89 research outputs found
Production of a chromium Bose-Einstein condensate
The recent achievement of Bose-Einstein condensation of chromium atoms [1]
has opened longed-for experimental access to a degenerate quantum gas with
long-range and anisotropic interaction. Due to the large magnetic moment of
chromium atoms of 6 {}B, in contrast to other Bose- Einstein condensates
(BECs), magnetic dipole-dipole interaction plays an important role in a
chromium BEC. Many new physical properties of degenerate gases arising from
these magnetic forces have been predicted in the past and can now be studied
experimentally. Besides these phenomena, the large dipole moment leads to a
breakdown of standard methods for the creation of a chromium BEC. Cooling and
trapping methods had to be adapted to the special electronic structure of
chromium to reach the regime of quantum degeneracy. Some of them apply
generally to gases with large dipolar forces. We present here a detailed
discussion of the experimental techniques which are used to create a chromium
BEC and alow us to produce pure condensates with up to {} atoms in an
optical dipole trap. We also describe the methods used to determine the
trapping parameters.Comment: 17 pages, 9 figure
Free Expansion of a Weakly-interacting Dipolar Fermi Gas
We theoretically investigate a polarized dipolar Fermi gas in free expansion.
The inter-particle dipolar interaction deforms phase-space distribution in trap
and also in the expansion. We exactly predict the minimal quadrupole
deformation in the expansion for the high-temperature Maxwell-Boltzmann and
zero-temperature Thomas-Fermi gases in the Hartree-Fock and Landau-Vlasov
approaches. In conclusion, we provide a proper approach to develop the
time-of-flight method for the weakly-interacting dipolar Fermi gas and also
reveal a scaling law associated with the Liouville's theorem in the long-time
behaviors of the both gases
Dipolar collisions of polar molecules in the quantum regime
Ultracold polar molecules offer the possibility of exploring quantum gases
with interparticle interactions that are strong, long-range, and spatially
anisotropic. This is in stark contrast to the dilute gases of ultracold atoms,
which have isotropic and extremely short-range, or "contact", interactions. The
large electric dipole moment of polar molecules can be tuned with an external
electric field; this provides unique opportunities such as control of ultracold
chemical reactions, quantum information processing, and the realization of
novel quantum many-body systems. In spite of intense experimental efforts aimed
at observing the influence of dipoles on ultracold molecules, only recently
have sufficiently high densities been achieved. Here, we report the observation
of dipolar collisions in an ultracold molecular gas prepared close to quantum
degeneracy. For modest values of an applied electric field, we observe a
dramatic increase in the loss rate of fermionic KRb molecules due to ultrcold
chemical reactions. We find that the loss rate has a steep power-law dependence
on the induced electric dipole moment, and we show that this dependence can be
understood with a relatively simple model based on quantum threshold laws for
scattering of fermionic polar molecules. We directly observe the spatial
anisotropy of the dipolar interaction as manifested in measurements of the
thermodynamics of the dipolar gas. These results demonstrate how the long-range
dipolar interaction can be used for electric-field control of chemical reaction
rates in an ultracold polar molecule gas. The large loss rates in an applied
electric field suggest that creating a long-lived ensemble of ultracold polar
molecules may require confinement in a two-dimensional trap geometry to
suppress the influence of the attractive dipolar interactions
Hydrodynamic excitations of trapped dipolar fermions
A single-component Fermi gas of polarized dipolar particles in a harmonic
trap can undergo a mechanical collapse due to the attractive part of the
dipole-dipole interaction. This phenomenon can be conveniently manipulated by
the shape of the external trapping potential. We investigate the signatures of
the instability by studying the spectrum of low-lying collective excitations of
the system in the hydrodynamic regime. To this end, we employ a time-dependent
variational method as well as exact numerical solutions of the hydrodynamic
equations of the system.Comment: 4 pages, 2 eps figures, final versio
A Storage Ring for Neutral Atoms
We have demonstrated a storage ring for ultra-cold neutral atoms. Atoms with
mean velocities of 1 m/s corresponding to kinetic energies of ~100 neV are
confined to a 2 cm diameter ring by magnetic forces produced by two
current-carrying wires. Up to 10^6 atoms are loaded at a time in the ring, and
7 revolutions are clearly observed. Additionally, we have demonstrated multiple
loading of the ring and deterministic manipulation of the longitudinal velocity
distribution of the atoms using applied laser pulses. Applications of this ring
include large area atom interferometers and cw monochromatic atomic beam
generation.Comment: 4 pages, 5 figure
Development of a strontium optical lattice clock for the SOC mission on the ISS
The ESA mission "Space Optical Clock" project aims at operating an optical
lattice clock on the ISS in approximately 2023. The scientific goals of the
mission are to perform tests of fundamental physics, to enable space-assisted
relativistic geodesy and to intercompare optical clocks on the ground using
microwave and optical links. The performance goal of the space clock is less
than uncertainty and
instability. Within an EU-FP7-funded project, a strontium optical lattice clock
demonstrator has been developed. Goal performances are instability below and fractional inaccuracy .
