Physics

Machine-learning-assisted correction of correlated qubit errors in a topological code. (arXiv:1705.07855v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

A fault-tolerant quantum computation requires an efficient means to detect and correct errors that accumulate in encoded quantum information. In the context of machine learning, neural networks are a promising new approach to quantum error correction. Here we show that a recurrent neural network can be trained, using only experimentally accessible data, to detect errors in a widely used topological code, the surface code, with a performance above that of the established minimum-weight perfect matching (or blossom) decoder. The performance gain is achieved because the neural network decoder can detect correlations between bit-flip (X) and phase-flip (Z) errors. The machine learning algorithm adapts to the physical system, hence no noise model is needed. The long short-term memory layers of the recurrent neural network maintain their performance over a large number of quantum error correction cycles, making it a practical decoder for forthcoming experimental realizations of the surface code.

Categories: Journals, Physics

Implementing Parrondo's paradox with two coin quantum walks. (arXiv:1702.05927v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

Parrondo's paradox is ubiquitous in games, ratchets and random walks.The apparent paradox, devised by J.~M.~R.~Parrondo, that two losing games $A$ and $B$ can produce an winning outcome has been adapted in many physical and biological systems to explain their working. However, proposals on demonstrating Parrondo's paradox using quantum walks failed {for large number of steps}. In this work, we show that instead of a single coin if we consider a two coin initial state which may or may not be entangled, we can observe a genuine Parrondo's paradox with quantum walks. Further we focus on reasons for this and pin down the asymmetry in initial two-coin state or asymmetry in shift operator, either of which are necessary for observing a genuine Parrondo's paradox. We extend our work to a 3-coin initial state too with similar results. The implications of our work for observing quantum ratchet like behavior using quantum walks is also discussed.

Categories: Journals, Physics

Generalized eigenstate typicality in translation-invariant quasifree fermionic models. (arXiv:1709.05569v3 [cond-mat.stat-mech] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

We demonstrate a generalized notion of eigenstate thermalization for translation-invariant quasifree fermionic models: the vast majority of eigenstates satisfying a finite number of suitable constraints (e.g. fixed energy and particle number) have the property that their reduced density matrix on small subsystems approximates the corresponding generalized Gibbs ensemble. To this end, we generalize analytic results by Lai and Yang (Phys. Rev. B 91, 081110 (2015)) and illustrate the claim numerically by example of the Jordan-Wigner transform of the XX spin chain.

Categories: Journals, Physics

Creation of Rydberg Polarons in a Bose Gas. (arXiv:1706.03717v4 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

We report spectroscopic observation of Rydberg polarons in an atomic Bose gas. Polarons are created by excitation of Rydberg atoms as impurities in a strontium Bose-Einstein condensate. They are distinguished from previously studied polarons by macroscopic occupation of bound molecular states that arise from scattering of the weakly bound Rydberg electron from ground-state atoms. The absence of a $p$-wave resonance in the low-energy electron-atom scattering in Sr introduces a universal behavior in the Rydberg spectral lineshape and in scaling of the spectral width (narrowing) with the Rydberg principal quantum number, $n$. Spectral features are described with a functional determinant approach (FDA) that solves an extended Fr\"{o}hlich Hamiltonian for a mobile impurity in a Bose gas. Excited states of polyatomic Rydberg molecules (trimers, tetrameters, and pentamers) are experimentally resolved and accurately reproduced with FDA.

Categories: Journals, Physics

Dual vibration configuration interaction (DVCI). A novel factorisation of molecular Hamiltonian for high performance infrared spectrum computation. (arXiv:1801.05216v2 [physics.comp-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

Here is presented an original program based on molecular Schrodinger equations. It is dedicated to target specific states of infrared vibrational spectrum in a very precise way with a minimal usage of memory. An eigensolver combined with a new probing technique accumulate information along the iterations so that desired eigenpairs rapidly tend towards the variational limit. Basis set is augmented from the maximal components of residual vectors that usually require the construction of a big matrix block that here is bypassed with a new factorisation of the Hamiltonian. The latest borrows the mathematical concept of duality and the second quantization formalism of quantum theory.

