arXiv subjects

Vladimir V. Kocharovsky

Publications and source records attributed to Vladimir V. Kocharovsky.

11 recordsLinked to original sources

arXiv2026

Anomalous Conductivity and Anisotropic Transport of Nonrelativistic Electrons in Plasma with Magnetostatic Weibel-Generated Turbulence

The anisotropic diffusion of electrons with various rigidity and the anomalous conductivity of a collisionless plasma in the presence of Weibel-generated quasi-static turbulent and uniform external magnetic fields are examined. Using an original code based on the Boris algorithm, the electron diffusion coefficients and the longitudinal, transverse, and Hall mobility factors are determined for a representative set of plasma parameters. It is shown that these values and their anisotropy depend strongly on the electron temperature, external magnetic field, average level of magnetic turbulence, and its spectrum. The physical origin and expected limits of such dependencies are indicated. Applications of the results are discussed in the case of coronal plasma, where the anomalous resistivity prevails over the collisional one and can be responsible for the redistribution of large-scale currents in magnetic loops and small-scale currents in the regions of reconnection of magnetic-field lines.

physics.plasm-ph↗

arXiv2024

Boson sampling with self-generation of squeezing via interaction of photons and atoms

We suggest a novel scheme for generating multimode squeezed states for the boson sampling implementation. The idea is to replace a commonly used linear interferometer by a multimode resonator containing a passive optical element consisting of two-level atoms dispersively interacting with photons and self-generating a squeezed compound state of both bosons -- photons and atoms. The suggested scheme does not need (a) on-demand external sources of photons in squeezed or Fock quantum states and (b) numerous interchannel couplers which introduce phase noise and losses that prevent scaling up the system and achieving quantum advantage. The idea is illustrated by a setup based on a Bose-Einstein-condensed gas confined in a multimode resonator, one of whose optical modes is in the classical coherent regime. The joint probability distribution of photon and/or noncondensed atom numbers is calculated via a matrix hafnian that, for certain parameters of the system, is hardly to be effectively calculated by classical computers. Such experiments are at reach via existing cavity-QED and cold-gas technology.

quant-ph↗

arXiv2022

Quantum supremacy of the many-body fluctuations in the occupations of the excited particle states in a Bose-Einstein-condensed gas

We find a universal analytic formula for a characteristic function (Fourier transform) of a joint probability distribution for the particle occupation numbers in a BEC gas and the Hafnian Master Theorem generalizing the famous Permanent Master Theorem of MacMahon. We suggest an appealing model, a multi-qubit BEC trap formed by a set of qubit potential wells, and discuss specifics of such an atomic boson-sampling system vs a photonic one. Finally, the process of many-body fluctuations in a BEC trap is #P-hard for computing. It could serve as a basis for demonstrating quantum supremacy of the many-body interacting systems over classical simulators.

cond-mat.stat-mech↗

arXiv2021

Exact recursive calculation of circulant permanents: A band of different diagonals inside a uniform matrix

We present a finite-order system of recurrence relations for a permanent of circulant matrices containing a band of k any-value diagonals on top of a uniform matrix (for k = 1, 2, and 3) as well as the method for deriving such recurrence relations which is based on the permanents of the matrices with defects. The proposed system of linear recurrence equations with variable coefficients provides a powerful tool for the analysis of the circulant permanents, their fast, linear time computing and finding their asymptotics in a large-matrix-size limit. The latter problem is an open fundamental problem. Its solution would be tremendously important for a unified analysis of a wide range of the nature's #P-hard problems, including problems in the physics of many-body systems, critical phenomena, quantum computing, quantum field theory, theory of chaos, fractals, theory of graphs, number theory, combinatorics, cryptography, etc.

cond-mat.stat-mech↗

arXiv2021

Bose-Einstein condensate fluctuations versus an interparticle interaction

We calculate the Bose-Einstein condensate (BEC) occupation statistics vs. the interparticle interaction in a dilute gas with a nonuniform condensate in a box trap within the Bogoliubov approach. The results are compared against the previously found BEC-occupation statistics in (i) an ideal gas and (ii) a weakly interacting gas with a uniform condensate. In particular, we reveal and explicitly describe an appearance of a nontrivial transition from the ideal gas to the Thomas-Fermi regime. The results include finding the main regimes of the BEC statistics - the anomalous non-Gaussian thermally-dominated fluctuations and the Gaussian quantum-dominated fluctuations - as well as a crossover between them and their manifestations in a mesoscopic system. Remarkably, we show that the effect of the boundary conditions, imposed at the box trap, on the BEC fluctuations does not vanish in the thermodynamic limit of a macroscopic system even in the presence of the interparticle interactions. Finally, we discuss a challenging problem of an experimental verification of the theory of the BEC fluctuations addressing a much deeper level of the many-body statistical physics than usually studied quantities related to the mean condensate occupation.

cond-mat.quant-gas↗

arXiv2016

Exact general solution to the three-dimensional Ising model and a self-consistency equation for the nearest-neighbors' correlations

We find an exact general solution to the three-dimensional (3D) Ising model via an exact self-consistency equation for nearest-neighbors' correlations. It is derived by means of an exact solution to the recurrence equations for partial contractions of creation and annihilation operators for constrained spin bosons in a Holstein-Primakoff representation. In particular, we calculate analytically the total irreducible self-energy, the order parameter, the correlation functions, and the joined occupation probabilities of spin bosons. The developed regular microscopic quantum-field-theory method has a potential for a full solution of a long-standing and still open problem of 3D critical phenomena.

