204 research outputs found
Typical state of an isolated quantum system with fixed energy and unrestricted participation of eigenstates
This work describes the statistics for the occupation numbers of quantum
levels in a large isolated quantum system, where all possible superpositions of
eigenstates are allowed, provided all these superpositions have the same fixed
energy. Such a condition is not equivalent to the conventional micro-canonical
condition, because the latter limits the participating eigenstates to a very
narrow energy window. The statistics is obtained analytically for both the
entire system and its small subsystem. In a significant departure from the
Boltzmann-Gibbs statistics, the average occupation numbers of quantum states
exhibit in the present case weak algebraic dependence on energy. In the
macroscopic limit, this dependence is routinely accompanied by the condensation
into the lowest energy quantum state. This work contains initial numerical
tests of the above statistics for finite systems, and also reports the
following numerical finding: When the basis states of large but finite random
matrix Hamiltonians are expanded in terms of eigenstates, the participation of
eigenstates in such an expansion obeys the newly obtained statistics. The above
statistics might be observable in small quantum systems, but for the
macroscopic systems, it rather reenforces doubts about self-sufficiency of
non-relativistic quantum mechanics for justifying the Boltzmann-Gibbs
equilibrium.Comment: 20 pages, 3 figure
Self-bound many-body states of quasi-one-dimensional dipolar Fermi gases: Exploiting Bose-Fermi mappings for generalized contact interactions
Using a combination of results from exact mappings and from mean-field theory
we explore the phase diagram of quasi-one-dimensional systems of identical
fermions with attractive dipolar interactions. We demonstrate that at low
density these systems provide a realization of a single-component
one-dimensional Fermi gas with a generalized contact interaction. Using an
exact duality between one-dimensional Fermi and Bose gases, we show that when
the dipole moment is strong enough, bound many-body states exist, and we
calculate the critical coupling strength for the emergence of these states. At
higher densities, the Hartree-Fock approximation is accurate, and by combining
the two approaches we determine the structure of the phase diagram. The
many-body bound states should be accessible in future experiments with
ultracold polar molecules
Spectral properties of quantum -body systems versus chaotic properties of their mean field approximations
We present numerical evidence that in a system of interacting bosons there
exists a correspondence between the spectral properties of the exact quantum
Hamiltonian and the dynamical chaos of the associated mean field evolution.
This correspondence, analogous to the usual quantum-classical correspondence,
is related to the formal parallel between the second quantization of the mean
field, which generates the exact dynamics of the quantum -body system, and
the first quantization of classical canonical coordinates. The limit of
infinite density and the thermodynamic limit are then briefly discussed.Comment: 15 pages RevTeX, 11 postscript figures included with psfig, uuencoded
gz-compressed .tar fil
Coherent States Expectation Values as Semiclassical Trajectories
We study the time evolution of the expectation value of the anharmonic
oscillator coordinate in a coherent state as a toy model for understanding the
semiclassical solutions in quantum field theory. By using the deformation
quantization techniques, we show that the coherent state expectation value can
be expanded in powers of such that the zeroth-order term is a classical
solution while the first-order correction is given as a phase-space Laplacian
acting on the classical solution. This is then compared to the effective action
solution for the one-dimensional \f^4 perturbative quantum field theory. We
find an agreement up to the order \l\hbar, where \l is the coupling
constant, while at the order \l^2 \hbar there is a disagreement. Hence the
coherent state expectation values define an alternative semiclassical dynamics
to that of the effective action. The coherent state semiclassical trajectories
are exactly computable and they can coincide with the effective action
trajectories in the case of two-dimensional integrable field theories.Comment: 20 pages, no figure
Chaotic properties of quantum many-body systems in the thermodynamic limit
By using numerical simulations, we investigate the dynamics of a quantum
system of interacting bosons. We find an increase of properly defined mixing
properties when the number of particles increases at constant density or the
interaction strength drives the system away from integrability. A
correspondence with the dynamical chaoticity of an associated -number system
is then used to infer properties of the quantum system in the thermodynamic
limit.Comment: 4 pages RevTeX, 4 postscript figures included with psfig; Completely
restructured version with new results on mixing properties added
Chaos in effective classical and quantum dynamics
We investigate the dynamics of classical and quantum N-component phi^4
oscillators in the presence of an external field. In the large N limit the
effective dynamics is described by two-degree-of-freedom classical Hamiltonian
systems. In the classical model we observe chaotic orbits for any value of the
external field, while in the quantum case chaos is strongly suppressed. A
simple explanation of this behaviour is found in the change in the structure of
the orbits induced by quantum corrections. Consistently with Heisenberg's
principle, quantum fluctuations are forced away from zero, removing in the
effective quantum dynamics a hyperbolic fixed point that is a major source of
chaos in the classical model.Comment: 6 pages, RevTeX, 5 figures, uses psfig, changed indroduction and
conclusions, added reference
Fluctuations in Stationary non Equilibrium States
In this paper we formulate a dynamical fluctuation theory for stationary non
equilibrium states (SNS) which covers situations in a nonlinear hydrodynamic
regime and is verified explicitly in stochastic models of interacting
particles. In our theory a crucial role is played by the time reversed
dynamics. Our results include the modification of the Onsager-Machlup theory in
the SNS, a general Hamilton-Jacobi equation for the macroscopic entropy and a
non equilibrium, non linear fluctuation dissipation relation valid for a wide
class of systems
Generalized Central Limit Theorem and Renormalization Group
We introduce a simple instance of the renormalization group transformation in
the Banach space of probability densities. By changing the scaling of the
renormalized variables we obtain, as fixed points of the transformation, the
L\'evy strictly stable laws. We also investigate the behavior of the
transformation around these fixed points and the domain of attraction for
different values of the scaling parameter. The physical interest of a
renormalization group approach to the generalized central limit theorem is
discussed.Comment: 16 pages, to appear in J. Stat. Phy
Asymmetric Landau-Zener tunneling in a periodic potential
Using a simple model for nonlinear Landau-Zener tunneling between two energy
bands of a Bose-Einstein condensate in a periodic potential, we find that the
tunneling rates for the two directions of tunneling are not the same. Tunneling
from the ground state to the excited state is enhanced by the nonlinearity,
whereas in the opposite direction it is suppressed. These findings are
confirmed by numerical simulations of the condensate dynamics. Measuring the
tunneling rates for a condensate of rubidium atoms in an optical lattice, we
have found experimental evidence for this asymmetry.Comment: 5 pages, 3 figure
Spatio-temporal modelling of COVID-19 incident cases using Richards’ curve: An application to the Italian regions
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