47 research outputs found
Direct Observation of the Superfluid Phase Transition in Ultracold Fermi Gases
Water freezes into ice, atomic spins spontaneously align in a magnet, liquid
helium becomes superfluid: Phase transitions are dramatic phenomena. However,
despite the drastic change in the system's behaviour, observing the transition
can sometimes be subtle. The hallmark of Bose-Einstein condensation (BEC) and
superfluidity in trapped, weakly interacting Bose gases is the sudden
appearance of a dense central core inside a thermal cloud. In strongly
interacting gases, such as the recently observed fermionic superfluids, this
clear separation between the superfluid and the normal parts of the cloud is no
longer given. Condensates of fermion pairs could be detected only using
magnetic field sweeps into the weakly interacting regime. The quantitative
description of these sweeps presents a major theoretical challenge. Here we
demonstrate that the superfluid phase transition can be directly observed by
sudden changes in the shape of the clouds, in complete analogy to the case of
weakly interacting Bose gases. By preparing unequal mixtures of the two spin
components involved in the pairing, we greatly enhance the contrast between the
superfluid core and the normal component. Furthermore, the non-interacting
wings of excess atoms serve as a direct and reliable thermometer. Even in the
normal state, strong interactions significantly deform the density profile of
the majority spin component. We show that it is these interactions which drive
the normal-to-superfluid transition at the critical population imbalance of
70(5)%.Comment: 16 pages (incl. Supplemental Material), 5 figure
Optical Lattices: Theory
This chapter presents an overview of the properties of a Bose-Einstein
condensate (BEC) trapped in a periodic potential. This system has attracted a
wide interest in the last years, and a few excellent reviews of the field have
already appeared in the literature (see, for instance, [1-3] and references
therein). For this reason, and because of the huge amount of published results,
we do not pretend here to be comprehensive, but we will be content to provide a
flavor of the richness of this subject, together with some useful references.
On the other hand, there are good reasons for our effort. Probably, the most
significant is that BEC in periodic potentials is a truly interdisciplinary
problem, with obvious connections with electrons in crystal lattices, polarons
and photons in optical fibers. Moreover, the BEC experimentalists have reached
such a high level of accuracy to create in the lab, so to speak, paradigmatic
Hamiltonians, which were first introduced as idealized theoretical models to
study, among others, dynamical instabilities or quantum phase transitions.Comment: Chapter 13 in Part VIII: "Optical Lattices" of "Emergent Nonlinear
Phenomena in Bose-Einstein Condensates: Theory and Experiment," edited by P.
G. Kevrekidis, D. J. Frantzeskakis, and R. Carretero-Gonzalez (Springer
Series on Atomic, Optical, and Plasma Physics, 2007) - pages 247-26
AEDGE: Atomic Experiment for Dark Matter and Gravity Exploration in Space
We propose in this White Paper a concept for a space experiment using cold
atoms to search for ultra-light dark matter, and to detect gravitational waves
in the frequency range between the most sensitive ranges of LISA and the
terrestrial LIGO/Virgo/KAGRA/INDIGO experiments. This interdisciplinary
experiment, called Atomic Experiment for Dark Matter and Gravity Exploration
(AEDGE), will also complement other planned searches for dark matter, and
exploit synergies with other gravitational wave detectors. We give examples of
the extended range of sensitivity to ultra-light dark matter offered by AEDGE,
and how its gravitational-wave measurements could explore the assembly of
super-massive black holes, first-order phase transitions in the early universe
and cosmic strings. AEDGE will be based upon technologies now being developed
for terrestrial experiments using cold atoms, and will benefit from the space
experience obtained with, e.g., LISA and cold atom experiments in microgravity.
