54 research outputs found

### Interference of atomic levels and superfluid -- Mott insulator phase transition in a two-component Bose-Einstein condensate

The superfluid -- Mott insulator phase transition in a Bose-Einstein
condensate of neutral atoms with doubly degenerate internal ground states in an
optical lattice is theoretically investigated. The optical lattice is created
by two counterpropagating linearly polarized laser beams with the angle
$\theta$ between the polarization vectors (lin-angle-lin configuration). The
phase diagram of the system and the critical values of the parameters are
worked out. It is shown that the sign of the detuning plays an important role
and that there is a strong suppression of the Mott transition in the case of
blue detuning. Varying the laser intensity and/or the angle $\theta$ one can
manipulate the Mott-insulator to superfluid quantum phase transition as well as
prepare the condensate in physically distinguishable "ferromagnetic" and
"antiferromagnetic" superfluid states

### Spin-1 bosons with coupled ground states in optical lattices

The superfluid--Mott-insulator phase transition of ultracold spin-1 bosons
with ferromagnetic and antiferromagnetic interactions in an optical lattice is
theoretically investigated. Two counterpropagating linearly polarized laser
beams with the angle $\theta$ between the polarization vectors
(lin-$\theta$-lin configuration), driving an $F_g=1$ to $F_e=1$ internal atomic
transition, create the optical lattice and at the same time couple atomic
ground states with magnetic quantum numbers $m=\pm 1$. Due to the coupling the
system can be described as a two-component one. At $\theta=0$ the system has a
continuous isospin symmetry, which can be spontaneously broken, thereby fixing
the number of particles in the atomic components. The phase diagram of the
system and the spectrum of collective excitations, which are density waves and
isospin waves, are worked out. In the case of ferromagnetic interactions, the
superfluid--Mott-insulator phase transition is always second order, but in the
case of antiferromagnetic interactions for some values of system parameters it
is first order and the superfluid and Mott phases can coexist. Varying the
angle $\theta$ one can control the populations of atomic components and
continuously turn on and tune their asymmetry

### Ultracold Bose atoms in intense laser fields: intensity- and density-dependent effects

Starting from the first principles of nonrelativistic QED we have derived the
system of Maxwell-Schr\"odinger equations, which can be used for theoretical
description of atom optical phenomena at high densities of atoms and high
intensities of the laser radiation. The role of multiple atomic transitions
between ground and excited states in atom optics has been investigated.
Nonlinear optical properties of interacting Bose gas are studied: formula for
the refractive index has been derived and the polariton spectrum of a
condensate interacting with an intense laser field has been investigated.Comment: 13 pages, LaTeX, Special issue on Bose-Einstein condensation of
trapped atom

### Dark solitons near the Mott-insulator--superfluid phase transition

Dark solitons of ultracold bosons in the vicinity of the
Mott-insulator--superfluid phase transition are studied. Making use of the
Gutzwiller ansatz we have found antisymmetric eigenstates corresponding to
standing solitons, as well as propagating solitons created by phase imprinting.
Near the phase boundary, superfluidity has either a particle or a hole
character depending on the system parameters, which greatly affects the
characteristics of both types of solitons. Within the insulating Mott regions,
soliton solutions are prohibited by lack of phase coherence between the lattice
sites. Linear and modulational stability show that the soliton solutions are
sensitive to small perturbations and, therefore, unstable. In general, their
lifetimes differ for on-site and off-site modes. For the on-site modes, there
are small areas between the Mott-insulator regions where the lifetime is very
large, and in particular much larger than that for the off-site modes.Comment: 10 pages, 12 figure

### Ultracold bosons with short-range interaction in regular optical lattices

During the last decade, many exciting phenomena have been experimentally
observed and theoretically predicted for ultracold atoms in optical lattices.
This paper reviews these rapid developments concentrating mainly on the theory.
Different types of the bosonic systems in homogeneous lattices of different
dimensions as well as in the presence of harmonic traps are considered. An
overview of the theoretical methods used for these investigations as well as of
the obtained results is given. Available experimental techniques are presented
and discussed in connection with theoretical considerations. Eigenstates of the
interacting bosons in homogeneous lattices and in the presence of harmonic
confinement are analysed. Their knowledge is essential for understanding of
quantum phase transitions at zero and finite temperature

