Majorana Quasi-Particles Protected by $\mathbb{Z}_2$ Angular Momentum Conservation. (arXiv:1702.04733v2 [cond-mat.str-el] UPDATED) Quantum Physics - Wed, 2017-05-24 09:45

We show how angular momentum conservation can stabilise a symmetry-protected quasi-topological phase of matter supporting Majorana quasi-particles as edge modes in one-dimensional cold atom gases. We investigate a number-conserving four-species Hubbard model in the presence of spin-orbit coupling. The latter reduces the global spin symmetry to an angular momentum parity symmetry, which provides an extremely robust protection mechanism that does not rely on any coupling to additional reservoirs. The emergence of Majorana edge modes is elucidated using field theory techniques, and corroborated by density-matrix-renormalization-group simulations. Our results pave the way toward the observation of Majorana edge modes with alkaline-earth-like fermions in optical lattices, where all basic ingredients for our recipe - spin-orbit coupling and strong inter-orbital interactions - have been experimentally realized over the last two years.

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Extracting work from quantum measurement in Maxwell demon engines. (arXiv:1702.01917v2 [quant-ph] UPDATED) Quantum Physics - Wed, 2017-05-24 09:45

The essence of both classical and quantum engines is to extract useful energy (work) from stochastic energy sources, e.g. thermal baths. In Maxwell's demon engines, work extraction is assisted by a feedback control based on measurements performed by a demon, whose memory is erased at some nonzero energy cost. Here we propose a new type of quantum Maxwell's demon engine where work is directly extracted from the measurement channel, such that no heat bath is required. We show that in the Zeno regime of frequent measurements, memory erasure costs eventually vanish. Our findings provide a new paradigm to analyze quantum heat engines and work extraction in the quantum world.

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Quantum Hall physics: Hierarchies and conformal field theory techniques

Reviews of Modern Physics - Tue, 2017-05-23 16:00

Author(s): T. H. Hansson, M. Hermanns, S. H. Simon, and S. F. Viefers

The quantum Hall effects by now are recognized as prime examples of the importance of topological considerations in condensed-matter physics. The fractional Quantum Hall effect in particular has proven to display a large number of topologically ordered states that have been classified and understood in terms of hierarchical schemes. This review explains the current understanding of such classifications, with particular emphasis on conformal-field-theory approaches.

[Rev. Mod. Phys. 89, 025005] Published Tue May 23, 2017

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Superradiant phase transition in the ultrastrong-coupling regime of the two-photon Dicke model

Author(s): L. Garbe, I. L. Egusquiza, E. Solano, C. Ciuti, T. Coudreau, P. Milman, and S. Felicetti

The controllability of current quantum technologies allows one to implement spin-boson models where two-photon couplings are the dominating terms of light-matter interaction. In this case, when the coupling strength becomes comparable with the characteristic frequencies, a spectral collapse can take…

[Phys. Rev. A 95, 053854] Published Tue May 23, 2017

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Quasiparticle Energy in a Strongly Interacting Homogeneous Bose-Einstein Condensate

Author(s): Raphael Lopes, Christoph Eigen, Adam Barker, Konrad G. H. Viebahn, Martin Robert-de-Saint-Vincent, Nir Navon, Zoran Hadzibabic, and Robert P. Smith

Using two-photon Bragg spectroscopy, we study the energy of particlelike excitations in a strongly interacting homogeneous Bose-Einstein condensate, and observe dramatic deviations from Bogoliubov theory. In particular, at large scattering length a the shift of the excitation resonance from the free…

[Phys. Rev. Lett. 118, 210401] Published Tue May 23, 2017

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Synopsis: Capillary Effect in Grains Explained

APS Physics - Tue, 2017-05-23 16:00

Numerical simulations show that a previously observed capillary-like action in vibrating grain systems is due to convective motion of the grains.  

[Physics] Published Tue May 23, 2017

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Entangling two oscillators with arbitrary asymmetric initial states

PRA: Quantum information - Tue, 2017-05-23 16:00

Author(s): Chui-Ping Yang, Qi-Ping Su, Shi-Biao Zheng, Franco Nori, and Siyuan Han

We present a Hamiltonian which can be used to convert any asymmetric state |φ〉a|ϕ〉b of two oscillators a and b into an entangled state via a single-step operation. Furthermore, with this Hamiltonian and only local operations, two oscillators, initially in any asymmetric initial states, can be entang…

[Phys. Rev. A 95, 052341] Published Tue May 23, 2017

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Universal quantum control in zero-field nuclear magnetic resonance

PRA: Quantum information - Tue, 2017-05-23 16:00

Author(s): Ji Bian, Min Jiang, Jiangyu Cui, Xiaomei Liu, Botao Chen, Yunlan Ji, Bo Zhang, John Blanchard, Xinhua Peng, and Jiangfeng Du

This paper describes a general method for the manipulation of nuclear spins in zero magnetic field. In the absence of magnetic fields, the spins lose the individual information on chemical shifts and inequivalent spins can only be distinguished by nuclear gyromagnetic ratios and spin-spin couplings.…

