Holography and thermalization in optical pump-probe spectroscopy. (arXiv:1708.08279v2 [hep-th] UPDATED) Quantum Physics - Wed, 2018-04-04 23:33

Using holography, we model experiments in which a 2+1D strange metal is pumped by a laser pulse into a highly excited state, after which the time evolution of the optical conductivity is probed. We consider a finite-density state with mildly broken translation invariance and excite it by oscillating electric field pulses. At zero density, the optical conductivity would assume its thermalized value immediately after the pumping has ended. At finite density, pulses with significant DC components give rise to slow exponential relaxation, governed by a vector quasinormal mode. In contrast, for high-frequency pulses the amplitude of the quasinormal mode is strongly suppressed, so that the optical conductivity assumes its thermalized value effectively instantaneously. This surprising prediction may provide a stimulus for taking up the challenge to realize these experiments in the laboratory. Such experiments would test a crucial open question faced by applied holography: Are its predictions artefacts of the large $N$ limit or do they enjoy sufficient UV independence to hold at least qualitatively in real-world systems?

Categories: Journals, Physics

Topological Maxwell Metal Bands in a Superconducting Qutrit. (arXiv:1709.05765v2 [quant-ph] UPDATED) Quantum Physics - Wed, 2018-04-04 23:33

We experimentally explore the topological Maxwell metal bands by mapping the momentum space of condensed-matter models to the tunable parameter space of superconducting quantum circuits. An exotic band structure that is effectively described by the spin-1 Maxwell equations is imaged. Three-fold degenerate points dubbed Maxwell points are observed in the Maxwell metal bands. Moreover, we engineer and observe the topological phase transition from the topological Maxwell metal to a trivial insulator, and report the first experiment to measure the Chern numbers that are higher than one.

Categories: Journals, Physics

Compact Neural Networks based on the Multiscale Entanglement Renormalization Ansatz. (arXiv:1711.03357v2 [cs.NE] UPDATED) Quantum Physics - Wed, 2018-04-04 23:33

This paper demonstrates a method for tensorizing neural networks based upon an efficient way of approximating scale invariant quantum states, the Multi-scale Entanglement Renormalization Ansatz (MERA). We employ MERA as a replacement for the fully connected layers in a convolutional neural network and test this implementation on the CIFAR-10 and CIFAR-100 datasets. The proposed method outperforms factorization using tensor trains, providing greater compression for the same level of accuracy and greater accuracy for the same level of compression. We demonstrate MERA layers with 14000 times fewer parameters and a reduction in accuracy of less than 1% compared to the equivalent fully connected layers, scaling like O(N).

Categories: Journals, Physics

Topological Phases in Nodeless Tetragonal Superconductors. (arXiv:1804.00007v1 [cond-mat.supr-con] CROSS LISTED) Quantum Physics - Wed, 2018-04-04 23:33

We compute the topological phase diagram of 2D tetragonal superconductors for the only possible nodeless pairing channels compatible with that crystal symmetry. Subject to a Zeeman field and spin-orbit coupling, we demonstrate that these superconductors show surprising topological features: non-trivial high Chern numbers, massive edge states, and zero-energy modes out of high symmetry points, even though the edge states remain topologically protected. Interestingly, one of these pairing symmetries, $d+id$, has been proposed to describe materials such as water-intercalated sodium cobaltates, bilayer silicene or highly doped monolayer graphene.

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The Einstein-Cartan-Dirac theory. (arXiv:1803.10621v2 [gr-qc] UPDATED) Quantum Physics - Wed, 2018-04-04 23:33

