124 research outputs found
Resonantly enhanced tunneling of Bose-Einstein condensates in periodic potentials
We report on measurements of resonantly enhanced tunneling of Bose-Einstein
condensates loaded into an optical lattice. By controlling the initial
conditions of our system we were able to observe resonant tunneling in the
ground and the first two excited states of the lattice wells. We also
investigated the effect of the intrinsic nonlinearity of the condensate on the
tunneling resonances.Comment: accepted for publication in Phys. Rev. Letter
Dynamical control of matter-wave tunneling in periodic potentials
We report on measurements of dynamical suppression of inter-well tunneling of
a Bose-Einstein condensate (BEC) in a strongly driven optical lattice. The
strong driving is a sinusoidal shaking of the lattice corresponding to a
time-varying linear potential, and the tunneling is measured by letting the BEC
freely expand in the lattice. The measured tunneling rate is reduced and, for
certain values of the shaking parameter, completely suppressed. Our results are
in excellent agreement with theoretical predictions. Furthermore, we have
verified that in general the strong shaking does not destroy the phase
coherence of the BEC, opening up the possibility of realizing quantum phase
transitions by using the shaking strength as the control parameter.Comment: 5 pages, 3 figure
Observation of photon-assisted tunneling in optical lattices
We have observed tunneling suppression and photon-assisted tunneling of
Bose-Einstein condensates in an optical lattice subjected to a constant force
plus a sinusoidal shaking. For a sufficiently large constant force, the ground
energy levels of the lattice are shifted out of resonance and tunneling is
suppressed; when the shaking is switched on, the levels are coupled by
low-frequency photons and tunneling resumes. Our results agree well with
theoretical predictions and demonstrate the usefulness of optical lattices for
studying solid-state phenomena.Comment: 5 pages, 3 figure
AC-induced superfluidity
We argue that a system of ultracold bosonic atoms in a tilted optical lattice
can become superfluid in response to resonant AC forcing. Among others, this
allows one to prepare a Bose-Einstein condensate in a state associated with a
negative effective mass. Our reasoning is backed by both exact numerical
simulations for systems consisting of few particles, and by a theoretical
approach based on Floquet-Fock states.Comment: Accepted for publication in Europhysics letters, 6 pages, 4 figures,
Changes in v2: reference 7 replaced by a more recent on
Trap modulation spectroscopy of the Mott-insulator transition in optical lattices
We introduce a new technique to probe the properties of an interacting cold
atomic gas that can be viewed as a dynamical compressibility measurement. We
apply this technique to the study of the superfluid to Mott insulator quantum
phase transition in one and three dimensions for a bosonic gas trapped in an
optical lattice. Excitations of the system are detected by time-of-flight
measurements. The experimental data for the one-dimensional case are in good
agreement with the results of a time-dependent density matrix renormalization
group calculation.Comment: 5 pages, 5 figure
Quasienergy spectra of a charged particle in planar honeycomb lattices
The low energy spectrum of a particle in planar honeycomb lattices is
conical, which leads to the unusual electronic properties of graphene. In this
letter we calculate the quasienergy spectra of a charged particle in honeycomb
lattices driven by a strong AC field, which is of fundamental importance for
its time-dependent dynamics. We find that depending on the amplitude, direction
and frequency of external field, many interesting phenomena may occur,
including band collapse, renormalization of velocity of ``light'', gap opening
etc.. Under suitable conditions, with increasing the magnitude of the AC field,
a series of phase transitions from gapless phases to gapped phases appear
alternatively. At the same time, the Dirac points may disappear or change to a
line. We suggest possible realization of the system in Honeycomb optical
lattices.Comment: 4+ pages, 5 figure
Tunneling control and localization for Bose-Einstein condensates in a frequency modulated optical lattice
The similarity between matter waves in periodic potential and solid-state
physics processes has triggered the interest in quantum simulation using
Bose-Fermi ultracold gases in optical lattices. The present work evidences the
similarity between electrons moving under the application of oscillating
electromagnetic fields and matter waves experiencing an optical lattice
modulated by a frequency difference, equivalent to a spatially shaken periodic
potential. We demonstrate that the tunneling properties of a Bose-Einstein
condensate in shaken periodic potentials can be precisely controlled. We take
additional crucial steps towards future applications of this method by proving
that the strong shaking of the optical lattice preserves the coherence of the
matter wavefunction and that the shaking parameters can be changed
adiabatically, even in the presence of interactions. We induce reversibly the
quantum phase transition to the Mott insulator in a driven periodic potential.Comment: Laser Physics (in press
Wave Function Renormalization Effects in Resonantly Enhanced Tunneling
We study the time evolution of ultra-cold atoms in an accelerated optical
lattice. For a Bose- Einstein condensate with a narrow quasi-momentum
distribution in a shallow optical lattice the decay of the survival probability
in the ground band has a step-like structure. In this regime we establish a
connection between the wave function renormalization parameter Z introduced in
[Phys. Rev. Lett. 86, 2699 (2001)] to characterize non-exponential decay and
the phenomenon of resonantly enhanced tunneling, where the decay rate is peaked
for particular values of the lattice depth and the accelerating force.Comment: 12 page
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