105 research outputs found
Study of light-assisted collisions between a few cold atoms in a microscopic dipole trap
We study light-assisted collisions in an ensemble containing a small number
(~3) of cold Rb87 atoms trapped in a microscopic dipole trap. Using our ability
to operate with one atom exactly in the trap, we measure the one-body heating
rate associated to a near-resonant laser excitation, and we use this
measurement to extract the two-body loss rate associated to light-assisted
collisions when a few atoms are present in the trap. Our measurements indicate
that the two-body loss rate can reach surprisingly large values beta>10^{-8}
cm^{3}.s^{-1} and varies rapidly with the trap depth and the parameters of the
excitation light.Comment: 6 pages, 7 figure
Measurement of the atom number distribution in an optical tweezer using single photon counting
We demonstrate in this paper a method to reconstruct the atom number
distribution of a cloud containing a few tens of cold atoms. The atoms are
first loaded from a magneto-optical trap into a microscopic optical dipole trap
and then released in a resonant light probe where they undergo a Brownian
motion and scatter photons. We count the number of photon events detected on an
image intensifier. Using the response of our detection system to a single atom
as a calibration, we extract the atom number distribution when the trap is
loaded with more than one atom. The atom number distribution is found to be
compatible with a Poisson distribution.Comment: 6 pages, 5 figure
Propagation of light through small clouds of cold interacting atoms
We demonstrate experimentally that a cloud of cold atoms with a size
comparable to the wavelength of light can induce large group delays on a laser
pulse when the laser is tightly focused on it and is close to an atomic
resonance. Delays as large as -10 ns are observed, corresponding to
"superluminal" propagation with negative group velocities as low as -300 m/s.
Strikingly, this large delay is associated with a moderate extinction owing to
the very small size of the cloud and to the light-induced interactions between
atoms. It implies that a large phase shift is imprinted on the continuous laser
beam, and opens interesting perspectives for applications to quantum
technologies.Comment: 5 pages, 3 figures Supplemental Material : 2 pages, 2 Figure
Evaporative cooling of a small number of atoms in a single-beam microscopic dipole trap
We demonstrate experimentally the evaporative cooling of a few hundred
rubidium 87 atoms in a single-beam microscopic dipole trap. Starting from 800
atoms at a temperature of 125microKelvins, we produce an unpolarized sample of
40 atoms at 110nK, within 3s. The phase-space density at the end of the
evaporation reaches unity, close to quantum degeneracy. The gain in phase-space
density after evaporation is 10^3. We find that the scaling laws used for much
larger numbers of atoms are still valid despite the small number of atoms
involved in the evaporative cooling process. We also compare our results to a
simple kinetic model describing the evaporation process and find good agreement
with the data.Comment: 7 pages, 5 figure
Controlling the cold collision shift in high precision atomic interferometry
We present here a new method based on a transfer of population by adiabatic
passage that allows to prepare cold atomic samples with a well defined ratio of
atomic density and atom number. This method is used to perform a measurement of
the cold collision frequency shift in a laser cooled cesium clock at the
percent level, which makes the evaluation of the cesium fountains accuracy at
the level realistic. With an improved set-up, the adiabatic passage
would allow measurements of atom number-dependent phase shifts at the
level in high precision experiments.Comment: 4 pages, 3 figures, 2 table
Homogenization of an ensemble of interacting resonant scatterers
We study theoretically the concept of homogenization in optics using an
ensemble of randomly distributed resonant stationary atoms with density .
The ensemble is dense enough for the usual condition for homogenization, viz.
, to be reached. Introducing the coherent and incoherent
scattered powers, we define two criteria to define the homogenization regime.
We find that when the excitation field is tuned in a broad frequency range
around the resonance, none of the criteria for homogenization is fulfilled,
meaning that the condition is not sufficient to
characterize the homogenized regime around the atomic resonance. We interpret
these results as a consequence of the light-induced dipole-dipole interactions
between the atoms, which implies a description of scattering in terms of
collective modes rather than as a sequence of individual scattering events.
Finally, we show that, although homogenization can never be reached for a dense
ensemble of randomly positioned laser-cooled atoms around resonance, it becomes
possible if one introduces spatial correlations in the positions of the atoms
or non-radiative losses, such as would be the case for organic molecules or
quantum dots coupled to a phonon bath.Comment: 9 pages, 5 figures. Corrected mistakes in reference
Sub-Poissonian atom number fluctuations using light-assisted collisions
We investigate experimentally the number statistics of a mesoscopic ensemble
of cold atoms in a microscopic dipole trap loaded from a magneto-optical trap,
and find that the atom number fluctuations are reduced with respect to a
Poisson distribution due to light-assisted two-body collisions. For numbers of
atoms N>2, we measure a reduction factor (Fano factor) of 0.72+/-0.07, which
differs from 1 by more than 4 standard deviations. We analyze this fact by a
general stochastic model describing the competition between the loading of the
trap from a reservoir of cold atoms and multi-atom losses, which leads to a
master equation. Applied to our experimental regime, this model indicates an
asymptotic value of 3/4 for the Fano factor at large N and in steady state. We
thus show that we have reached the ultimate level of reduction in number
fluctuations in our system.Comment: 4 pages, 3 figure
Observation of suppression of light scattering induced by dipole-dipole interactions in a cold atomic ensemble
We study the emergence of collective scattering in the presence of
dipole-dipole interactions when we illuminate a cold cloud of rubidium atoms
with a near-resonant and weak intensity laser. The size of the atomic sample is
comparable to the wavelength of light. When we gradually increase the atom
number from 1 to 450, we observe a broadening of the line, a small red shift
and, consistently with these, a strong suppression of the scattered light with
respect to the noninteracting atom case. Numerical simulations, which include
the internal atomic level structure, agree with the data.Comment: 5 pages, 5 figure
Imaging a single atom in a time-of-flight experiment
We perform fluorescence imaging of a single 87Rb atom after its release from
an optical dipole trap. The time-of-flight expansion of the atomic spatial
density distribution is observed by accumulating many single atom images. The
position of the atom is revealed with a spatial resolution close to 1
micrometer by a single photon event, induced by a short resonant probe. The
expansion yields a measure of the temperature of a single atom, which is in
very good agreement with the value obtained by an independent measurement based
on a release-and-recapture method. The analysis presented in this paper
provides a way of calibrating an imaging system useful for experimental studies
involving a few atoms confined in a dipole trap.Comment: 14 pages, 8 figure
Collisional shifts in optical-lattice atom clocks
We theoretically study the effects of elastic collisions on the determination
of frequency standards via Ramsey fringe spectroscopy in optical-lattice atom
clocks. Interparticle interactions of bosonic atoms in multiply-occupied
lattice sites can cause a linear frequency shift, as well as generate
asymmetric Ramsey fringe patterns and reduce fringe visibility due to
interparticle entanglement. We propose a method of reducing these collisional
effects in an optical lattice by introducing a phase difference of
between the Ramsey driving fields in adjacent sites. This configuration
suppresses site to site hopping due to interference of two tunneling pathways,
without degrading fringe visibility. Consequently, the probability of double
occupancy is reduced, leading to cancellation of collisional shifts.Comment: 15 pages, 11 figure
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