There are various generalizations of Einstein's theory of gravity (GR); one of which is the Einstein-Cartan (EC) theory. It modifies the geometrical structure of manifold and relaxes the notion of affine connection being symmetric. The theory is also called $U_4$ theory of gravitation; where the underlying manifold is not Riemannian. The non-Riemannian part of the space-time is sourced by the spin density of matter. Here mass and spin both play the dynamical role. We consider the minimal coupling of Dirac field with EC theory; thereby calling the full theory as Einstein-Cartan-Dirac (ECD) theory. In the recent works by T.P Singh titled "A new length scale in quantum gravity", the idea of new unified; mass dependent length scale $L_{cs}$ in quantum gravity has been proposed. We discuss this idea and formulate ECD theory in both - standard length scales as well as this new length scale. We found the non-relativistic limit of ECD theory using WKB-like expansion in $\sqrt{\hbar}/c$ of the ECD field equations with both the length scales. At leading order, ECD equations with standard length scales give Schr\"{o}dinger-Newton equation. With $L_{cs}$, in the low mass limit, it gives source-free Poisson equation and for higher mass limit, it reduces to Poisson equation with delta function source. Based on this, a falsifiable test of the idea of $L_{cs}$ has been proposed. Next, we formulate ECD theory with both the length scales (especially the Dirac equation (Hehl-Datta equation) and Contorsion spin coefficients) in Newman-Penrose (NP) formalism. The idea of $L_{cs}$ suggests a symmetry between small and large masses. Formulating ECD theory with $L_{cs}$ in NP formalism is desirable because NP formalism happens to be the common vocabulary for the description of low masses (Dirac theory) and high masses (gravity theories). A duality between Curvature and torsion has also been discussed.

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Spectral statistics in spatially extended chaotic quantum many-body systems. (arXiv:1803.03841v2 [cond-mat.stat-mech] UPDATED) Quantum Physics - Wed, 2018-04-04 23:33

We study spectral statistics in spatially extended chaotic quantum many-body systems, using simple lattice Floquet models without time-reversal symmetry. Computing the spectral form factor $K(t)$ analytically and numerically, we show that it follows random matrix theory (RMT) at times longer than a many-body Thouless time, $t_{\rm Th}$. We obtain a striking dependence of $t_{\rm Th}$ on the spatial dimension $d$ and size of the system. For $d>1$, $t_{\rm Th}$ is finite in the thermodynamic limit and set by the inter-site coupling strength. By contrast, in one dimension $t_{\rm Th}$ diverges with system size, and for large systems there is a wide window in which spectral correlations are not of RMT form.

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On the family of Wigner functions for N-level quantum system. (arXiv:1708.05981v3 [quant-ph] UPDATED) Quantum Physics - Wed, 2018-04-04 23:33

The family of unitary non-equivalent Weyl-Stratonovich kernels determining the Wigner probability distribution function of an arbitrary N-level quantum system is constructed.

Categories: Journals, Physics

Time crystal platform: from quasi-crystal structures in time to systems with exotic interactions. (arXiv:1710.10087v3 [cond-mat.quant-gas] UPDATED) Quantum Physics - Wed, 2018-04-04 23:33

Time crystals are quantum many-body systems which, due to interactions between particles, are able to spontaneously self-organize their motion in a periodic way in time by analogy with the formation of crystalline structures in space in condensed matter physics. In solid state physics properties of space crystals are often investigated with the help of external potentials that are spatially periodic and reflect various crystalline structures. A similar approach can be applied for time crystals, as periodically driven systems constitute counterparts of spatially periodic systems, but in the time domain. Here we show that condensed matter problems ranging from single particles in potentials of quasi-crystal structure to many-body systems with exotic long-range interactions can be realized in the time domain with an appropriate periodic driving. Moreover, it is possible to create molecules where atoms are bound together due to destructive interference if the atomic scattering length is modulated in time.

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On the contact geometry and the Poisson geometry of the ideal gas. (arXiv:1802.04091v2 [math-ph] UPDATED) Quantum Physics - Wed, 2018-04-04 23:33

We elaborate on existing notions of contact geometry and Poisson geometry as applied to the classical ideal gas. Specifically we observe that it is possible to describe its dynamics using a 3-dimensional contact submanifold of the standard 5-dimensional contact manifold used in the literature. This reflects the fact that the internal energy of the ideal gas depends exclusively on its temperature. We also present a Poisson algebra of thermodynamic operators for a quantum-like description of the classical ideal gas. The central element of this Poisson algebra is proportional to Boltzmann's constant. A Hilbert space of states is identified and a system of wave equations governing the wavefunction is found. Expectation values for the operators representing pressure, volume and temperature are found to satisfy the classical equations of state.