### Surface effects influencing the single-atom spontaneous emission in a linear atomic chain

As a contribution to quantum optics in the vicinity of surfaces we study the single atom spontaneous emission in a linear chain of two-level atoms. The electromagnetic field is thereby treated with the help of integro-differential equations which take into account the interaction with the other atoms in the chain. The life time of the excited atom, the frequency shift of the atomic transition and the angular distribution of emitted photons are worked out. They depend on the position of the emitting atom. As compared with the single atom in free space, considerable modifications occur for atoms a few interatomic distances away from the ends of the chain

### Microscopic theory of the interaction of ultracold dense Bose and Fermi gases with electromagnetic field

We present the rigorous microscopic quantum theory of the interaction of
ultracold Bose and Fermi gases with the electromagnetic field of vacuum and
laser photons. The main attention has been paid to the consistent consideration
of dynamical dipole-dipole interactions. The theory developed is shown to be
consistent with the general principles of the canonical quantization of
electromagnetic field in a medium. Starting from the first principles of QED we
have derived the general system of Maxwell-Bloch equations for atomic creation
and annihilation operators and the propagation equation for the laser field
which can be used for the self-consistent analysis of various linear and
nonlinear phenomena in atom optics at high densities of the atomic system. All
known equations which are used for the description of the behaviour of an
ultracold atomic ensemble in a radiation field can be obtained from our general
system of equations in a low-density limit.Comment: 6 pages, RevTeX, invited talk at the 8TH International Workshop on
Laser Physic

### Local-field effect in atom optics of two-component Bose-Einstein condensates

Starting from the first principles of nonrelativistic QED we have developed
the quantum theory of the interaction of a two-component ultracold atomic
ensemble with the electromagnetic field of vacuum and laser photons. The main
attention has been paid to the consistent consideration of dynamical
dipole-dipole interactions in the radiation field. Taking into account
local-field effects we have derived the system of Maxwell-Bloch equations.
Optical properties of the two-component Bose gas are investigated. It is shown
that the refractive index of the gas is given by the Maxwell-Garnett formula.
All equations which are used up to now for the description of the behavior of
an ultracold atomic ensemble in a radiation field can be obtained from our
general system of equations in the low-density limit. Raman-Nath diffraction of
the two-component atomic beam is investigated on the basis of our general
system of equations.Comment: 12 pages, LaTeX, talk at the 9th International Workshop on Laser
Physic

### Phase diagram of quasi-two-dimensional bosons in laser speckle potential

We have studied the phase diagram of a quasi-two-dimensional interacting Bose
gas at zero temperature in the presence of random potential created by laser
speckles. The superfluid fraction and the fraction of particles with zero
momentum are obtained within the mean-field Gross-Pitaevskii theory and in
diffusion Monte Carlo simulations. We find a transition from the superfluid to
the insulating state, when the strength of the disorder grows. Estimations of
the critical parameters are compared with the predictions of the percolation
theory in the Thomas-Fermi approximation. Analytical expressions for the
zero-momentum fraction and the superfluid fraction are derived in the limit of
weak disorder and weak interactions within the framework of the Bogoliubov
theory. Limits of validity of various approximations are discussed.Comment: 5 pages, 4 figures; v2 - published versio

### Local-field approach to the interaction of an ultracold dense Bose gas with a light field

The propagation of the electromagnetic field of a laser through a dense Bose
gas is examined and nonlinear operator equations for the motion of the center
of mass of the atoms are derived. The goal is to present a self-consistent set
of coupled Maxwell-Bloch equations for atomic and electromagnetic fields
generalized to include the atomic center-of-mass motion. Two effects are
considered: The ultracold gas forms a medium for the Maxwell field which
modifies its propagation properties. Combined herewith is the influence of the
dipole-dipole interaction between atoms which leads to a density dependent
shift of the atomic transition frequency. It is expressed in a position
dependent detuning and is the reason for the nonlinearity. This results in a
direct and physically transparent way from the quantum field theoretical
version of the local-field approach to electrodynamics in quantum media. The
equations for the matter fields are general. Previously published nonlinear
equations are obtained as limiting cases. As an atom optical application the
scattering of a dense beam of a Bose gas is studied in the Raman-Nath regime.
The main conclusion is that for increasing density of the gas the dipole-dipole
interaction suppresses or enhances the scattering depending on the sign of the
detuning.Comment: 19 pages, RevTeX, to be published in the Physical Review

- â€¦