excitations of the BEC, with a characteristic powerlaw tail that appears as a consequence of the universal physics of contact interactions. We discuss both the direct rf measurement, in which the impurity is initially in an interacting state, and the inverse rf measurement, in which the impurity is initially in a noninteracting state. In the latter case, in order to calculate the rf spectrum, we solve the problem of polaron formation: a mobile impurity is suddenly introduced in a BEC, and dynamically gets dressed by Bogoliubov phonons. Our solution is based on a timedependent variational ansatz of coherent states of Bogoliubov phonons, which becomes exact when the impurity is localized. Moreover, we show that such an ansatz compares well with a semiclassical estimate of the propagation amplitude of a mobile impurity in the BEC. Our technique can be extended to cases when both initial and final impurity states are interacting with the BEC.


Fritsch, K., Kermarrec, E., Ross, K. A., Qiu, Y., Copley, J. R. D., Pomaranski, D., et al. (2014). Temperature and magnetic field dependence of spinice correlations in the pyrochlore magnet Tb2Ti2O7. Phys. Rev. B, 90(1), 9 pp.
Abstract: We present a parametric study of the diffuse magnetic scattering at (1/2, 1/2, 1/2) positions in reciprocal space, ascribed to a frozen antiferromagnetic spin ice state in singlecrystalline Tb2Ti2O7. Our highresolution neutron scattering measurements show that the elastic (0.02 meV < E < 0.02 meV) (1/2, 1/2, 1/2) scattering develops strongly below approximate to 275 mK, and correlates with the opening of a spin gap of approximate to 0.06 to 0.08 meV over most of the Brillouin zone. The nature of the transition at 275 mK has many characteristics of spin glass behavior, consistent with acsusceptibility measurements. The application of a magnetic field of 0.075 T applied along the [1 (1) over bar0] direction destroys the (1/2, 1/2, 1/2) elastic scattering, revealing the fragility of this shortrange ordered ground state. We construct a refined HT phase diagram for Tb2Ti2O7 and [1 (1) over bar0] fields which incorporates this frozen spin ice regime and the antiferromagnetic longrange order previously known to be induced in relatively large fields. Specific heat measurements on the same crystal reveal a sharp anomaly at Tc approximate to 450 mK and no indication of a transition near approximate to 275 mK. We conclude that the higher temperature specific heat peak is not related to the magnetic ordering but is likely a signal of other, nonmagnetic dipole correlations.


Agnew, M., Bolduc, E., Resch, K. J., FrankeArnold, S., & Leach, J. (2014). Discriminating SinglePhoton States Unambiguously in High Dimensions. Phys. Rev. Lett., 113(2), 5 pp.
Abstract: The ability to uniquely identify a quantum state is integral to quantum science, but for nonorthogonal states, quantum mechanics precludes deterministic, errorfree discrimination. However, using the nondeterministic protocol of unambiguous state discrimination enables the errorfree differentiation of states, at the cost of a lower frequency of success. We discriminate experimentally between nonorthogonal, highdimensional states encoded in single photons; our results range from dimension d = 2 to d = 14. We quantify the performance of our method by comparing the total measured error rate to the theoretical rate predicted by minimumerror state discrimination. For the chosen states, we find a lower error rate by more than 1 standard deviation for dimensions up to d = 12. This method will find immediate application in highdimensional implementations of quantum information protocols, such as quantum cryptography.


Wang, J., Byrd, J. N., Simbotin, I., & Cote, R. (2014). Tuning Ultracold Chemical Reactions via RydbergDressed Interactions. Phys. Rev. Lett., 113(2), 5 pp.
Abstract: We show that ultracold chemical reactions with an activation barrier can be tuned using Rydbergdressed interactions. Scattering in the ultracold regime is sensitive to longrange interactions, especially when weakly bound (or quasibound) states exist near the collision threshold. We investigate how, by Rydberg dressing a reactant, one enhances its polarizability and modifies the longrange van der Waals collision complex, which can alter chemical reaction rates by shifting the position of nearthreshold bound states. We carry out a full quantum mechanical scattering calculation for the benchmark system H2 + D, and show that resonances can be moved substantially and that rate coefficients at cold and ultracold temperatures can be increased by several orders of magnitude.


