A novel derivation of quantum propagator useful for time-dependent trapping and control. (arXiv:1612.07674v3 [quant-ph] UPDATED) Quantum Physics - 4 hours 5 min ago

A novel derivation of quantum propagator of a system described by a general quadratic Lagrangian is presented in the framework of Heisenberg equations of motion. The general corresponding density matrix is obtained for a derived quantum harmonic oscillator and a particle confined in a one dimensional Paul trap. Total mean energy, work and absorbed heat, Wigner function and excitation probabilities are found explicitly. The method presented here is based on the Heisenberg representation of position and momentum operators and can be generalized to a system consisting of a set of linearly interacting harmonic oscillators straightforwardly.

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Totally Destructive Many-Particle Interference

Author(s): Christoph Dittel, Gabriel Dufour, Mattia Walschaers, Gregor Weihs, Andreas Buchleitner, and Robert Keil

In a general, multimode scattering setup, we show how the permutation symmetry of a many-particle input state determines those scattering unitaries that exhibit strictly suppressed many-particle transition events. We formulate purely algebraic suppression laws that identify these events and show tha...

[Phys. Rev. Lett. 120, 240404] Published Fri Jun 15, 2018

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Latent Space Purification via Neural Density Operators

Author(s): Giacomo Torlai and Roger G. Melko

Machine learning is actively being explored for its potential to design, validate, and even hybridize with near-term quantum devices. A central question is whether neural networks can provide a tractable representation of a given quantum state of interest. When true, stochastic neural networks can b...

[Phys. Rev. Lett. 120, 240503] Published Fri Jun 15, 2018

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Exact Confirmation of 1D Nonlinear Fluctuating Hydrodynamics for a Two-Species Exclusion Process

Author(s): Zeying Chen, Jan de Gier, Iori Hiki, and Tomohiro Sasamoto

Nonlinear fluctuating hydrodynamics correctly predicts the long time behavior for multi-component systems.

[Phys. Rev. Lett. 120, 240601] Published Fri Jun 15, 2018

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Bistability of a slow mechanical oscillator coupled to a laser-driven two-level system

Author(s): Fabio Pistolesi

It has been recently proposed that single molecule spectroscopy could be employed to detect the motion of nanomechanical resonators. Estimates of the coupling constant (g) between the molecular two-level system and the oscillator indicate that it can reach values much larger than the mechanical reso...

[Phys. Rev. A 97, 063833] Published Fri Jun 15, 2018

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Light emission by accelerated electric, toroidal, and anapole dipolar sources

Author(s): V. Savinov

Emission of electromagnetic radiation by accelerated particles with electric, toroidal, and anapole dipole moments is analyzed. It is shown that ellipticity of the emitted light can be used to differentiate between electric and toroidal dipole sources and that anapoles, elementary neutral nonradiati...

[Phys. Rev. A 97, 063834] Published Fri Jun 15, 2018

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Energy conservation in self-phase modulation

Author(s): P. Béjot and J. Kasparian

Spectral broadening of ultrashort laser pulses is simultaneously described by either self-phase modulation (SPM) or four-wave mixing (FWM). The latter implies the instantaneous conservation of both the photon number and energy, while the former describes a time-dependent frequency shift, implying a ...

[Phys. Rev. A 97, 063835] Published Fri Jun 15, 2018

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Focus: Puzzling Tropical Wind Pattern Generated with Simple Model

APS Physics - Fri, 2018-06-15 12:00

Author(s): Mark Buchanan

2D simulations of the atmosphere, with few assumptions, can generate a slowly oscillating, tropical wind pattern that has puzzled atmospheric scientists.

[Physics 11, 62] Published Fri Jun 15, 2018

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Scale-invariant freezing of entanglement

PRA: Quantum information - Fri, 2018-06-15 12:00

Author(s): Titas Chanda, Tamoghna Das, Debasis Sadhukhan, Amit Kumar Pal, Aditi Sen(De), and Ujjwal Sen

We show that bipartite entanglement in an one-dimensional quantum spin model undergoing time evolution due to local Markovian environments can be frozen over time. We demonstrate this by using a number of paradigmatic quantum spin models including the anisotropic XY model in the presence of a unifor...

[Phys. Rev. A 97, 062324] Published Fri Jun 15, 2018

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Asymptotic quantum algorithm for the Toeplitz systems

PRA: Quantum information - Fri, 2018-06-15 12:00

Author(s): Lin-Chun Wan, Chao-Hua Yu, Shi-Jie Pan, Fei Gao, Qiao-Yan Wen, and Su-Juan Qin

Solving the Toeplitz systems, which involves finding the vector x such that Tnx=b given an n×n Toeplitz matrix Tn and a vector b, has a variety of applications in mathematics and engineering. In this paper, we present a quantum algorithm for solving the linear equations of Toeplitz matrices, in whic...