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Dissipative tunneling by means of scaled trajectories. (arXiv:1801.09137v2 [quant-ph] UPDATED) Quantum Physics - Wed, 2018-04-04 23:33

Dissipative quantum tunnelling through an inverted parabolic barrier is considered in the presence of an electric field. A Schr\"odinger-Langevin or Kostin quantum classical transition wave equation is used and applied resulting in a scaled differential equation of motion. A Gaussian wave packet solution to the resulting scaled Kostin nonlinear equation is assumed and compared to the same solution for the scaled linear Caldirola-Kanai equation. The resulting scaled trajectories are obtained at different dynamical regimes and friction cases, showing the gradual decoherence process in this open dynamics. Theoretical results show that the transmission probabilities are always higher in the Kostin approach than in the Caldirola-Kanai approach in the presence or not of an external electric field. This discrepancy should be understood due to the presence of an environment since the corresponding open dynamics should be governed by nonlinear quantum equations, whereas the second approach is issued from an effective Hamiltonian within a linear theory.

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Characterization of topological states via dual multipartite entanglement. (arXiv:1712.05286v3 [quant-ph] UPDATED) Quantum Physics - Wed, 2018-04-04 23:33

We demonstrate that multipartite entanglement is able to characterize one-dimensional symmetry-protected topological order, which is witnessed by the scaling behavior of the quantum Fisher information of the ground state with respect to the spin operators defined in the dual lattice. We investigate an extended Kitaev chain with a $\mathbf{Z}$ symmetry identified equivalently by winding numbers and paired Majorana zero modes at each end. The topological phases with high winding numbers are detected by the scaling coefficient of the quantum Fisher information density with respect to generators in different dual lattices. Containing richer properties and more complex structures than bipartite entanglement, the dual multipartite entanglement of the topological state has promising applications in robust quantum computation and quantum metrology, and can be generalized to identify topological order in the Kitaev honeycomb model.

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<i>Colloquium</i>: Excitons in atomically thin transition metal dichalcogenides

Reviews of Modern Physics - Wed, 2018-04-04 12:00

Author(s): Gang Wang, Alexey Chernikov, Mikhail M. Glazov, Tony F. Heinz, Xavier Marie, Thierry Amand, and Bernhard Urbaszek

Most two-dimensional (2D) semiconductors are interesting materials as quantum confinement enhances the Coulomb interaction between carriers, leading to a strong attraction between conduction electrons and valence holes, forming stable excitons and the optical response of 2D semiconductors can be extraordinary. In this Colloquium the progress and open questions in the study of excitons in 2D semiconductors from both the experimental and theoretical perspectives are reviewed.

[Rev. Mod. Phys. 90, 021001] Published Wed Apr 04, 2018

Categories: Journals, Physics

Enhancement of the spontaneous emission in subwavelength quasi-two-dimensional waveguides and resonators

Author(s): Mikhail Tokman, Zhongqu Long, Sultan AlMutairi, Yongrui Wang, Mikhail Belkin, and Alexey Belyanin

We consider a quantum-electrodynamic problem of the spontaneous emission from a two-dimensional (2D) emitter, such as a quantum well or a 2D semiconductor, placed in a quasi-2D waveguide or cavity with subwavelength confinement in one direction. We apply the Heisenberg-Langevin approach, which inclu...

[Phys. Rev. A 97, 043801] Published Wed Apr 04, 2018

Categories: Journals, Physics

Synopsis: The Geometry of Arctic Ponds

APS Physics - Wed, 2018-04-04 12:00

A geometric model of meltwater ponds may help predict how the polar ice caps might evolve under future climate changes.

[Physics] Published Wed Apr 04, 2018

Categories: Physics

Evaluation of the non-Gaussianity of two-mode entangled states over a bosonic memory channel via cumulant theory and quadrature detection

PRA: Quantum information - Wed, 2018-04-04 12:00

Author(s): Shao-Hua Xiang, Wei Wen, Yu-Jing Zhao, and Ke-Hui Song

We study the properties of the cumulants of multimode boson operators and introduce the phase-averaged quadrature cumulants as the measure of the non-Gaussianity of multimode quantum states. Using this measure, we investigate the non-Gaussianity of two classes of two-mode non-Gaussian states: photon...

