674 research outputs found
Periodically-driven quantum systems: Effective Hamiltonians and engineered gauge fields
Driving a quantum system periodically in time can profoundly alter its
long-time dynamics and trigger topological order. Such schemes are particularly
promising for generating non-trivial energy bands and gauge structures in
quantum-matter systems. Here, we develop a general formalism that captures the
essential features ruling the dynamics: the effective Hamiltonian, but also the
effects related to the initial phase of the modulation and the micro-motion.
This framework allows for the identification of driving schemes, based on
general N-step modulations, which lead to configurations relevant for quantum
simulation. In particular, we explore methods to generate synthetic spin-orbit
couplings and magnetic fields in cold-atom setups.Comment: 25 pages, 6 figures, includes Appendices (A-K). An erroneous factor
of two has been corrected in the last term of Eq. C10 (Appendix C); this typo
had no impact on the rest of the articl
All-optical production of 7Li Bose-Einstein condensation using Feshbach resonances
We show an all-optical method of making 7Li condensate using tunability of
the scattering length in the proximity of a Feshbach resonance. We report the
observation of two new Feshbach resonances on |F = 1;mF = 0> state. The narrow
(broad) resonance of 7 G (34 G) width is detected at 831 +- 4 G (884 +4 -13 G).
Position of the scattering length zero crossing between the resonances is found
at 836 +- 4 G. The broad resonance is shown to be favorable for run away
evaporation which we perform in a crossed-beam optical dipole trap. Starting
directly form the phase space density of a magneto-optical trap we observe a
Bose-Einstein condensation threshold in less than 3 s of forced evaporation.Comment: 5 pages, 5 figure
Spontaneous excitation of an accelerated hydrogen atom coupled with electromagnetic vacuum fluctuations
We consider a multilevel hydrogen atom in interaction with the quantum
electromagnetic field and separately calculate the contributions of the vacuum
fluctuation and radiation reaction to the rate of change of the mean atomic
energy of the atom for uniform acceleration. It is found that the acceleration
disturbs the vacuum fluctuations in such a way that the delicate balance
between the contributions of vacuum fluctuation and radiation reaction that
exists for inertial atoms is broken, so that the transitions to higher-lying
states from ground state are possible even in vacuum. In contrast to the case
of an atom interacting with a scalar field, the contributions of both
electromagnetic vacuum fluctuations and radiation reaction to the spontaneous
emission rate are affected by the acceleration, and furthermore the
contribution of the vacuum fluctuations contains a non-thermal
acceleration-dependent correction, which is possibly observable.Comment: 8 pages, Revtex4, accepted for publication in PR
Modification of energy shifts of atoms by the presence of a boundary in a thermal bath and the Casimir-Polder force
We study the modification by the presence of a plane wall of energy level
shifts of two-level atoms which are in multipolar coupling with quantized
electromagnetic fields in a thermal bath in a formalism which separates the
contributions of thermal fluctuations and radiation reaction and allows a
distinct treatment to atoms in the ground and excited states. The position
dependent energy shifts give rise to an induced force acting on the atoms. We
are able to identify three different regimes where the force shows distinct
features and examine, in all regimes, the behaviors of this force in both the
low temperature limit and the high temperature limit for both the ground state
and excited state atoms, thus providing some physical insights into the
atom-wall interaction at finite temperature. In particular, we show that both
the magnitude and the direction of the force acting on an atom may have a clear
dependence on atomic the polarization directions. In certain cases, a change of
relative ratio of polarizations in different directions may result in a change
of direction of the force.Comment: 29 pages, 3 figure
Spontaneous excitation of an accelerated multilevel atom in dipole coupling to the derivative of a scalar field
We study the spontaneous excitation of an accelerated multilevel atom in
dipole coupling to the derivative of a massless quantum scalar field and
separately calculate the contributions of the vacuum fluctuation and radiation
reaction to the rate of change of the mean atomic energy of the atom. It is
found that, in contrast to the case where a monopole like interaction between
the atom and the field is assumed, there appear extra corrections proportional
to the acceleration squared, in addition to corrections which can be viewed as
a result of an ambient thermal bath at the Unruh temperature, as compared with
the inertial case, and the acceleration induced correction terms show
anisotropy with the contribution from longitudinal polarization being four
times that from the transverse polarization for isotropically polarized
accelerated atoms. Our results suggest that the effect of acceleration on the
rate of change of the mean atomic energy is dependent not only on the quantum
field to which the atom is coupled, but also on the type of the interaction
even if the same quantum scalar field is considered.Comment: 11 pages, no figure
Spontaneous absorption of an accelerated hydrogen atom near a conducting plane in vacuum
We study, in the multipolar coupling scheme, a uniformly accelerated
multilevel hydrogen atom in interaction with the quantum electromagnetic field
near a conducting boundary and separately calculate the contributions of the
vacuum fluctuation and radiation reaction to the rate of change of the mean
atomic energy. It is found that the perfect balance between the contributions
of vacuum fluctuations and radiation reaction that ensures the stability of
ground-state atoms is disturbed, making spontaneous transition of ground-state
atoms to excited states possible in vacuum with a conducting boundary. The
boundary-induced contribution is effectively a nonthermal correction, which
enhances or weakens the nonthermal effect already present in the unbounded
case, thus possibly making the effect easier to observe. An interesting feature
worth being noted is that the nonthermal corrections may vanish for atoms on
some particular trajectories.Comment: 19 pages, no figures, Revtex
Vortex formation in a stirred Bose-Einstein condensate
Using a focused laser beam we stir a Bose-Einstein condensate of 87Rb
confined in a magnetic trap and observe the formation of a vortex for a
stirring frequency exceeding a critical value. At larger rotation frequencies
we produce states of the condensate for which up to four vortices are
simultaneously present. We have also measured the lifetime of the single vortex
state after turning off the stirring laser beam.Comment: 4 pages, 3 figure
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