505 research outputs found
Clock spectroscopy of interacting bosons in deep optical lattices
We report on high-resolution optical spectroscopy of interacting bosonic
Yb atoms in deep optical lattices with negligible tunneling. We prepare
Mott insulator phases with singly- and doubly-occupied isolated sites and probe
the atoms using an ultra-narrow "clock" transition. Atoms in singly-occupied
sites undergo long-lived Rabi oscillations. Atoms in doubly-occupied sites are
strongly affected by interatomic interactions, and we measure their inelastic
decay rates and energy shifts. We deduce from these measurements all relevant
collisional parameters involving both clock states, in particular the intra-
and inter-state scattering lengths
Non-linear Relaxation of Interacting Bosons Coherently Driven on a Narrow Optical Transition
We study the dynamics of a two-component Bose-Einstein condensate (BEC) of
Yb atoms coherently driven on a narrow optical transition. The
excitation transfers the BEC to a superposition of states with different
internal and momentum quantum numbers. We observe a crossover with decreasing
driving strength between a regime of damped oscillations, where coherent
driving prevails, and an incoherent regime, where relaxation takes over.
Several relaxation mechanisms are involved: inelastic losses involving two
excited atoms, leading to a non-exponential decay of populations; Doppler
broadening due to the finite momentum width of the BEC and inhomogeneous
elastic interactions, both leading to dephasing and to damping of the
oscillations. We compare our observations to a two-component Gross-Pitaevskii
(GP) model that fully includes these effects. For small or moderate densities,
the damping of the oscillations is mostly due to Doppler broadening. In this
regime, we find excellent agreement between the model and the experimental
results. For higher densities, the role of interactions increases and so does
the damping rate of the oscillations. The damping in the GP model is less
pronounced than in the experiment, possibly a hint for many-body effects not
captured by the mean-field description.Comment: 7 pages, 4 figures; supplementary material available as ancillary
fil
Momentum spectroscopy of 1D phase fluctuations in Bose-Einstein condensates
We measure the axial momentum distribution of Bose-Einstein condensates with
an aspect ratio of 152 using Bragg spectroscopy. We observe the Lorentzian
momentum distribution characteristic of one-dimensional phase fluctuations. The
temperature dependence of the width of this distribution provides a
quantitative test of quasi-condensate theory. In addition, we observe a
condensate length consistent with the absence of density fluctuations, even
when phase fluctuations are large.Comment: 4 pages, 3 figures; submitted to Phys. Rev. Let
Correlated directional atomic clouds via four-heterowave mixing
We investigate the coherence properties of pairs of counter-propagating
atomic clouds, produced in superradiant Rayleigh scattering off atomic
condensates. It is shown that these clouds exhibit long-range spatial coherence
and strong nonclassical density cross-correlations, which make this scheme a
promising candidate for the production of highly directional nonclassically
correlated atomic pulses.Comment: 12 pages, 3 figure
Measuring topology in a laser-coupled honeycomb lattice: From Chern insulators to topological semi-metals
Ultracold fermions trapped in a honeycomb optical lattice constitute a
versatile setup to experimentally realize the Haldane model [Phys. Rev. Lett.
61, 2015 (1988)]. In this system, a non-uniform synthetic magnetic flux can be
engineered through laser-induced methods, explicitly breaking time-reversal
symmetry. This potentially opens a bulk gap in the energy spectrum, which is
associated with a non-trivial topological order, i.e., a non-zero Chern number.
In this work, we consider the possibility of producing and identifying such a
robust Chern insulator in the laser-coupled honeycomb lattice. We explore a
large parameter space spanned by experimentally controllable parameters and
obtain a variety of phase diagrams, clearly identifying the accessible
topologically non-trivial regimes. We discuss the signatures of Chern
insulators in cold-atom systems, considering available detection methods. We
also highlight the existence of topological semi-metals in this system, which
are gapless phases characterized by non-zero winding numbers, not present in
Haldane's original model.Comment: 30 pages, 12 figures, 4 Appendice
Adiabatic loading of a Bose-Einstein condensate in a 3D optical lattice
We experimentally investigate the adiabatic loading of a Bose-Einstein
condensate into an optical lattice potential. The generation of excitations
during the ramp is detected by a corresponding decrease in the visibility of
the interference pattern observed after free expansion of the cloud. We focus
on the superfluid regime, where we show that the limiting time scale is related
to the redistribution of atoms across the lattice by single-particle tunneling
Momentum Spectroscopy of Phase Fluctuations of an Elongated Bose-Einstein Condensate
We have measured the momentum distribution of an elongated BEC (aspect ratio
of 152), for temperatures below the critical temperature. The corresponding
coherence length is significantly smaller than the condensate length in a wide
range of temperature, in quantitative agreement with theoretical predictions.
The Lorentzian shape of the momentum spectrum supports the image of a phase
fluctuating quasicondensate.Comment: Proceedings of the International Conference on Laser Spectroscopy
(ICOLS 03), Cairns, Australia, july 200
The equation of state of ultracold Bose and Fermi gases: a few examples
We describe a powerful method for determining the equation of state of an
ultracold gas from in situ images. The method provides a measurement of the
local pressure of an harmonically trapped gas and we give several applications
to Bose and Fermi gases. We obtain the grand-canonical equation of state of a
spin-balanced Fermi gas with resonant interactions as a function of
temperature. We compare our equation of state with an equation of state
measured by the Tokyo group, that reveals a significant difference in the
high-temperature regime. The normal phase, at low temperature, is well
described by a Landau Fermi liquid model, and we observe a clear thermodynamic
signature of the superfluid transition. In a second part we apply the same
procedure to Bose gases. From a single image of a quasi ideal Bose gas we
determine the equation of state from the classical to the condensed regime.
Finally the method is applied to a Bose gas in a 3D optical lattice in the Mott
insulator regime. Our equation of state directly reveals the Mott insulator
behavior and is suited to investigate finite-temperature effects.Comment: 14 pages, 6 figure
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