## Physics

### Configuration weight function method to solve the many-body Schr\"{o}dinger equation. (arXiv:1806.06473v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

A method to solve the Schr\"{o}dinger equation based on the use of constant particle-particle interaction potential surfaces is proposed. The many-body wave function is presented in configuration interaction form with coefficients - configuration weight functions - dependent on the total interaction potential. A set of linear ordinary differential equations for the configuration weight functions was developed and solved for particles in a infinite well and He-like ions. The results demonstrate that the method is variational and provides upper bound for energy of the ground state; even in its lowest two-body interaction potential surfaces approximation, it is more accurate than the conventional configuration interaction method and demonstrates a better convergence with a basis set increase. For He-like ions one configuration approximation with non-interaction electrons functions are used as basis set the calculated energies are below the Hartree-Fock limit. In three configuration approximations the accuracy of energy calculation is close to CI accuracy with 35 configuration taking into account. Four configurations give the energies below CI method and slightly below precise calculation with Hylleraas type wave functions.

Categories: Journals, Physics

### Cold Atom targets in the femtosecond laser field. (arXiv:1806.06510v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

We have built up a magneto-optical trap (MOT) recoil ion momentum spectroscopy (RIMS) (MOTRIMS) apparatus for studies of Rb atom in strong infrared laser field. Three cold Rb target modes namely two-dimensional (2D) MOT, 2D molasses MOT, and 3D MOT can be provided in the setup and the profiles for the density and the velocity of 3D MOT are characterized using absorption imaging and photoionization approaches. The momentum distributions of Rb${^+}$ for three types of atom targets are detected with standard RIMS method and the resolution of 0.15 a.u. is achieved along the extraction direction.

Categories: Journals, Physics

### A Universal Quantum Computing Virtual Machine. (arXiv:1806.06511v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

A small-scale quantum computer with full universal quantum computing capability is necessary for various practical aims and testing applications.

Here we report a 34-qubit quantum virtual machine (QtVM) based on a medium server. Our QtVM can run quantum assembly language with graphic interfaces. The QtVM is implemented with single qubit rotation gate, single to multiple controlled not gates to realize the universal quantum computation. Remarkably, it can realize a series of basic functions, such as, the "if" conditional programming language based on single-shot projective measurement results, "for" iteration programming language, build in arithmetic calculation. The measurement can be single-shot and arbitrary number of multi-shot types. In addition, there is in principle no limitation on number of logic gates implemented for quantum computation. By using QtVM, we demonstrate the simulation of dynamical quantum phase transition of transverse field Ising model by quantum circuits, where 34 qubits and $6.8\times 10^4$ gates are realized. We also show the realization of programmable Shor algorithm for factoring 15 and 35.

Categories: Journals, Physics

### Signatures of quantum spin liquids in small lattices. (arXiv:1806.06542v1 [cond-mat.str-el])

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

Quantum spin liquids remain one of the most challenging subjects of quantum magnetism. Characterized by massive degenerate ground states that have long range entanglement and are locally indistinguishable, highly demanding numerical techniques are often needed to describe them. Here we propose an easy computational method based on exact diagonalization with engineered boundary conditions to unveil their most significant features in small lattices. We derive the quantum phase diagram of diverse antiferromagnetic Heisenberg models in the triangular lattice. For all studied cases, our results are in accordance with the previous results obtained by means of sophisticated variational methods.

Categories: Journals, Physics

### Polarization-dependent property of dipole-dipole interaction mediated by localized surface plasmon of an Ag nanosphere. (arXiv:1806.06546v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

The effect of the dipole polarization on the quantum dipole dipole interaction near an Ag nanosphere (ANS) is investigated. A theoretical formalism in terms of classical Green function is developed for the transfer rate and the potential energy of the dipole dipole interaction (DDI) between two polarized dipoles. It is found that a linear transition dipole can couple to a left circularly polarized transition dipole much stronger than to a right circularly polarized transition dipole. This polarization selectivity exists over a wide frequency range and is robust against the variation of the dipoles' position or the radius of the ANS. In contrast, a right circularly polarized transition dipole, can change sharply from coupling strongly to another right circularly polarized dipole to coupling strongly to a left circularly polarized dipole with varying frequency. However, if the two dipoles are placed in the same radial direction of the sphere, the right circularly polarized transition dipole can only couple to the dipole with the same polarization while not to the left circularly polarized transition dipole. These findings may be used in solid-state quantum-information processing based on the DDI.