[Phys. Rev. A 97, 062322] Published Fri Jun 15, 2018

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Randomized benchmarking with restricted gate sets

PRA: Quantum information - Fri, 2018-06-15 12:00

Author(s): Winton G. Brown and Bryan Eastin

Standard randomized benchmarking protocols entail sampling from a unitary 2-design, which is not always practical. In this article we examine randomized benchmarking protocols based on subgroups of the Clifford group that are not unitary 2-designs. We introduce a general method for analyzing such pr...

[Phys. Rev. A 97, 062323] Published Fri Jun 15, 2018

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The Lyapunov Spectrum of Quantum Thermalisation. (arXiv:1806.05204v1 [cond-mat.str-el]) Quantum Physics - Fri, 2018-06-15 10:03

The eigenstate thermalisation hypothesis resolves the paradox of emergent thermal or classical behaviour in a closed quantum system by focussing upon local observations. This permits the remainder of the system to act as a bath, thermalisation arising due to a process of de-phasing that gradually reveals the thermal nature of local observables measured in an eigenstate. This is very different from thermalisation in closed classical systems, which is driven by dynamical chaos. We show how quantum thermalisation in closed systems can be recast in a way that is directly related to classical thermalisation. Local observables can be accurately captured by projecting states onto a suitable variational manifold. Evolving on this manifold using the time-dependent variational principle projects the quantum dynamics onto a (semi-)classical Hamiltonian dynamics. Thermalisation in this setting is driven by dynamical chaos. We carry out this procedure for an infinite spin chain in two ways --- using the matrix product state ansatz for the wavefunction and for the thermofield double purification of the density matrix --- and extract the full Lyapunov spectrum of the resulting chaotic dynamics. This provides an alternative perspective upon eigenstate thermalisation, pre-thermalisation and integrability.

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A group with at least subexponential hyperlinear profile. (arXiv:1806.05267v1 [math.GR]) Quantum Physics - Fri, 2018-06-15 10:03

The hyperlinear profile of a group measures the growth rate of the dimension of unitary approximations to the group. We construct a finitely-presented group whose hyperlinear profile is at least subexponential, i.e. at least $\exp(1/\epsilon^{k})$ for some $0 < k < 1$. We use this group to give an example of a two-player non-local game requiring subexponential Hilbert space dimension to play near-perfectly.

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High-efficiency measurement of an artificial atom embedded in a parametric amplifier. (arXiv:1806.05276v1 [quant-ph]) Quantum Physics - Fri, 2018-06-15 10:03

A crucial limit to measurement efficiencies of superconducting circuits comes from losses involved when coupling to an external quantum amplifier. Here, we realize a device circumventing this problem by directly embedding a two-level artificial atom, comprised of a transmon qubit, within a flux-pumped Josephson parametric amplifier. Surprisingly, this configuration is able to enhance dispersive measurement without exposing the qubit to appreciable excess backaction. This is accomplished by engineering the circuit to permit high-power operation that reduces information loss to unmonitored channels associated with the amplification and squeezing of quantum noise. By mitigating the effects of off-chip losses downstream, the on-chip gain of this device produces end-to-end measurement efficiencies of up to 80 percent. Our theoretical model accurately describes the observed interplay of gain and measurement backaction, and delineates the parameter space for future improvement. The device is compatible with standard fabrication and measurement techniques, and thus provides a route for definitive investigations of fundamental quantum effects and quantum control protocols.

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Spectral form factors and late time quantum chaos. (arXiv:1806.05316v1 [hep-th]) Quantum Physics - Fri, 2018-06-15 10:03

This is a collection of notes that are about spectral form factors of standard ensembles in the random matrix theory, written for the practical usage of current study of late time quantum chaos. More precisely, we consider Gaussian Unitary Ensemble (GUE), Gaussian Orthogonal Ensemble (GOE), Gaussian Symplectic Ensemble (GSE), Wishart-Laguerre Unitary Ensemble (LUE), Wishart-Laguerre Orthogonal Ensemble (LOE), and Wishart-Laguerre Symplectic Ensemble (LSE). These results and their physics applications cover a three-fold classification of late time quantum chaos in terms of spectral form factors.

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Quantum theory is not only about information. (arXiv:1806.05323v1 [quant-ph]) Quantum Physics - Fri, 2018-06-15 10:03

In his recent book Bananaworld. Quantum mechanics for primates, Jeff Bub revives and provides a mature version of his influential information theoretic interpretation of Quantum Theory (QT). In this paper, I test Bub s conjecture that QT should be interpreted as a theory about information, by examining whether his information theoretic interpretation has the resources to explain (or explain away) quantum conundrums. The discussion of Bub s theses will also serve to investigate, more in general, whether other approaches succeed in defending the claim that QT is about quantum information. First of all, I argue that Bub s interpretation of QT as a principle theory fails to fully explain quantum nonlocality. Secondly, I argue that a constructive interpretation, where the quantum state is interpreted ontically as information, also fails at providing a full explanation of quantum correlations. Finally, while epistemic interpretations might succeed in this respect, I argue that such a success comes at the price of rejecting some in between the most basic scientific standards of physical theories.

