602 research outputs found
Creation of effective magnetic fields in optical lattices: The Hofstadter butterfly for cold neutral atoms
We investigate the dynamics of neutral atoms in a 2D optical lattice which
traps two distinct internal states of the atoms in different columns. Two Raman
lasers are used to coherently transfer atoms from one internal state to the
other, thereby causing hopping between the different columns. By adjusting the
laser parameters appropriately we can induce a non vanishing phase of particles
moving along a closed path on the lattice. This phase is proportional to the
enclosed area and we thus simulate a magnetic flux through the lattice. This
setup is described by a Hamiltonian identical to the one for electrons on a
lattice subject to a magnetic field and thus allows us to study this equivalent
situation under very well defined controllable conditions. We consider the
limiting case of huge magnetic fields -- which is not experimentally accessible
for electrons in metals -- where a fractal band structure, the Hofstadter
butterfly, characterizes the system.Comment: 6 pages, RevTe
Finite Temperature Phase Diagram in Rotating Bosonic Optical Lattice
Finite temperature phase boundary between superfluid phase and normal state
is analytically derived by studying the stability of normal state in rotating
bosonic optical lattice. We also prove that the oscillation behavior of
critical hopping matrix directly follows the upper boundary of Hofstadter
butterfly as the function of effective magnetic field.Comment: 10 pages, 2 figure
Rotating states for trapped bosons in an optical lattice
Rotational states for trapped bosons in an optical lattice are studied in the
framework of the Hubbard model. Critical frequencies are calculated and the
main parameter regimes are identified. Transitions are observed from edge
superfluids to vortex lattices with Mott insulating cores, and subsequently to
lattices of interstitial vortices. The former transition coincides with the
Mott transition. Changes in symmetry of the vortex lattices are observed as a
function of lattice depth. Predictions for experimental signatures are
presented.Comment: 6 pages, 6 figures, accepted for publication in EP
Controlling two-species Mott-insulator phses in an optical lattice to form an array of dipolar molecules
We consider the transfer of a two-species Bose-Einstein condensate into an
optical lattice with a density such that that a Mott-insulator state with one
atom per species per lattice site is obtained in the deep lattice regime.
Depending on collision parameters the result could be either a `mixed' or a
`separated' Mott-insulator phase. Such a `mixed' two-species insulator could
then be photo-associated into an array of dipolar molecules suitable for
quantum computation or the formation of a dipolar molecular condensate. For the
case of a Rb-K two-species BEC, however, the large inter-species
scattering length makes obtaining the desired `mixed' Mott insulator phase
difficult. To overcome this difficulty we investigate the effect of varying the
lattice frequency on the mean-field interaction and find a favorable parameter
regime under which a lattice of dipolar molecules could be generated
Quantum phases of electric dipole ensembles in atom chips
We present how a phase factor is generated when an electric dipole moves
along a closed trajectory inside a magnetic field gradient. The similarity of
this situation with charged particles in a magnetic field can be employed to
simulate condensed matter models, such as the quantum Hall effect and chiral
spin Hamiltonians, with ultra cold atoms integrated on atom chips. To
illustrate this we consider a triangular configuration of a two dimensional
optical lattice, where the chiral spin Hamiltonian can be generated between any three
neighbours on a lattice yielding an experimentally implementable chiral ground
state.Comment: 4 pages, 2 figures, REVTEX. Title slightly changed and conclusions
extende
Correlated motion of two atoms trapped in a single mode cavity field
We study the motion of two atoms trapped at distant positions in the field of
a driven standing wave high-Q optical resonator. Even without any direct
atom-atom interaction the atoms are coupled through their position dependent
influence on the intracavity field. For sufficiently good trapping and low
cavity losses the atomic motion becomes significantly correlated and the two
particles oscillate in their wells preferentially with a 90 degrees relative
phase shift. The onset of correlations seriously limits cavity cooling
efficiency, raising the achievable temperature to the Doppler limit. The
physical origin of the correlation can be traced back to a cavity mediated
cross-friction, i.e. a friction force on one particle depending on the velocity
of the second particle. Choosing appropriate operating conditions allows for
engineering these long range correlations. In addition this cross-friction
effect can provide a basis for sympathetic cooling of distant trapped clouds.Comment: 10 pages, 9 figures, accepted for publication in Phys. Rev. A. Minor
grammatical changes to previous versio
Testing quantum nonlocality by generalized quasiprobability functions
We derive a Bell inequality based on a generalized quasiprobability function
which is parameterized by one non-positive real value. Two types of known Bell
inequalities formulated in terms of the Wigner and Q functions are included as
limiting cases. We investigate violations of our Bell inequalities for single
photon entangled states and two-mode squeezed vacuum states when varying the
detector efficiency. We show that the Bell inequality for the Q function allows
the lowest detection efficiency for violations of local realism.Comment: 6 pages, 3 figure
Superfluid-Insulator transition of ultracold atoms in an optical lattice in the presence of a synthetic magnetic field
We study the Mott insulator-superfluid transition of ultracold bosonic atoms
in a two-dimensional square optical lattice in the presence of a synthetic
magnetic field with p/q (p and q being co-prime integers) flux quanta passing
through each lattice plaquette. We show that on approach to the transition from
the Mott side, the momentum distribution of the bosons exhibits q precursor
peaks within the first magnetic Brillouin zone. We also provide an effective
theory for the transition and show that it involves q interacting boson fields.
We construct, from a mean-field analysis of this effective theory, the
superfluid ground states near the transition and compute, for q=2,3, both the
gapped and the gapless collective modes of these states. We suggest experiments
to test our theory.Comment: 4 pages, 4 figs; v
Quantum Logic for Trapped Atoms via Molecular Hyperfine Interactions
We study the deterministic entanglement of a pair of neutral atoms trapped in
an optical lattice by coupling to excited-state molecular hyperfine potentials.
Information can be encoded in the ground-state hyperfine levels and processed
by bringing atoms together pair-wise to perform quantum logical operations
through induced electric dipole-dipole interactions. The possibility of
executing both diagonal and exchange type entangling gates is demonstrated for
two three-level atoms and a figure of merit is derived for the fidelity of
entanglement. The fidelity for executing a CPHASE gate is calculated for two
87Rb atoms, including hyperfine structure and finite atomic localization. The
main source of decoherence is spontaneous emission, which can be minimized for
interaction times fast compared to the scattering rate and for sufficiently
separated atomic wavepackets. Additionally, coherent couplings to states
outside the logical basis can be constrained by the state dependent trapping
potential.Comment: Submitted to Physical Review
Two-component Bose gas in an optical lattice at single-particle filling
The Bose-Hubbard model of a two-fold degenerate Bose gas is studied in an
optical lattice with one particle per site and virtual tunneling to empty and
doubly-occupied sites. An effective Hamiltonian for this system is derived
within a continued-fraction approach. The ground state of the effective model
is studied in mean-field approximation for a modulated optical lattice. A
dimerized mean-field state gives a Mott insulator whereas the lattice without
modulations develops long-range correlated phase fluctuations due to a
Goldstone mode. This result is discussed in comparison with the superfluid and
the Mott-insulating state of a single-component hard-core Bose.Comment: 11 page
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