Categories: Journals, Physics

### Three types of discrete energy eigenvalues in complex PT-symmetric scattering potentials. (arXiv:1806.06578v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

For complex PT-symmetric scattering potentials (CPTSSPs) $V(x)$, we show that complex $k$-poles of transmission amplitude $t(k)$ or zeros of ${\cal M}_{22}(k)=1/t(k)$ of the type $\pm k_1+ik_2, k_2\ge 0$ are physical which alone are sufficient to yield three types of discrete energy eigenvalues of the potential. These discrete energies are real negative, complex conjugate pairs of eigenvalues(CCPE:${\cal E}_n \pm i \gamma_n$) and real positive energy called spectral singularity (SS) at $E=E_*$. Based on four analytically solvable and other numerically solved models, we conjecture that for a CPTSSP, the real positive energy $E_* =k_*^2$ (SS), when $V_2$ (the strength of the imaginary part of $V(x)$) equals $v_*$, is unique (single) and it is the upper bound to the CCPEs such that ${\cal E}_l \approx < E_*$, here ${\cal E}_l$ corresponds to the last of CCPEs. When there are exceptional points ($g_1<g_2<g_3<..<g_l$) in $V(x)$, $v_* >>g_l$. We show that when $V_2 >v_*$ ($V(x)$ is slightly more non-Hermitian), the SS disappears and the last of CCPEs appears as though SS is split in to ${\cal E}_l\pm i \gamma_l$, where ${\cal E}_l \approx E_*$. When $V_2 < v_*$ $(V(x)$ is slightly less non-Hermitian), one pair of CCPE disappears but others do remain with ${\cal E}_l <<E_*$. This is as though SS is not split.

Categories: Journals, Physics

### Probabilistic Fault-Tolerant Universal Quantum Computation and Sampling Problems in Continuous Variables. (arXiv:1806.06618v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

Continuous-Variable (CV) devices are a promising platform for demonstrating large-scale quantum information protocols. In this framework, we define a general quantum computational model based on a CV hardware. It consists of vacuum input states, a finite set of gates - including non-Gaussian elements - and homodyne detection. We show that this model incorporates encodings sufficient for probabilistic fault-tolerant universal quantum computing. Furthermore, we show that this model can be adapted to yield sampling problems that cannot be simulated efficiently with a classical computer, unless the polynomial hierarchy collapses. This allows us to provide a simple paradigm for short-term experiments to probe quantum advantage relying on Gaussian states, homodyne detection and some form of non-Gaussian evolution. We finally address the recently introduced model of Instantaneous Quantum Computing in CV, and prove that the hardness statement is robust with respect to some experimentally relevant simplifications in the definition of that model.

Categories: Journals, Physics

### Disproving hidden variable models with spin magnitude conservation. (arXiv:1806.06637v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

The squares of the three components of the spin-s operators sum up to $s(s+1)$. However, a similar relation is rarely satisfied by the set of possible spin projections onto mutually orthogonal directions. This has fundamental consequences if one tries to construct a hidden variable (HV) theory describing measurements of spin projections. We propose a test of local HV-models in which spin magnitudes are conserved. These additional constraints imply that the corresponding inequalities are violated within quantum theory by larger classes of states than in the case of standard Bell inequalities. We conclude that in any HV-theory pertaining to measurements on a spin one can find situations in which either HV-assignments do not represent a physical reality of a spin vector, but rather provide a deterministic algorithm for prediction of the measurement outcomes, or HV-assignments represent a physical reality, but the spin cannot be considered as a vector of fixed length.

Categories: Journals, Physics

### On the use of deep neural networks in optical communications. (arXiv:1806.06663v1 [physics.app-ph])

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

Information transfer rates in optical communications may be dramatically increased by making use of spatially non-Gaussian states of light. Here we demonstrate the ability of deep neural networks to classify numerically-generated, noisy Laguerre-Gauss modes of up to 100 quanta of orbital angular momentum with near-unity fidelity. The scheme relies only on the intensity profile of the detected modes, allowing for considerable simplification of current measurement schemes required to sort the states containing increasing degrees of orbital angular momentum. We also present results that show the strength of deep neural networks in the classification of experimental superpositions of Laguerre-Gauss modes when the networks are trained solely using simulated images. It is anticipated that these results will allow for an enhancement of current optical communications technologies.

