arXiv subjects

Alexander Pikovski

Publications and source records attributed to Alexander Pikovski.

9 recordsLinked to original sources

arXiv2026

Benchmarking of algorithms for set partitions

Set partitions are arrangements of distinct objects into groups. The problem of listing all set partitions arises in a variety of settings, in particular in combinatorial optimization tasks. After a brief review, we give practical approximate formulas for determining the number of set partitions, both for small and large set sizes. Several algorithms for enumerating all set partitions are reviewed, and benchmarking tests were conducted. The algorithm of Djokic et al. is recommended for practical use.

cs.DS↗

arXiv2019

Cooper pair polaritons in cold fermionic atoms within a cavity

We formulate a Bardeen-Cooper-Schriffer (BCS) theory of quasiparticles in a degenerate Fermi gas strongly coupled to photons in a optical cavity. The elementary photonic excitations of the system are cavity polaritons, which consist of a cavity photon and an excitation of an atom within the Fermi sea. The excitation of the atom out of the Fermi sea leaves behind a hole, which together results in a loosely bound Cooper pair, allowing for the system to be written by a BCS wavefunction. As the density of the excitations is increased, the excited atom and hole become more strongly bound, crossing over into the molecular regime. This thus realizes an alternative BCS to BEC crossover scenario, where the participating species are quasiparticle excitations in a Fermi sea consisting of excited atoms and holes.

cond-mat.quant-gas↗

arXiv2015

Comparing energy levels in isotropic and anisotropic potentials

Qualitative information about the quantized energy levels of a system can be of great value. We study the relationship between the bound-state energies of an anisotropic potential and those of its spherical average. It is shown that the two ground-state energies satisfy an inequality, and there is a similar inequality for the first excited states.

quant-ph↗

arXiv2015

Dynamics of correlations in two-dimensional quantum spin models with long-range interactions: A phase-space Monte-Carlo study

Interacting quantum spin models are remarkably useful for describing different types of physical, chemical, and biological systems. Significant understanding of their equilibrium properties has been achieved to date, especially for the case of spin models with short-range couplings. However, progress towards the development of a comparable understanding in long-range interacting models, in particular out-of-equilibrium, remains limited. In a recent work, we proposed a semiclassical numerical method to study spin models, the discrete truncated Wigner approximation (DTWA), and demonstrated its capability to correctly capture the dynamics of one- and two-point correlations in one dimensional (1D) systems. Here we go one step forward and use the DTWA method to study the dynamics of correlations in 2D systems with many spins and different types of long-range couplings, in regimes where other numerical methods are generally unreliable. We compute spatial and time-dependent correlations for spin-couplings that decay with distance as a power-law and determine the velocity at which correlations propagate through the system. Sharp changes in the behavior of those velocities are found as a function of the power-law decay exponent. Our predictions are relevant for a broad range of systems including solid state materials, atom-photon systems and ultracold gases of polar molecules, trapped ions, Rydberg, and magnetic atoms. We validate the DTWA predictions for small 2D systems and 1D systems, but ultimately, in the spirt of quantum simulation, experiments will be needed to confirm our predictions for large 2D systems.

quant-ph↗

arXiv2015

Many-Body Quantum Spin Dynamics with Monte Carlo Trajectories on a Discrete Phase Space

Interacting spin systems are of fundamental relevance in different areas of physics, as well as in quantum information science, and biology. These spin models represent the simplest, yet not fully understood, manifestation of quantum many-body systems. An important outstanding problem is the efficient numerical computation of dynamics in large spin systems. Here we propose a new semiclassical method to study many-body spin dynamics in generic spin lattice models. The method is based on a discrete Monte Carlo sampling in phase-space in the framework of the so-called truncated Wigner approximation. Comparisons with analytical and numerically exact calculations demonstrate the power of the technique. They show that it correctly reproduces the dynamics of one- and two-point correlations and spin squeezing at short times, thus capturing entanglement. Our results open the possibility to study the quantum dynamics accessible to recent experiments in regimes where other numerical methods are inapplicable.

quant-ph↗

arXiv2014

A differentiation formula, with application to the two-dimensional Schr\"odinger equation

A method for obtaining discretization formulas for the derivatives of a function is presented, which relies on a generalization of divided differences. These modified divided differences essentially correspond to a change of the dependent variable. This method is applied to the numerical solution of the eigenvalue problem for the two-dimensional Schr\"odinger equation, where standard methods converge very slowly while the approach proposed here gives accurate results.

physics.comp-ph↗

arXiv2014

On cold gases with anisotropic interactions

A cold gas of particles with anisotropic interactions of general form, due to a polarizing field, is studied. Special cases are atoms or molecules with dipole-dipole or quadrupole-quadrupole interactions. It is shown that the angular dependence of an observable on the direction of the polarizing field is largely determined by symmetry. For a gas in a confined quasi two-dimensional geometry, the effective interaction is calculated in general form. Some examples of dipole and quadrupole gases are considered. It is concluded that when anisotropic forces are studied in a general manner, one can obtain simpler results and better understanding for some problems.

cond-mat.quant-gas↗

arXiv2012

Averaged collision and reaction rates in a two-species gas of ultracold fermions

Reactive or elastic two-body collisions in an ultracold gas are affected by quantum statistics. In this paper, we study ensemble-averaged collision rates for a two-species fermionic gas. The two species may have different masses, densities and temperatures. We investigate how averaged collision rates are affected by the presence of Fermi spheres in the initial states; Pauli blocking of final states is not considered. It is shown that, independently on the details of the collision, Fermi-averaged collision rates deviate from Boltzmann-averaged ones, particularly for a gas with strong imbalance of masses or densities.

cond-mat.quant-gas↗

arXiv2010

Two-dimensional scattering and bound states of polar molecules in bilayers

Low-energy two-dimensional scattering is particularly sensitive to the existence and properties of weakly-bound states. We show that interaction potentials $V(r)$ with vanishing zero-momentum Born approximation $\int d^2r V(r)=0$ lead to an anomalously weak bound state which crucially modifies the two-dimensional scattering properties. This anomalous case is especially relevant in the context of polar molecules in bilayer arrangements.

cond-mat.quant-gas↗