For the design of the clock, techniques and approaches suitable for later space
application are used, such as modular design, diode lasers, low power
consumption subunits, and compact dimensions. The Sr clock apparatus is fully
operational, and the clock transition in Sr was observed with linewidth
as small as 9 Hz.Comment: 12 pages, 8 figures, SPIE Photonics Europe 201
Controlling the quantum stereodynamics of ultracold bimolecular reactions
Chemical reaction rates often depend strongly on stereodynamics, namely the
orientation and movement of molecules in three-dimensional space. An ultracold
molecular gas, with a temperature below 1 uK, provides a highly unusual regime
for chemistry, where polar molecules can easily be oriented using an external
electric field and where, moreover, the motion of two colliding molecules is
strictly quantized. Recently, atom-exchange reactions were observed in a
trapped ultracold gas of KRb molecules. In an external electric field, these
exothermic and barrierless bimolecular reactions, KRb+KRb -> K2+Rb2, occur at a
rate that rises steeply with increasing dipole moment. Here we show that the
quantum stereodynamics of the ultracold collisions can be exploited to suppress
the bimolecular chemical reaction rate by nearly two orders of magnitude. We
use an optical lattice trap to confine the fermionic polar molecules in a
quasi-two-dimensional, pancake-like geometry, with the dipoles oriented along
the tight confinement direction. With the combination of sufficiently tight
confinement and Fermi statistics of the molecules, two polar molecules can
approach each other only in a "side-by-side" collision, where the chemical
reaction rate is suppressed by the repulsive dipole-dipole interaction. We show
that the suppression of the bimolecular reaction rate requires quantum-state
control of both the internal and external degrees of freedom of the molecules.
The suppression of chemical reactions for polar molecules in a
quasi-two-dimensional trap opens the way for investigation of a dipolar
molecular quantum gas. Because of the strong, long-range character of the
dipole-dipole interactions, such a gas brings fundamentally new abilities to
quantum-gas-based studies of strongly correlated many-body physics, where
quantum phase transitions and new states of matter can emerge.Comment: 19 pages, 4 figure
Strong dipolar effects in a quantum ferrofluid
We report on the realization of a Chromium Bose-Einstein condensate (BEC)
with strong dipolar interaction. By using a Feshbach resonance, we reduce the
usual isotropic contact interaction, such that the anisotropic magnetic
dipole-dipole interaction between 52Cr atoms becomes comparable in strength.
This induces a change of the aspect ratio of the cloud, and, for strong dipolar
interaction, the inversion of ellipticity during expansion - the usual "smoking
gun" evidence for BEC - can even be suppressed. These effects are accounted for
by taking into account the dipolar interaction in the superfluid hydrodynamic
equations governing the dynamics of the gas, in the same way as classical
ferrofluids can be described by including dipolar terms in the classical
hydrodynamic equations. Our results are a first step in the exploration of the
unique properties of quantum ferrofluids.Comment: Final, published versio
Development of a strontium optical lattice clock for the SOC mission on the ISS
Ultra-precise optical clocks in space will allow new studies in fundamental
physics and astronomy. Within an European Space Agency (ESA) program, the Space
Optical Clocks (SOC) project aims to install and to operate an optical lattice
clock on the International Space Station (ISS) towards the end of this decade.
It would be a natural follow-on to the ACES mission, improving its performance
by at least one order of magnitude. The payload is planned to include an
optical lattice clock, as well as a frequency comb, a microwave link, and an
optical link for comparisons of the ISS clock with ground clocks located in
several countries and continents. Within the EU-FP7-SPACE-2010-1 project no.
263500, during the years 2011-2015 a compact, modular and robust strontium
lattice optical clock demonstrator has been developed. Goal performance is a
fractional frequency instability below 1x10^{-15}, tau^{-1/2} and a fractional
inaccuracy below 5x10^{-17}. Here we describe the current status of the
apparatus' development, including the laser subsystems. Robust preparation of
cold {88}^Sr atoms in a second stage magneto-optical trap (MOT) is achieved.Comment: 27 Pages, 15 figures, Comptes Rendus Physique 201
The Space Optical Clocks Project: Development of high-performance transportable and breadboard optical clocks and advanced subsystems
The use of ultra-precise optical clocks in space ("master clocks") will allow
for a range of new applications in the fields of fundamental physics (tests of
Einstein's theory of General Relativity, time and frequency metrology by means
of the comparison of distant terrestrial clocks), geophysics (mapping of the
gravitational potential of Earth), and astronomy (providing local oscillators
for radio ranging and interferometry in space). Within the ELIPS-3 program of
ESA, the "Space Optical Clocks" (SOC) project aims to install and to operate an
optical lattice clock on the ISS towards the end of this decade, as a natural
follow-on to the ACES mission, improving its performance by at least one order
of magnitude. The payload is planned to include an optical lattice clock, as
well as a frequency comb, a microwave link, and an optical link for comparisons
of the ISS clock with ground clocks located in several countries and
continents. Undertaking a necessary step towards optical clocks in space, the
EU-FP7-SPACE-2010-1 project no. 263500 (SOC2) (2011-2015) aims at two
"engineering confidence", accurate transportable lattice optical clock
demonstrators having relative frequency instability below 1\times10^-15 at 1 s
integration time and relative inaccuracy below 5\times10^-17. This goal
performance is about 2 and 1 orders better in instability and inaccuracy,
respectively, than today's best transportable clocks. The devices will be based
on trapped neutral ytterbium and strontium atoms. One device will be a
breadboard. The two systems will be validated in laboratory environments and
their performance will be established by comparison with laboratory optical
clocks and primary frequency standards. In this paper we present the project
and the results achieved during the first year.Comment: Contribution to European Frequency and Time Forum 2012, Gothenburg,
Swede
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