This paper is based on a submission (v1) in response to the Call for White
Papers for the Voyage 2050 long-term plan in the ESA Science Programme. ESA
limited the number of White Paper authors to 30. However, in this version (v2)
we have welcomed as supporting authors participants in the Workshop on Atomic
Experiments for Dark Matter and Gravity Exploration held at CERN: ({\tt
https://indico.cern.ch/event/830432/}), as well as other interested scientists,
and have incorporated additional material
Search for gravitational waves associated with gamma-ray bursts detected by Fermi and Swift during the LIGO–Virgo run O3b
We search for gravitational-wave signals associated with gamma-ray bursts (GRBs) detected by the Fermi and Swift satellites during the second half of the third observing run of Advanced LIGO and Advanced Virgo (2019 November 1 15:00 UTC–2020 March 27 17:00 UTC). We conduct two independent searches: a generic gravitational-wave transients search to analyze 86 GRBs and an analysis to target binary mergers with at least one neutron star as short GRB progenitors for 17 events. We find no significant evidence for gravitational-wave signals associated with any of these GRBs. A weighted binomial test of the combined results finds no evidence for subthreshold gravitational-wave signals associated with this GRB ensemble either. We use several source types and signal morphologies during the searches, resulting in lower bounds on the estimated distance to each GRB. Finally, we constrain the population of low-luminosity short GRBs using results from the first to the third observing runs of Advanced LIGO and Advanced Virgo. The resulting population is in accordance with the local binary neutron star merger rate
Atom resonant tunnelling through a moving barrier
We study the one-dimensional dynamical behaviour of an atomic wavepacket moving across an opaque and spatially oscillating optical barrier. The tunnelling probability shows a dramatic dependence on the frequency of the barrier oscillation. Transmission of the atomic wavepacket is, in fact, dominated by inelastic or elastic effects, depending on whether the oscillation period is comparable to or shorter than the atom-barrier interaction time. In the elastic regime, in particular, we find that full transparency becomes possible at energies where the stationary barrier would otherwise be essentially opaque. We predict that both inelastic and elastic resonant tunnelling regimes could be observed with atomic condensates impinging on a spatially oscillating optical barrier. A few applications will also be discussed
Inelastic time-dependent tunnelling of matter waves
Tunnelling of ultra-cold atoms across a periodically displaced optical barrier exhibits characteristic inelastic effects as well as remarkable transparency at energies for which the same barrier, if stationary, would be opaque. Such phenomena could be observed with sodium Bose-Einstein condensate wavepackets and could be exploited for atom-laser sideband generation
Resonance superfluidity in a quantum degenerate Fermi gas
We consider the superfluid phase transition that arises when a Feshbach resonance pairing occurs in a dilute Fermi gas. This is related to the phenomenon of superconductivity described by the seminal Bardeen-Cooper- Schrieffer theory. In superconductivity, the phase transition is caused by a coupling between pairs of electrons within the medium. This coupling is perturbative and leads to a critical temperature Tc which is small compared to the Fermi temperature TF. Even high-Tc superconductors typically have a critical temperature which is two orders of magnitude below TF. Here we describe a resonance pairing mechanism in a quantum degenerate gas of potassium (40K) atoms which leads to superfluidity in a novel regime -- a regime that promises the unique opportunity to experimentally study the cross-over from the Bardeen- Cooper-Schrieffer phase of weakly-coupled fermions to the Bose-Einstein condensate of strongly-bound composite bosons. We find that the transition to a superfluid phase is possible at the high critical temperature of about 0.5 TF. It should be straightforward to verify this prediction, since these temperatures can already be achieved experimentally
Effects of atomic interactions on the resonant tunnelling of sodium condensates
We investigate the influence of atomic interactions on the tunnelling of sodium condensates across a spatially oscillating optical barrier. In the limit of very fast barrier oscillations, in which resonant tunnelling via a metastable state takes place, the interactions affect the position and lineshape of the transmission peak. We anticipate that the possibility of modulating the interactions in a tunnelling condensate can be exploited to achieve nonlinear effects such as optical limiting and bistability