[Phys. Rev. A 95, 052342] Published Tue May 23, 2017

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Observation of Quantum Forces in the Aharonov-Bohm effect. (arXiv:1705.07139v1 [quant-ph]) Quantum Physics - Tue, 2017-05-23 08:53

Decades ago, the idea that electrons can be affected by electromagnetic potentials without experiencing force-fields changed our thinking with the advent of the Aharonov-Bohm effect. Zeilingers theorem, which casts the absence of force fields in quantum terms as the dispersionless nature of the Aharonov-Bohm effect, cemented the idea in textbooks. Surprisingly, in the late 1990s a quantum force was predicted for the Aharonov-Bohm physical system by Shelankov, and elucidated by Berry. Here we show experimentally that this force is indeed present. Nevertheless, the observation does not change the understanding that potentials can act without force-fields, as we show theoretically that Zeilingers theorem on the dispersionless nature is not generally applicable to the Aharonov-Bohm system.

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Demonstration of a quantum key distribution network across urban fiber channels. (arXiv:1705.07154v1 [quant-ph]) Quantum Physics - Tue, 2017-05-23 08:53

We present the results of realizing a quantum key distribution (QKD) network with the use the standard fiber channels in Moscow. The developed QKD network is based on the trusted repeater paradigm. The QKD network allows establishing a common key between users over an intermediate trustworthy node. In our experiment, the developed QKD network connects users with two different QKD setups: phase-encoding and polarization-encoding schemes. The generated keys can be used for continuous key renewal in the currently available symmetric cipher devices with key refresh time up to 14 seconds.

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Exotic looped trajectories in double-slit experiments with matter waves. (arXiv:1705.07156v1 [quant-ph]) Quantum Physics - Tue, 2017-05-23 08:53

We study the observation of exotic looped trajectories in double-slit experiments with matter waves. We consider the relative intensity at $x=0$ as a function of the time-of-flight from the double-slit to the screen inside the interferometer. This allows us to define a fringe visibility associated to the contribution to the interference pattern given by exotic lopped trajectories. We demonstrate that the Sorkin parameter is given in terms of this visibility and of the axial phases which include the Gouy phase. We verify how this parameter can be obtained by measuring the relative intensity at the screen. We show that the effect of exotic looped trajectories can be significantly increased by simply adjusting the parameters of the double-slit apparatus. Applying our results to the case of neutron interferometry, we obtain a maximum Sorkin parameter of the order of $|\kappa_{max}|\approx0.2$, which is the value of the fringe visibility.

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Multiphoton Effects Enhanced Due to Ultrafast Photon-Number Fluctuations. (arXiv:1705.07159v1 [quant-ph]) Quantum Physics - Tue, 2017-05-23 08:53

Multi-photon processes are the essence of nonlinear optics. Optical harmonics generation and multi-photon absorption, ionization, polymerization or spectroscopy are widely used in practical applications. Generally, the rate of an n-photon effect scales as the n-th order autocorrelation function of the incident light, which is high for light with strong photon-number fluctuations. Therefore `noisy' light sources are much more efficient for multi-photon effects than coherent sources with the same mean power, pulse duration and repetition rate. Here we generate optical harmonics of order 2-4 from bright squeezed vacuum (BSV), a state of light consisting of only quantum noise with no coherent component. We observe up to two orders of magnitude enhancement in the generation of optical harmonics due to ultrafast photon-number fluctuations. This feature is especially important for the nonlinear optics of fragile structures where the use of a `noisy' pump can considerably increase the effect without overcoming the damage threshold.

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Theoretical and computational aspects of entanglement. (arXiv:1705.07160v1 [quant-ph]) Quantum Physics - Tue, 2017-05-23 08:53

We show that the two notions of entanglement: the maximum of the geometric measure of entanglement and the maximum of the nuclear norm is attained for the same states. We affirm the conjecture of Higuchi-Sudberry on the maximum entangled state of four qubits. We introduce the notion of d-density tensor for mixed d-partite states. We show that d-density tensor is separable if and only if its nuclear norm is $1$. We suggest an alternating method for computing the nuclear norm of tensors. We apply the above results to symmetric tensors. We give many numerical examples.

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Shadow wave function with a symmetric kernel. (arXiv:1705.07170v1 [quant-ph]) Quantum Physics - Tue, 2017-05-23 08:53

A shadow wave function with an explicit symmetric kernel is introduced. As a consequence the atoms exchange in the system is enhanced. Basic properties of this class of trial functions are kept and quantities it can describe are easily estimated. The effectiveness of this approach is analized by computing properties of interest in a system formed from 4He atoms.

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Astro-comb calibrator using a turn-key laser frequency comb. (arXiv:1705.07192v1 [astro-ph.IM]) Quantum Physics - Tue, 2017-05-23 08:53

Using a turn-key Ti:sapphire femtosecond laser frequency comb and an off-the-shelf supercontinuum device, we report the generation of a 16 GHz frequency comb spanning an 80 nm band about a center wavelength of 570 nm. The light from this turn-key astro-comb is used to calibrate the HARPS-N astrophysical spectrograph for precision radial velocity measurements. The comb-calibrated spectrograph achieves a stability of $\sim$ 1 cm/s within half an hour of averaging time. We also use the turn-key astro-comb to perform calibration of solar spectra obtained with a compact telescope, and to study intrapixel sensitivity variations on the CCD of the spectrograph.