Categories: Journals, Physics

HoloQuantum Network Theory. (arXiv:1801.05286v2 [physics.gen-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

The complete general fundamental theory of the dynamical hypergraphs whose `all' mathematical structures are quantized, HoloQuantum Network Theory, is originally defined and formulated based upon a unique system of nine principles. HoloQuantum Networks are the quantum states of a (0+1) dimensional purely-information-theoretic quantum many body system, made of a complete set of the distinctively-interacting qubits of the absences-or-presences, which formulate the most complete unitary evolutions of the most general superpositions of the arbitrarily-structured hypergraphs. All the defining interactions and the complete total Hamiltonian of the quantum many body system of HoloQuantum Network Theory are uniquely obtained upon realizing the dynamical hypergraphical well-definedness by all the `cascade operators', the quantum-hypergraphical isomorphisms, the U(1) symmetry for the global-phase redundancies of the multi-qubit wavefunctions, the minimally broken symmetry of the qubits-equal-treatment, a Wheelerian maximal randomness, and the `covariant completeness'. By the axiomatic definition and construction of the theory, HoloQuantum Networks are all the time-dependent purely-information-theoretic wavefunctions, and mixed states, of every in-principle-realizable quantum many body system of the arbitrarily-chosen quantum objects and their arbitrarily-chosen quantum relations. Being so, we propose HoloQuantuam Network Theory as the most-fundamental most-complete form-invariant `it-from-qubit' theory of `All Quantum Natures'.

Categories: Journals, Physics

Expectation values of $p^2$ and $p^4$ in the square well potential. (arXiv:1801.04730v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

Position and momentum representations of a wavefunction $\psi(x)$ and $\phi(p)$, respectively are physically equivalent yet mathematically in a given case one may be easier or more transparent than the other. This disparity may be so much so that one has to device a special strategy to get the quantity of interest in one of them. We revisit finite square well (FSW) in this regard. Circumventing the the problems of discontinuity of second and higher derivatives of $\psi(x)$ we obtain simple analytic expressions of $<\!p^2\!>$ and $<\!p^4\!>$. But it is the surprising fall-off of $\phi(p)$ as $p^{-6}$ that reveals and restricts $<\!p^s\!>$ to be finite and non-zero only for $s=2,4$. In finding $<\!p^s\!>(s=2,4)$ from $\phi(p)$, $p$-integrals are improper which for time-being, have been evaluated numerically to show the agreement between two representations.

Categories: Journals, Physics

Optimizing deep-space optical communication under power constraints. (arXiv:1801.03947v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

We investigate theoretically the efficiency of deep-space optical communication in the presence of background noise. With decreasing average signal power spectral density, a scaling gap opens up between optimized simple-decoded pulse position modulation and generalized on-off keying with direct detection. The scaling of the latter follows the quantum mechanical capacity of an optical channel with additive Gaussian noise. Efficient communication is found to require a highly imbalanced distribution of instantaneous signal power. This condition can be alleviated through the use of structured receivers which exploit optical interference over multiple time bins to concentrate the signal power before the detection stage.

Categories: Journals, Physics

Towards a quantum time mirror for nonrelativistic wave packets. (arXiv:1801.01818v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

We propose a method -- a quantum time mirror (QTM) -- for simulating a partial time-reversal of the free-space motion of a nonrelativistic quantum wave packet. The method is based on a short-time spatially-homogeneous perturbation to the wave packet dynamics, achieved by adding a nonlinear time-dependent term to the underlying Schr\"odinger equation. Numerical calculations, supporting our analytical considerations, demonstrate the effectiveness of the proposed QTM for generating a time-reversed echo image of initially localized matter-wave packets in one and two spatial dimensions. We also discuss possible experimental realizations of the proposed QTM.