Repellin, C., Neupert, T., Papic, Z., & Regnault, N. (2014). Singlemode approximation for fractional Chern insulators and the fractional quantum Hall effect on the torus. Phys. Rev. B, 90(4), 11 pp.
Abstract: We analyze the collective magnetoroton excitations of bosonic Laughlin v = 1/2 fractional quantum Hall (FQH) states on the torus and of their analog on the lattice, the fractional Chern insulators (FCIs). We show that, by applying the appropriate mapping of momentum quantum numbers between the two systems, the magnetoroton mode can be identified in FCIs and that it contains the same number of states as in the FQH case. Further, we numerically test the singlemode approximation to the magnetoroton mode for both the FQH and FCI cases. This proves particularly challenging for the FCI because its eigenstates have a lower translational symmetry than the FQH states. In spite of this, we construct the FCI singlemode approximation such that it carries the same momenta as the FQH states, allowing for a direct comparison between the two systems. We show that the singlemode approximation captures well a dispersive subset of the magnetoroton excitations both for the FQH and the FCI cases. We find remarkable quantitative agreement between the two systems. For example, the manybody excitation gap extrapolates to almost the same value in the thermodynamic limit.


Leung, D., Li, K., Smith, G., & Smolin, J. A. (2014). Maximal Privacy without Coherence. Phys. Rev. Lett., 113(3), 5 pp.
Abstract: Privacy is a fundamental feature of quantum mechanics. A coherently transmitted quantum state is inherently private. Remarkably, coherent quantum communication is not a prerequisite for privacy: there are quantum channels that are too noisy to transmit any quantum information reliably that can nevertheless send private classical information. Here, we ask how much private classical information a channel can transmit if it has little quantum capacity. We present a class of channels Nd with input dimension d(2), quantum capacity Q(Nd) <= 1, and private capacity P(Nd) = log d. These channels asymptotically saturate an interesting inequality P(N) <= (1/2) [log d(A) + Q(N)] for any channel N with input dimension d(A) and capture the essence of privacy stripped of the confounding influence of coherence.


Ito, T. (2014). Parallelization of entanglementresistant multiprover interactive proofs. Inf. Process. Lett., 114(10), 579–583.
Abstract: Multiprover interactive proof systems are said to be entanglementresistant if the soundness holds even when provers are allowed to share an arbitrary quantum state before the interaction starts. This letter proves that every entanglementresistant multiprover interactive proof system can be parallelized to two rounds without ruining its entanglement resistance at the expense of adding one prover. (C) 2014 Elsevier B.V. All rights reserved.
Keywords: Computational complexity; Quantum computing; Interactive proof; Quantum nonlocality


Lu, D. W., Brodutch, A., Li, J., Li, H., & Laflamme, R. (2014). Experimental realization of postselected weak measurements on an NMR quantum processor. New J. Phys., 16, 12 pp.
Abstract: The ability to postselect the outcomes of an experiment is a useful theoretical concept and experimental tool. In the context of weak measurements, postselection can lead to surprising results such as complex weak values outside the range of eigenvalues. Usually postselection is realized by a projective measurement, which is hard to implement in ensemble systems such as NMR. We demonstrate the first experiment of a weak measurement with postselection on an NMR quantum information processor. Our setup is used for measuring complex weak values and weak values outside the range of eigenvalues. The scheme for overcoming the problem of postselection in an ensemble quantum computer is general and can be applied to any circuitbased implementation. This experiment paves the way for studying and exploiting postselection and weak measurements in systems where projective measurements are hard to realize experimentally.
Keywords: weak measurement; NMR quantum information processing; quantum circuit


Jayakumar, H., Predojevic, A., Kauten, T., Huber, T., Solomon, G. S., & Weihs, G. (2014). Timebin entangled photons from a quantum dot. Nat. Commun., 5, 5 pp.
Abstract: Longdistance quantum communication is one of the prime goals in the field of quantum information science. With information encoded in the quantum state of photons, existing telecommunication fibre networks can be effectively used as a transport medium. To achieve this goal, a source of robust entangled singlephoton pairs is required. Here we report the realization of a source of timebin entangled photon pairs utilizing the biexcitonexciton cascade in a III/V selfassembled quantum dot. We analyse the generated photon pairs by an inherently phasestable interferometry technique, facilitating uninterrupted long integration times. We confirm the entanglement by performing quantum state tomography of the emitted photons, which yields a fidelity of 0.69(3) and a concurrence of 0.41(6) for our realization of timeenergy entanglement from a single quantum emitter.


