74 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
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
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
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
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
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
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
Energy distribution and cooling of a single atom in an optical tweezer
We investigate experimentally the energy distribution of a single rubidium
atom trapped in a strongly focused dipole trap under various cooling regimes.
Using two different methods to measure the mean energy of the atom, we show
that the energy distribution of the radiatively cooled atom is close to
thermal. We then demonstrate how to reduce the energy of the single atom, first
by adiabatic cooling, and then by truncating the Boltzmann distribution of the
single atom. This provides a non-deterministic way to prepare atoms at low
microKelvin temperatures, close to the ground state of the trapping potential.Comment: 9 pages, 6 figures, published in PR
Diffraction limited optics for single atom manipulation
We present an optical system designed to capture and observe a single neutral
atom in an optical dipole trap, created by focussing a laser beam using a large
numerical aperture N.A.=0.5 aspheric lens. We experimentally evaluate the
performance of the optical system and show that it is diffraction limited over
a broad spectral range (~ 200 nm) with a large transverse field (+/- 25
microns). The optical tweezer created at the focal point of the lens is able to
trap single atoms of 87Rb and to detect them individually with a large
collection efficiency. We measure the oscillation frequency of the atom in the
dipole trap, and use this value as an independent determination of the waist of
the optical tweezer. Finally, we produce with the same lens two dipole traps
separated by 2.2 microns and show that the imaging system can resolve the two
atoms.Comment: 8 pages, 9 figures; typos corrected and references adde
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