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Causality and quantum theory. (arXiv:1705.07201v1 [quant-ph]) Quantum Physics - Tue, 2017-05-23 08:53

We begin with a brief summary of issues encountered involving causality in quantum theory, placing careful emphasis on the assumptions involved in results such as the EPR paradox and Bell's inequality. We critique some solutions to the resulting paradox, including Rovelli's relational quantum mechanics and the many-worlds interpretation. We then discuss how a spacetime manifold could come about on the classical level out of a quantum system, by constructing a space with a topology out of the algebra of observables, and show that even with an hypothesis of superluminal causation enforcing consistent measurements of entangled states, a causal cone structure arises on the classical level. Finally, we discuss the possibility that causality as understood in classical relativistic physics may be an emergent symmetry which does not hold on the quantum level.

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Bath energy for correlated initial states versus information flow in local dephasing channels. (arXiv:1705.07218v1 [quant-ph]) Quantum Physics - Tue, 2017-05-23 08:53

Variations of the bath energy are compared with the information flow in local dephasing channels. Special correlated initial conditions are prepared from the thermal equilibrium of the whole system, by performing a selective measurement on the qubit. The spectral densities under study are ohmic-like at low frequencies and include logarithmic perturbations of the power-law profiles. The bath and the correlation energy alternately increase or decrease, monotonically, over long times, according to the value of the ohmicity parameter, following logarithmic and power laws. Consider initial conditions such that the environment is in a thermal state, factorized from the state of the qubit. In the super-ohmic regime the long-time features of the information flow are transferred to the bath and correlation energy, by changing the initial condition from the factorized to the specially correlated, even with different temperatures. In fact, the low-frequency structures of the spectral density that provide information backflow with the factorized initial condition, induce increasing (decreasing) bath (correlation) energy with the specially correlated initial configuration. By performing the same change of initial conditions, the spectral properties providing information loss, produce decrease (increase) of the bath (correlation) energy.

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Periodic quantum graphs from the Bethe-Sommerfeld perspective. (arXiv:1705.07306v1 [math-ph]) Quantum Physics - Tue, 2017-05-23 08:53

The paper is concerned with the number of open gaps in spectra of periodic quantum graphs. The well-known conjecture by Bethe and Sommerfeld (1933) says that the number of open spectral gaps for a system periodic in more than one direction is finite. To the date its validity is established for numerous systems, however, it is known that quantum graphs do not comply with this law as their spectra have typically infinitely many gaps, or no gaps at all. These facts gave rise to the question about the existence of quantum graphs with the `Bethe-Sommerfeld property', that is, featuring a nonzero finite number of gaps in the spectrum. In this paper we prove that the said property is impossible for graphs with the vertex couplings which are either scale-invariant or associated to scale-invariant ones in a particular way. On the other hand, we demonstrate that quantum graphs with a finite number of open gaps do indeed exist. We illustrate this phenomenon on an example of a rectangular lattice with a $\delta$ coupling at the vertices and a suitable irrational ratio of the edges. Our result allows to find explicitly a quantum graph with any prescribed exact number of gaps, which is the first such example to the date.

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The Quantum Monad on Relational Structures. (arXiv:1705.07310v1 [cs.LO]) Quantum Physics - Tue, 2017-05-23 08:53

Homomorphisms between relational structures play a central role in finite model theory, constraint satisfaction and database theory. A central theme in quantum computation is to show how quantum resources can be used to gain advantage in information processing tasks. In particular, non-local games have been used to exhibit quantum advantage in boolean constraint satisfaction, and to obtain quantum versions of graph invariants such as the chromatic number. We show how quantum strategies for homomorphism games between relational structures can be viewed as Kleisli morphisms for a quantum monad on the (classical) category of relational structures and homomorphisms. We show a general connection between these notions and state-independent quantum realizations of strong contextuality in the Abramsky-Brandenburger formulation of contextuality. We use these results to exhibit a wide range of examples of contextuality-powered quantum advantage, and to unify several apparently diverse strands of previous work.

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Realization of an atomically thin mirror using monolayer MoSe2. (arXiv:1705.07317v1 [cond-mat.mes-hall]) Quantum Physics - Tue, 2017-05-23 08:53

Advent of new materials such as van der Waals heterostructures, propels new research directions in condensed matter physics and enables development of novel devices with unique functionalities. Here, we show experimentally that a monolayer of MoSe 2 embedded in a charge controlled heterostructure can be used to realize an electrically tunable atomically-thin mirror, that effects 90% extinction of an incident field that is resonant with its exciton transition. The corresponding maximum reflection coefficient of 45% is only limited by the ratio of the radiative decay rate to the linewidth of exciton transition and is independent of incident light intensity up to 400 Watts/cm 2 . We demonstrate that the reflectivity of the mirror can be drastically modified by applying a gate voltage that modifies the monolayer charge density. Our findings could find applications ranging from fast programmable spatial light modulators to suspended ultra-light mirrors for optomechanical devices.

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