Categories: Journals, Physics

Study of the Q.Q Interaction- Single Particle Behavior to Elliott's Rotations. (arXiv:1710.06218v15 [nucl-th] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

We perform shell model calculations using a quadrupole-quadrupole interaction (Q.Q).We show results in single j shell spaces and the full S-D shell . We show that one gets useful results with Q.Q in both spaces.. We emphasize the importance of the choice of single particle energies in order to obtain the results of Elliott using a Q.Q interaction without the momentum terms. We show a J(J+1) spectrum for a ground state band but with B(E2)'s different from the rotational model. We also show excited J(J-1), J((J-1) and J(J+3) spectra.We find spectra starting with J=0 which have both even J and odd J members.

Categories: Journals, Physics

Optimal entangling operations between deterministic blocks of qubits encoded into single photons. (arXiv:1711.01319v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

Here, we numerically simulate probabilistic elementary entangling operations between rail-encoded photons for the purpose of scalable universal quantum computation or communication. We propose grouping logical qubits into single-photon blocks wherein single-qubit rotations and the CNOT gate are fully deterministic and simple to implement. Inter-block communication is then allowed through said probabilistic entangling operations. We find a promising trend in the increasing probability of successful inter-block communication as we increase the number of optical modes operated on by our elementary entangling operations.

Categories: Journals, Physics

On the waiting time in quantum repeaters with probabilistic entanglement swapping. (arXiv:1710.06214v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

The standard approach to realize a quantum repeater relies upon probabilistic but heralded entangled state manipulations and the storage of quantum states while waiting for successful events. In the literature on this class of repeaters, calculating repeater rates has typically depended on approximations assuming sufficiently small probabilities. Here we propose an exact and systematic approach including an algorithm based on Markov chains theory to compute the average waiting time (and hence the transmission rates) of quantum repeaters with arbitrary numbers of links. For up to four repeater segments, we present the exact rate formulae for arbitrary entanglement swapping probabilities. The effect of finite memory times is also considered and the relative influence of the classical communication (of heralded signals) is shown to grow significantly for larger probabilities. Conversely, we demonstrate that for small swapping probabilities the standard deviation of the waiting time is too large to be ignored in certain applications. We test and verify our analytical results through additional numerical simulations.

Categories: Journals, Physics

Controllability in tunable chains of coupled harmonic oscillators. (arXiv:1710.03145v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

We prove that temporal control of the strengths of springs connecting $N$ harmonic oscillators in a chain provides complete access to all Gaussian states of $N-1$ collective modes. The proof relies on the construction of a suitable basis of cradle modes for the system. An iterative algorithm to reach any desired Gaussian state requires at most $3N(N-1)/2$ operations. We illustrate this capability by engineering squeezed pseudo-phonon states - highly non-local, strongly correlated states that may result from various nonlinear processes. Tunable chains of coupled harmonic oscillators can be implemented by a number of current state-of-the-art experimental platforms, including cold atoms in lattice potentials, arrays of mechanical micro-oscillators, and coupled optical waveguides.

Categories: Journals, Physics

Entangling distant solid-state spins via thermal phonons. (arXiv:1710.01455v2 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Thu, 2018-01-18 05:04

The implementation of quantum entangling gates between qubits is essential to achieve scalable quantum computation. Here, we propose a robust scheme to realize an entangling gate for distant solid-state spins via a mechanical oscillator in its thermal equilibrium state. By appropriate Hamiltonian engineering and usage of a protected subspace, we show that the proposed scheme is able to significantly reduce the thermal effect of the mechanical oscillator on the spins. In particular, we demonstrate that a high entangling gate fidelity can be achieved even for a relatively high thermal occupation. Our scheme can thus relax the requirement for ground-state cooling of the mechanical oscillator, and may find applications in scalable quantum information processing in hybrid solid-state architectures.