Categories: Journals, Physics

### Relativistic treatment of the energy shifts caused by static electromagnetic effects on free electrons. (arXiv:1806.06686v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

Magnetic effects on free electron systems have been studied extensively in the context of spin-to-orbital angular momentum conversion. Starting from the Dirac equation, we derive a fully relativistic expression for the energy of free electrons in the presence of a spatiotemporally constant, weak electromagnetic field. The expectation value of the maximum energy shift, which is completely independent of the electron spin-polarization coefficients, is computed perturbatively to first order. This effect is orders of magnitude larger than that predicted by the quantum mechanical Zeeman shift. We then show, in the non-relativistic limit, how to discriminate between achiral and completely polarized states and discuss possible mesoscopic and macroscopic manifestations of electron spin states across many orders of magnitude in the physical world.

Categories: Journals, Physics

### State-preparation-and-measurement tomography of a two-qubit system. (arXiv:1806.06697v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

We experimentally demonstrate that loop state-preparation-and-measurement (SPAM) tomography is capable of detecting correlated errors in a two-qubit system. We prepare photon pairs in a state that approximates a Werner state, which may or may not be entangled. By performing measurements with multiple different detector settings we are able to detect correlated errors between two single-qubit measurements performed in different locations. No assumptions are made concerning either the state preparations or the measurements, other than that the dimensions of the states and the positive-operator-valued measures describing the detectors are known. The only other needed information is experimentally measured expectation values, which are analyzed for self-consistency. This demonstrates that loop SPAM tomography is a useful technique for detecting errors that would degrade the performance of multiple-qubit quantum information processors.

Categories: Journals, Physics

### Circuit-based Modular Implementation of Quantum Ghost Imaging. (arXiv:1806.06702v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

Efforts on enhancing the ghost imaging speed and quality are intensified when the debate around the nature of ghost imaging (quantum vs. classical) is suspended for a while. Accordingly, most of the studies these years in the field fall into the improvement regarding these two targets by utilizing the different imaging mediums. Nevertheless, back to the raging debate occurred but with different focus, to overcome the inherent difficulties in the classical imaging domain, if we are able to utilize the superiority that quantum information science offers us, the ghost imaging experiment may be implemented more practically. In this study, a quantum circuit implementation of ghost imaging experiment is proposed, where the speckle patterns and phase mask are encoded by utilizing the quantum representation of images. To do this, we formulated several quantum models, i.e. quantum accumulator, quantum multiplier, and quantum divider. We believe this study will provide a new impetus to explore the implementation of ghost imaging using quantum computing resources.

Categories: Journals, Physics

### PT Symmetric Floquet Topological Phase in SSH Model. (arXiv:1806.06722v1 [quant-ph])

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

We consider periodically modulated Su-Schrieffer-Heeger (SSH) model with gain and loss. This model, which can be realized with current technology in photonics using waveguides, allows us to study Floquet topological insulating phase. By using Floquet theory, we find the quasi-energy spectrum of this one dimensional PT symmetric topological insulator. We show that stable Floquet topological phase exists in our model provided that oscillation frequency is large and the non-Hermitian degree is below than a critical value.

Categories: Journals, Physics

### ISA-Pol: Distributed polarizabilities and dispersion models from a basis-space implementation of the iterated stockholder atoms procedure. (arXiv:1806.06737v1 [physics.chem-ph])

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

Recently we have developed a robust, basis-space implementation of the iterated stockholder atoms (BS-ISA) approach for defining atoms in a molecule. This approach has been shown to yield rapidly convergent distributed multipole expansions with a well-defined basis-set limit. Here we use this method as the basis of a new approach, termed ISA-Pol, for obtaining non-local distributed frequency-dependent polarizabilities. We demonstrate how ISA-Pol can be combined with localization methods to obtain distributed dispersion models that share the many unique properties of the ISA: These models have a well-defined basis-set limit, lead to very accurate dispersion energies, and, remarkably, satisfy commonly used combination rules to a good accuracy. As these models are based on the ISA, they can be expected to respond to chemical and physical changes naturally, and thus they may serve as the basis for the next generation of polarization and dispersion models for ab initio force-field development.