cond-mat.stat-mech↗

arXiv2016

Microscopic theory of phase transitions in a critical region

The problem of finding a microscopic theory of phase transitions across a critical point is a central unsolved problem in theoretical physics. We find a general solution to that problem and present it here for the cases of Bose-Einstein condensation in an interacting gas and ferromagnetism in a lattice of spins, interacting via a Heisenberg or Ising Hamiltonian. For Bose-Einstein condensation, we present the exact, valid for the entire critical region, equations for the Green functions and order parameter, that is a critical-region extension of the Beliaev-Popov and Gross-Pitaevskii equations. For the magnetic phase transition, we find an exact theory in terms of constrained bosons in a lattice and obtain similar equations for the Green functions and order parameter. In particular, we outline an exact solution for the three-dimensional Ising model.

cond-mat.stat-mech↗

arXiv2015

Self-similar analytical solution of the critical fluctuations problem for the Bose-Einstein condensation in an ideal gas

Paper is published in J. Phys. A: Math. Theor. 43 (2010) 225001, doi:10.1088/1751-8113/43/22/225001. Exact analytical solution for the universal probability distribution of the order parameter fluctuations as well as for the universal statistical and thermodynamic functions of an ideal gas in the whole critical region of Bose-Einstein condensation is obtained. A universal constraint nonlinearity is found that is responsible for all nontrivial critical phenomena of the BEC phase transition. Simple analytical approximations, which describe the universal structure of the critical region in terms of confluent hypergeometric or parabolic cylinder functions, as well as asymptotics of the exact solution are derived. The results for the order parameter, all higher-order moments of BEC fluctuations, and thermodynamic quantities, including specific heat, perfectly match the known asymptotics outside critical region as well as the phenomenological renormalization-group ansatz with known critical exponents in the close vicinity of the critical point. Thus, a full analytical solution to a long-standing problem of finding a universal structure of the lambda-point for BEC in an ideal gas is found.

cond-mat.stat-mech↗

arXiv2015

Analytical theory of mesoscopic Bose-Einstein condensation in ideal gas

We find universal structure and scaling of BEC statistics and thermodynamics for mesoscopic canonical-ensemble ideal gas in a trap for any parameters, including critical region. We identify universal constraint-cut-off mechanism that makes BEC fluctuations non-Gaussian and is responsible for critical phenomena. Main result is analytical solution to problem of critical phenomena. It is derived by calculating universal distribution of noncondensate occupation (Landau function) and then universal functions for physical quantities. We find asymptotics of that solution and its approximations which describe universal structure of critical region in terms of parabolic cylinder or confluent hypergeometric functions. Results for order parameter, statistics, and thermodynamics match known asymptotics outside critical region. We suggest 2-level and 3-level trap models and find their exact solutions in terms of cut-off negative binomial distribution (that tends to cut-off gamma distribution in continuous limit) and confluent hypergeometric distribution. We introduce a regular refinement scheme for condensate statistics approximations on the basis of infrared universality of higher-order cumulants and method of superposition and show how to model BEC statistics in actual traps. We find that 3-level trap model with matching the first 4 or 5 cumulants is enough to yield remarkably accurate results in whole critical region. We derive exact multinomial expansion for noncondensate occupation distribution and find its high temperature asymptotics (Poisson distribution). We demonstrate that critical exponents and a few known terms of Taylor expansion of universal functions, calculated previously from fitting finite-size simulations within renorm-group theory, can be obtained from presented solutions.

cond-mat.stat-mech↗

arXiv2015

Microscopic theory of a phase transition in a critical region: Bose-Einstein condensation in an interacting gas

We present a microscopic theory of the second order phase transition in an interacting Bose gas that allows one to describe formation of an ordered condensate phase from a disordered phase across an entire critical region continuously. We derive the exact fundamental equations for a condensate wave function and the Green functions, which are valid both inside and outside the critical region. They are reduced to the usual Gross-Pitaevskii and Beliaev-Popov equations in a low-temperature limit outside the critical region. The theory is readily extendable to other phase transitions, in particular, in the physics of condensed matter and quantum fields.

cond-mat.stat-mech↗

arXiv2006

Fluctuations in Ideal and Interacting Bose-Einstein Condensates: From the laser phase transition analogy to squeezed states and Bogoliubov quasiparticles

We review the phenomenon of equilibrium fluctuations in the number of condensed atoms in a trap containing N atoms total. We start with a history of the Bose-Einstein distribution, the Einstein-Uhlenbeck debate concerning the rounding of the mean number of condensed atoms near a critical temperature, and a discussion of the relations between statistics of BEC fluctuations in the grand canonical, canonical, and microcanonical ensembles. Next we discuss different approaches capable of providing approximate analytical results and physical insight into the problem of fluctuations. In particular, we describe the master equation (similar to the quantum theory of the laser) and canonical-ensemble quasiparticle approaches which give the most accurate and physically transparent picture of the BEC fluctuations. In the last part we describe condensate fluctuations in the interacting Bose gas. In particular, we show that the canonical-ensemble quasiparticle approach works very well for the interacting gases and yields analytical formulas for the characteristic function and all moments of the condensate fluctuations. In most cases the ground-state occupation fluctuations are anomalously large and are not Gaussian even in the thermodynamic limit. We clarify a crossover between the ideal and weakly-interacting-gas statistics which is governed by a pair-correlation squeezing mechanism.

cond-mat.stat-mech↗