[Phys. Rev. A 97, 042303] Published Wed Apr 04, 2018

Categories: Journals, Physics

Simulating and assessing boson sampling experiments with phase-space representations

PRA: Quantum information - Wed, 2018-04-04 12:00

Author(s): Bogdan Opanchuk, Laura Rosales-Zárate, Margaret D. Reid, and Peter D. Drummond

The search for new, application-specific quantum computers designed to outperform any classical computer is driven by the ending of Moore's law and the quantum advantages potentially obtainable. Photonic networks are promising examples, with experimental demonstrations and potential for obtaining a ...

[Phys. Rev. A 97, 042304] Published Wed Apr 04, 2018

Categories: Journals, Physics

Experimental tests of coherence and entanglement conservation under unitary evolutions

PRA: Quantum information - Wed, 2018-04-04 12:00

Author(s): Antonín Černoch, Karol Bartkiewicz, Karel Lemr, and Jan Soubusta

We experimentally demonstrate the migration of coherence between composite quantum systems and their subsystems. The quantum systems are implemented using polarization states of photons in two experimental setups. The first setup is based on a linear optical controlled-phase quantum gate and the sec...

[Phys. Rev. A 97, 042305] Published Wed Apr 04, 2018

Categories: Journals, Physics

Unconditional preparation of nonclassical states via linear-and-quadratic optomechanics. (arXiv:1804.00014v1 [quant-ph]) Quantum Physics - Tue, 2018-04-03 22:45

Reservoir engineering enables the robust preparation of pure quantum states in noisy environments. We show how a new family of quantum states of a mechanical oscillator can be stabilized in a cavity that is parametrically coupled to both the mechanical displacement and the displacement squared. The cavity is driven with three tones, on the red sideband, on the cavity resonance and on the second blue sideband. The states so stabilized are (squeezed and displaced) superpositions of a finite number of phonons. They show the unique feature of encompassing two prototypes of nonclassicality for bosonic systems: by adjusting the strength of the drives, one can in fact move from a single-phonon- to a Schrodinger-cat-like state. The scheme is deterministic, supersedes the need for measurement-and-feedback loops and does not require initialization of the oscillator to the ground state. As such, it enables the unconditional preparation of nonclassical states of a macroscopic object.

Categories: Journals, Physics

Non-Hermitian coherent states for finite-dimensional systems. (arXiv:1804.00051v1 [math-ph]) Quantum Physics - Tue, 2018-04-03 22:45

We introduce Gilmore-Perelomov coherent states for non-unitary representations of non-compact groups, and discuss the main similarities and differences with respect to ordinary unitary Gilmore-Perelomov coherent states. The example of coherent states for the non-unitary finite dimensional representations of $SU(1,1)$ is considered and they are used to describe the propagation of light in coupled PT-symmetric optical devices.

Categories: Journals, Physics

Spontaneous and Stimulated Radiative emission of Modulated Free-Electron Quantum wavepackets - Semiclassical Analysis. (arXiv:1804.00053v1 [quant-ph]) Quantum Physics - Tue, 2018-04-03 22:45

Here we present a semiclassical analysis of spontaneous and stimulated radiative emission from unmodulated and optically-modulated electron quantum wavepackets. We show that the radiative emission/absorption and corresponding deceleration/acceleration of the wavepackets depend on the controllable 'history-dependent' wavepacket size. The characteristics of the radiative interaction when a wavepacket of size (duration) is short relative to the radiation wavelength, are close to the predictions of the classical point-particle modeling. On the other hand, in the long-sized wavepacket limit, the interaction is quantum-mechanical, and it diminishes exponentially at high frequency. We exemplify these effects through the scheme of Smith-Purcell radiation, and demonstrate that if the wavepacket is optically-modulated and periodically-bunched, it exhibits finite radiative emission at harmonics of the modulation frequency beyond the limit of high-frequency cutoff. Besides, the radiation analysis is further extended to the cases of superradiant emission from a beam of phase-correlated modulated electron wavepackets. This wavepacket-dependent emission process shows the radiative features of classical-to-quantum transition, indicates a way for measuring the quantum electron wavepacket size and suggests a new direction for exploring light-matter interaction.

Categories: Journals, Physics
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