370 research outputs found
Resonant control of spin dynamics in ultracold quantum gases by microwave dressing
We study experimentally interaction-driven spin oscillations in optical
lattices in the presence of an off-resonant microwave field. We show that the
energy shift induced by this microwave field can be used to control the spin
oscillations by tuning the system either into resonance to achieve near-unity
contrast or far away from resonance to suppress the oscillations. Finally, we
propose a scheme based on this technique to create a flat sample with either
singly- or doubly-occupied sites, starting from an inhomogeneous Mott
insulator, where singly- and doubly-occupied sites coexist.Comment: 4 pages, 5 figure
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
Measurement of the thorium-228 activity in solutions cavitated by ultrasonic sound
We show that cavitation of a solution of thorium-228 in water does not induce
its transformation at a faster rate than the natural radioactive decay. We
measured the activity of a thorium-228 solution in water before, and after, it
was subjected to a cavitation at 44 kHz and W for 90 minutes in order to
observe any change in the thorium half-life. The results were compared to the
original activity of the sample and we observed no change. Our results and
conclusions conflict with those in a recent paper by F. Cardone et. al. [Phys.
Lett. A 373 (2009) 1956-1958].Comment: 6 pages, 1 figure, 2 tables, v1 submitted to Physics Letters A. v2:
minor corrections, change caption for tables (include comment for counter
efficiency with uncertainty) and symbols for beta-alph
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
Topological superfluids on a lattice with non-Abelian gauge fields
Two-component fermionic superfluids on a lattice with an external non-Abelian
gauge field give access to a variety of topological phases in presence of a
sufficiently large spin imbalance. We address here the important issue of
superfluidity breakdown induced by spin imbalance by a self-consistent
calculation of the pairing gap, showing which of the predicted phases will be
experimentally accessible. We present the full topological phase diagram, and
we analyze the connection between Chern numbers and the existence of
topologically protected and non-protected edge modes. The Chern numbers are
calculated via a very efficient and simple method.Comment: 6 pages, 5 figures to be published in Europhysics Letter
Low energy neutron propagation in MCNPX and GEANT4
Simulations of neutron background from rock for underground experiments are
presented. Neutron propagation through two types of rock, lead and hydrocarbon
material is discussed. The results show a reasonably good agreement between
GEANT4, MCNPX and GEANT3 in transporting low-energy neutrons.Comment: 9 Figure
Flux lattices reformulated
We theoretically explore the optical flux lattices produced for ultra-cold
atoms subject to laser fields where both the atom-light coupling and the
effective detuning are spatially periodic. We analyze the geometric vector
potential and the magnetic flux it generates, as well as the accompanying
geometric scalar potential. We show how to understand the gauge-dependent
Aharonov-Bohm singularities in the vector potential, and calculate the
continuous magnetic flux through the elementary cell in terms of these
singularities. The analysis is illustrated with a square optical flux lattice.
We conclude with an explicit laser configuration yielding such a lattice using
a set of five properly chosen beams with two counterpropagating pairs (one
along the x axes and the other y axes), together with a single beam along the z
axis. We show that this lattice is not phase-stable, and identify the one
phase-difference that affects the magnetic flux. Thus armed with realistic
laser setup, we directly compute the Chern number of the lowest Bloch band to
identify the region where the non- zero magnetic flux produces a topologically
non-trivial band structure.Comment: 22 pages, 7 figure
Geometric Phases generated by the non-trivial spatial topology of static vector fields coupled to a neutral spin-endowed particle. Application to 171Yb atoms trapped in a 2D optical lattice
We have constructed the geometric phases emerging from the non-trivial
topology of a space-dependent magnetic field, interacting with the spin
magnetic moment of a neutral particle. Our basic tool is the local unitary
transformation which recasts the magnetic spin interaction under a diagonal
form. Rewriting the kinetic term in the "rotated" frame requires the
introduction of non-Abelian covariant derivatives, involving the gradients of
the Euler angles which define the orientation of the local field. Within the
rotated frame, we have built a perturbation scheme,assuming that the
longitudinal non-Abelian field component dominates the transverse ones, to be
evaluated to second-order. The geometry embedded in the longitudinal gauge
vector field and its curl, the geometric magnetic field, is described by the
associated Aharonov-Bohm phase. As an illustration, we study the physics of
cold 171Yb atoms dressed by two sets of circularly polarized beams, forming
square or triangular 2D optical lattices. The geometric field is computed
explicitly from the Euler angles. The magnitude of 2nd-order corrections due to
transverse fields can be reduced to the percent level by a choice of light
intensity which keeps the dressed atom loss rate below 5 s^{-1}. An auxiliary
optical lattice confines the atoms within 2D domains where the geometric field
is pointing upward.Comment: 12 pages, 4 figures. Comments and one figure added about the effect
of the additional scalar potential (sec. V.B). To be published in J. Phys.
A:Math. Theo
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