Muralidharan, S., Kim, J., Lutkenhaus, N., Lukin, M. D., & Jiang, L. (2014). Ultrafast and FaultTolerant Quantum Communication across Long Distances. Phys. Rev. Lett., 112(25), 6 pp.
Abstract: Quantum repeaters (QRs) provide a way of enabling long distance quantum communication by establishing entangled qubits between remote locations. In this Letter, we investigate a new approach to QRs in which quantum information can be faithfully transmitted via a noisy channel without the use of long distance teleportation, thus eliminating the need to establish remote entangled links. Our approach makes use of small encoding blocks to faulttolerantly correct both operational and photon loss errors. We describe a way to optimize the resource requirement for these QRs with the aim of the generation of a secure key. Numerical calculations indicate that the number of quantum memory bits at each repeater station required for the generation of one secure key has favorable polylogarithmic scaling with the distance across which the communication is desired.


Yang, H., & Casals, M. (2014). Wavefront twisting by rotating black holes: Orbital angular momentum generation and phase coherent detection. Phys. Rev. D, 90(2), 15 pp.
Abstract: In this paper we study wave propagation and scattering near a black hole. In particular, we assume a coherent emission source near the black hole and investigate the wavefront distortion as seen by a distant observer. By ignoring the spin nature of the electromagnetic radiation we model it by a complex scalar field. Then, the propagating wave near the observer can be decomposed using the LaguerreGaussian mode basis and its wavefront distortion can be characterized by the decomposition coefficient. We find that this decomposition spectrum is symmetric with respect to the azimuthal quantum number in the case that the wave source is located near a nonrotating (Schwarzschild) black hole, whereas the spectrum is generically asymmetric if the host black hole is rotating (Kerr). The spectral asymmetry, or the net orbital angular momentum carried by the wave, is intimately related to the blackhole spin and mass, the wave frequency and the locations of the source and the observer. We present semianalytical expressions and numerical results for these parameter dependences. If the emitted radiation is temporally coherent, our results show that the secondary images (arising from the orbiting of the wavefront around the black hole) of the source can be almost as bright as its primary image. Separately, in the case of temporally incoherent radiation, we show that the nonfundamental spectrum components in the primary image could be resolved by spatially separated telescopes, although that would be degenerate with the telescope direction. Finally, our results suggest that the blackholeinduced spectral asymmetry is generally too weak to be observed in radio astronomy, even if the observer is located near an optical caustic.


Piggott, A. Y., Lagoudakis, K. G., Sarmiento, T., Bajcsy, M., Shambat, G., & Vuckovic, J. (2014). Photooxidative tuning of individual and coupled GaAs photonic crystal cavities. Opt. Express, 22(12), 15017–15023.
Abstract: We demonstrate a photoinduced oxidation technique for tuning GaAs photonic crystal cavities using a lowpower 390 nm pulsed laser. The laser oxidizes a small (< 1 μm) diameter spot, reducing the local index of refraction and blueshifting the cavity. The tuning progress can be actively monitored in real time. We also demonstrate tuning an individual cavity within a pair of proximitycoupled cavities, showing that this method can be used to tune individual cavities in a cavity network, with applications in quantum simulations and quantum computing. (C) 2014 Optical Society of America


MartinMartinez, E., & Louko, J. (2014). Particle detectors and the zero mode of a quantum field. Phys. Rev. D, 90(2), 15 pp.
Abstract: We study the impact of the zero mode of a quantum field on the evolution of a particle detector. For a massless scalar field in a periodic cavity, we show that the impact of the zero mode on the UnruhDeWitt detector and its derivativecoupling generalization is necessarily nonvanishing but can be made negligible in some limits, including those commonly occurring in nonrelativistic quantum optics. For the derivativecoupling detector this can be accomplished by just tuning the zero mode's initial state, but the standard UnruhDeWitt detector requires a more subtle and careful tuning. Applications include an inertial detector with arbitrary velocity, where we demonstrate the regularity of the ultrarelativistic limit, and a detector with uniform acceleration.