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It's a Matter of Principle. Scientific Explanation in Information-Theoretic Reconstructions of Quantum Theory. (arXiv:1806.05324v1 [quant-ph]) Quantum Physics - Fri, 2018-06-15 10:03

The aim of this paper is to explore the ways in which Axiomatic Reconstructions of Quantum Theory in terms of Information-Theoretic principles (ARQITs) can contribute to explaining and understanding quantum phenomena, as well as to study their explanatory limitations. This is achieved in part by offering an account of the kind of explanation that axiomatic reconstructions of quantum theory provide, and re-evaluating the epistemic status of the program in light of this explanation. As illustrative cases studies, I take Clifton's, Bub's and Halvorson's characterization theorem and Popescu's and Rohrlich's toy models, and their explanatory contribution with respect to quantum non-locality. On the one hand, I argue that ARQITs can aspire to provide genuine explanations of (some aspects of) quantum non-locality. On the other hand, I argue that such explanations cannot rule out a mechanical quantum theory.

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Unconventional Rydberg pumping and applications in quantum information processing. (arXiv:1806.05336v1 [quant-ph]) Quantum Physics - Fri, 2018-06-15 10:03

We propose a mechanism of unconventional Rydberg pumping (URP) via simultaneously driving each Rydberg atom by two classical fields with different strengths of Rabi frequencies. This mechanism differs from the general Rydberg blockade or Rydberg antiblockade since it is closely related to the ground states of atoms, i.e. two atoms in the same ground state are stable while two atoms in different ground states are resonantly excited. Furthermore, we find the URP can be employed to simplify some special quantum information processing tasks, such as implementation of a three-qubit controlled phase gate with only a single Rabi oscillation, preparation of two- and three-dimensional steady-state entanglement with two identical atoms, and realization of the autonomous quantum error correction in a Rydberg-atom-cavity system. The feasibility of the above applications is certified explicitly by the state-of-the-art technology.

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Entanglement dynamics for uniformly accelerated two-level atoms in the presence of a reflecting boundary. (arXiv:1806.05344v1 [quant-ph]) Quantum Physics - Fri, 2018-06-15 10:03

We study the entanglement dynamics for two uniformly accelerated two-level atoms in interaction with a bath of fluctuating electromagnetic fields in vacuum in the presence of a reflecting boundary. We consider two different alignments of atoms, i.e. parallel and vertical alignments with respect to the boundary. In particular, we focus on the effects of the boundary, and acceleration on the entanglement dynamics, which are closely related to the orientations of polarization. For the parallel case, the initial entanglement of two transversely polarizable atoms very close to the boundary can be preserved as if it were a closed system, while for two vertically polarizable atoms, the concurrence evolves two times as fast as that in the free space. In the presence of a boundary, entanglement revival is possible for two atoms initially in the symmetric state depending on the orientations of the atomic polarizations, which is in sharp contrast to the fact that the concurrence always decays monotonically in the free space. Interestingly, two initially separable atoms, for which entanglement generation can never happen in the free space with any given acceleration and separation, can get entangled in the presence of a boundary if they are aligned parallel to the boundary. The birth time of entanglement can be noticeably advanced or postponed for the parallel two-atom system placed close to the boundary, while the maximal concurrence during evolution can be significantly enhanced when the atoms are vertically aligned. Moreover, two inertial atoms with different polarizations remain separable all the time, while as the acceleration increases, the delayed birth of entanglement happens, and the nonzero concurrence can be enhanced.

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Integral formulation of the quantum mechanics in the phase space. (arXiv:1806.05383v1 [quant-ph]) Quantum Physics - Fri, 2018-06-15 10:03

A formulation of quantum mechanics is introduced based on a $2D$-dimensional phase-space wave function $\text{\reflectbox{\text{p}}}\mkern-3mu\text{p}\left(q,p\right)$ which might be computed from the position-space wave function $\psi\left(q\right)$ with a transformation related to the Gabor transformation. The equation of motion for conservative systems can be written in the form of the Schr\"{o}dinger equation with a $4D$-dimensional Hamiltonian with classical terms on the diagonal and complex off-diagonal couplings. The Hamiltonian does not contain any differential operators and the quantization is achieved by replacing $q$ and $p$ with $2D$-dimensional counterparts $\left(q+q'\right)/2$ and $\left(p+p'\right)/2$ and by using a complex-valued factor $e^{i\left(q\cdot p'-q'\cdot p\right)/2}$ in phase-space integrals. Despite the fact that the formulation increases the dimensionality, it might provide a way towards exact multi-dimensional computations as it may be evaluated directly with Monte-Carlo algorithms.

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