Categories: Journals, Physics

Analog gravity by an optical vortex: Resonance enhancement of Hawking radiation

Author(s): Marco Ornigotti, Shimshon Bar-Ad, Alexander Szameit, and Victor Fleurov

Propagation of coherent light in a Kerr nonlinear medium can be mapped onto a flow of an equivalent fluid. Here we use this mapping to model the conditions in the vicinity of a rotating black hole as a Laguerre–Gauss vortex beam. We describe weak fluctuations of the phase and amplitude of the electr...


[Phys. Rev. A 97, 013823] Published Wed Jan 17, 2018

Categories: Journals, Physics

Modal analysis of wave propagation in dispersive media

Author(s): M. Ismail Abdelrahman and B. Gralak

Surveys on wave propagation in dispersive media have been limited since the pioneering work of Sommerfeld [Ann. Phys. 349, 177 (1914)] by the presence of branches in the integral expression of the wave function. In this article a method is proposed to eliminate these critical branches and hence to e...


[Phys. Rev. A 97, 013824] Published Wed Jan 17, 2018

Categories: Journals, Physics

Dissipative phase transition in the open quantum Rabi model

Author(s): Myung-Joong Hwang, Peter Rabl, and Martin B. Plenio

We demonstrate that the open quantum Rabi model (QRM) exhibits a second-order dissipative phase transition (DPT) and propose a method to observe this transition with trapped ions. The interplay between the ultrastrong qubit-oscillator coupling and the oscillator damping brings the system into a stea...


[Phys. Rev. A 97, 013825] Published Wed Jan 17, 2018

Categories: Journals, Physics

Viewpoint: A Possible Failure of Determinism in General Relativity

APS Physics - Wed, 2018-01-17 11:00

Author(s): Harvey Reall

A numerical analysis of perturbations of a charged black hole suggests that the usual predictability of the laws of physics can fail in general relativity.


[Physics 11, 6] Published Wed Jan 17, 2018

Categories: Physics

Correction Four-Component Dirac-Coulomb Using Gaussian Basis-Set and Gaussian Model Distribution for Super Heavy Element (Z=115). (arXiv:1801.04914v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-01-17 04:45

In this paper, we consider the Dirac-Coulomb equation for many-particles, to describe the interaction between electrons in the system having many electrons. The four- component wave function will expanding into a finite basis-set, using Gaussian basis function technique, in order to describe the upper and lower two components of the 4- spinors, respectively. Gaussian basis-set type dyall.v2z has been adopted to describe the correlation and polarization of 4-component wave function. The small component Gaussian basis functions have been generated from large component Gaussian basis functions using kinetic balance relation. The considered techniques have been applied for super heavy element 115 Uup, in which the nuclei has large charge and the inner spinors s 1/2 is strongly contracted. To solve the problem resulted from singularity at the origin for the 1s 1/2 spinors. We adopting the Gaussian charge distribution model to describe the charge of nuclei. To calculate accurate properties of the atomic levels, we used Dirac-Hartree-Fock method, which have more flexibility through Gaussian basis- set to treat relativistic quantum calculation for system has many-particle.

Categories: Journals, Physics

Time invariant $\mathcal{PT}$-product and phase locking in $\mathcal{PT}$-symmetric lattice models. (arXiv:1801.04918v1 [quant-ph])

arXiv.org: Quantum Physics - Wed, 2018-01-17 04:45

Over the past decade, non-Hermitian, $\mathcal{PT}$-symmetric Hamiltonians have been investigated as candidates for both, a fundamental, unitary, quantum theory, and open systems with a non-unitary time evolution. In this paper, we investigate the implications of the former approach in the context of the latter. Motivated by the invariance of the $\mathcal{PT}$ (inner) product under time evolution, we discuss the dynamics of wave-function phases in a wide range of $\mathcal{PT}$-symmetric lattice models. In particular, we numerically show that, starting with a random initial state, a universal, gain-site location dependent locking between wave function phases at adjacent sites occurs in the $\mathcal{PT}$-symmetry broken region. Our results pave the way towards understanding the physically observable implications of time-invariants in the non-unitary dynamics produced by $\mathcal{PT}$-symmetric Hamiltonians.

Categories: Journals, Physics
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