Categories: Journals, Physics

### Manipulating quantum materials with quantum light. (arXiv:1806.06752v1 [cond-mat.str-el])

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

We show that the macroscopic magnetic and electronic properties of strongly correlated electron systems can be manipulated by coupling them to a cavity mode. As a paradigmatic example we consider the Fermi-Hubbard model and find that the electron-cavity coupling enhances the magnetic interaction between the electron spins in the ground-state manifold, introduces a next-nearest neighbour hopping and mediates a long-range electron-electron interaction between distant sites. In the manifold of states with one photon or one doublon excitation the cavity results in the formation of polariton branches. The vacuum Rabi splitting of the two outermost branches is collectively enhanced and can exceed the width of the first excited Hubbard band. The cavity-mediated modifications of the material properties in the ground and first excited manifolds can be experimentally observed via measurements of the magnetic susceptibility and the optical conductivity, respectively.

Categories: Journals, Physics

### A semiclassical ramp in SYK and in gravity. (arXiv:1806.06840v1 [hep-th])

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

In finite entropy systems, real-time partition functions do not decay to zero at late time. Instead, assuming random matrix universality, suitable averages exhibit a growing "ramp" and "plateau" structure. Deriving this non-decaying behavior in a large $N$ collective field description is a challenge related to one version of the black hole information problem. We describe a candidate semiclassical explanation of the ramp for the SYK model and for black holes. In SYK, this is a two-replica nonperturbative saddle point for the large $N$ collective fields, with zero action and a compact zero mode that leads to a linearly growing ramp. In the black hole context, the solution is a two-sided black hole that is periodically identified under a Killing time translation. We discuss but do not resolve some puzzles that arise.

Categories: Journals, Physics

### Quantum mechanics as a solutions to the classical self-force problem. (arXiv:0902.4606v7 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

It is argued that, contrary to conventional wisdom, no trustworthy universal self-force/radiative corrections to the Lorentz force equation, can be derived from the basic tenets of classical electrodynamics. This concords with the apparent randomness observed in quantum mechanical scattering experiments and with the absence of any experimental support for such universality. In a recent paper [11], the statistical effect of radiative corrections to the motion of charged bodies has been derived from the basic tenets and does take a universal form, described by quantum mechanical wave equations---again conforming with experiment. As that derivation assumes nothing about the size, mass or composition of the body, it is conjectured that quantum mechanics is the appropriate framework for dealing also with radiative corrections to the motion of macroscopic bodies.

Categories: Journals, Physics

### Quantum random flip-flop based on random photon emitter and its applications. (arXiv:1308.5719v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

We propose, experimentally realize and study possible applications of a new type of logic element: random flip-flop. By definition it operates similarly to a conventional flip-flop except that it functions with probability of 1/2 otherwise it does nothing. We demonstrate one practical realization of the random flip-flop based on optical quantum random number generator and discuss possible usages of such a device in computers, cryptographic hardware and testing equipment.

Categories: Journals, Physics

### Time as a stochastic process. (arXiv:1411.1741v6 [physics.gen-ph] UPDATED)

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

In the paper we consider an interesting possibility of a time as a stochastic process in quantum mechanics.In order to do it we reconsider time as a mechanical quantity in classical mechanics and afterwards we quantize it. We consider continuous and discrete time.

Categories: Journals, Physics

### Work and reversibility in quantum thermodynamics. (arXiv:1506.08145v3 [quant-ph] UPDATED)

arXiv.org: Quantum Physics - Tue, 2018-06-19 11:45

It is a central question in quantum thermodynamics to determine how irreversible is a process that transforms an initial state $\rho$ to a final state $\sigma$, and whether such irreversibility can be thought of as a useful resource. For example, we might ask how much work can be obtained by thermalizing $\rho$ to a thermal state $\sigma$ at temperature $T$ of an ambient heat bath. Here, we show that, for different sets of resource-theoretic thermodynamic operations, the amount of entropy produced along a transition is characterized by how reversible the process is. More specifically, this entropy production depends on how well we can return the state $\sigma$ to its original form $\rho$ without investing any work. At the same time, the entropy production can be linked to the work that can be extracted along a given transition, and we explore the consequences that this fact has for our results. We also exhibit an explicit reversal operation in terms of the Petz recovery channel coming from quantum information theory. Our result establishes a quantitative link between the reversibility of thermodynamical processes and the corresponding work gain.

Categories: Journals, Physics