12,520 research outputs found
Coherence and Josephson oscillations between two tunnel-coupled one-dimensional atomic quasicondensates at finite temperature
We revisit the theory of tunnel-coupled atomic quasicondensates in
double-well elongated traps at finite temperatures. Using the
functional-integral approach, we calculate the relative-phase correlation
function beyond the harmonic limit of small fluctuations of the relative phase
and its conjugate relative-density variable. We show that the thermal
fluctuations of the relative phase between the two quasicondensates decrease
the frequency of Josephson oscillations and even wash out these oscillations
for small values of the tunnel coupling.Comment: revtex4, 4 figures (.eps
Single-atom quantum memory with degenerate atomic levels
The storage and retrieval of a single-photon polarization q-bit by means of
STIRAP through the atoms with degenerate levels is studied theoretically for
arbitrary polarization of the driving laser field and arbitrary values of the
angular momenta of resonant atomic levels. The dependence of the probability of
long-term photon storage on the polarization of the driving field and on the
initial atomic state is examined.Comment: 12 pages, 1 figure. arXiv admin note: text overlap with
arXiv:1203.439
Fermion Pair Production From an Electric Field Varying in Two Dimensions
The Hamiltonian describing fermion pair production from an arbitrarily
time-varying electric field in two dimensions is studied using a
group-theoretic approach. We show that this Hamiltonian can be encompassed by
two, commuting SU(2) algebras, and that the two-dimensional problem can
therefore be reduced to two one-dimensional problems. We compare the group
structure for the two-dimensional problem with that previously derived for the
one-dimensional problem, and verify that the Schwinger result is obtained under
the appropriate conditions.Comment: Latex, 14 pages of text. Full postscript version available via the
worldwide web at http://nucth.physics.wisc.edu/ or by anonymous ftp from
ftp://nucth.physics.wisc.edu:/pub/preprints
Secularly growing loop corrections in strong electric fields
We calculate one--loop corrections to the vertexes and propagators of photons
and charged particles in the strong electric field backgrounds. We use the
Schwinger--Keldysh diagrammatic technique. We observe that photon's Keldysh
propagator receives growing with time infrared contribution. As the result,
loop corrections are not suppressed in comparison with tree--level
contribution. This effect substantially changes the standard picture of the
pair production. To sum up leading IR corrections from all loops we consider
the infrared limit of the Dyson--Schwinger equations and reduce them to a
single kinetic equation.Comment: 16 pages, no figures; Minor correction
Vortex mass in a superfluid at low frequencies
An inertial mass of a vortex can be calculated by driving it round in a
circle with a steadily revolving pinning potential. We show that in the low
frequency limit this gives precisely the same formula that was used by Baym and
Chandler, but find that the result is not unique and depends on the force field
used to cause the acceleration. We apply this method to the Gross-Pitaevskii
model, and derive a simple formula for the vortex mass. We study both the long
range and short range properties of the solution. We agree with earlier results
that the non-zero compressibility leads to a divergent mass. From the
short-range behavior of the solution we find that the mass is sensitive to the
form of the pinning potential, and diverges logarithmically when the radius of
this potential tends to zero.Comment: 4 page
Ground state energy of a dilute two-dimensional Bose gas from the Bogoliubov free energy functional
We extend the analysis of the Bogoliubov free energy functional to two
dimensions at very low temperatures. For sufficiently weak interactions, we
prove two term asymptotics for the ground state energy.Comment: revised versio
Berezinskii-Kosterlitz-Thouless transition in two-dimensional dipole systems
The superfluid to normal fluid transition of dipolar bosons in two dimensions
is studied throughout the whole density range using path integral Monte Carlo
simulations and summarized in the phase diagram. While at low densities, we
find good agreement with the universal results depending only on the scattering
length , at moderate and high densities, the transition temperature is
strongly affected by interactions and the elementary excitation spectrum. The
results are expected to be of relevance to dipolar atomic and molecular systems
and indirect excitons in quantum wells
A Gapless Theory of Bose-Einstein Condensation in Dilute Gases at Finite Temperature
In this paper we develop a gapless theory of BEC which can be applied to both
trapped and homogeneous gases at zero and finite temperature. The many-body
Hamiltonian for the system is written in a form which is approximately
quadratic with higher order cubic and quartic terms. The quadratic part is
diagonalized exactly by transforming to a quasiparticle basis, while the
non-quadratic terms are dealt with using first and second order perturbation
theory. The conventional treatment of these terms, based on factorization
approximations, is shown to be inconsistent.
Infra-red divergences can appear in individual terms of the perturbation
expansion, but we show analytically that the total contribution beyond
quadratic order is finite. The resulting excitation spectrum is gapless and the
energy shifts are small for a dilute gas away from the critical region,
justifying the use of perturbation theory. Ultra-violet divergences can appear
if a contact potential is used to describe particle interactions. We show that
the use of this potential as an approximation to the two-body T-matrix leads
naturally to a high-energy renormalization.
The theory developed in this paper is therefore well-defined at both low and
high energy and provides a systematic description of Bose-Einstein condensation
in dilute gases. It can therefore be used to calculate the energies and decay
rates of the excitations of the system at temperatures approaching the phase
transition.Comment: 39 pages of Revtex. 1 figur
The Zel'dovich effect and evolution of atomic Rydberg spectra along the Periodic Table
In 1959 Ya. B. Zel'dovich predicted that the bound-state spectrum of the
non-relativistic Coulomb problem distorted at small distances by a short-range
potential undergoes a peculiar reconstruction whenever this potential alone
supports a low-energy scattering resonance. However documented experimental
evidence of this effect has been lacking. Previous theoretical studies of this
phenomenon were confined to the regime where the range of the short-ranged
potential is much smaller than Bohr's radius of the Coulomb field. We go beyond
this limitation by restricting ourselves to highly-excited s states. This
allows us to demonstrate that along the Periodic Table of elements the
Zel'dovich effect manifests itself as systematic periodic variation of the
Rydberg spectra with a period proportional to the cubic root of the atomic
number. This dependence, which is supported by analysis of experimental and
numerical data, has its origin in the binding properties of the ionic core of
the atom.Comment: 17 pages, 12 figure
Statistical properties of one dimensional Bose gas
Monte Carlo method within, so called, classical fields approximation is
applied to one dimensional weakly interacting repulsive Bose gas trapped in a
harmonic potential. Equilibrium statistical properties of the condensate are
calculated within a canonical ensemble. We also calculate experimentally
relevant low order correlation functions of the whole gas
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