An ultracold Fermi gas of ^6Li atoms was converted into an
ultracold gas of ^6Li_2 molecules. This was accomplished
by adiabatic passage through a magnetically tuned scattering
resonance between pairs of free atoms and a bound molecular
state (“Feshbach resonance”). More than 10^5 molecules
were produced with 50% efficiency and confined to an
optical trap. The molecules are formed with a high degree of
vibrational excitation, so they would normally be expected
to collisionally decay into molecules with lower vibrational
energy, causing rapid heating and destruction of the
ultracold gas. However, we have observed lifetimes of ~1 s,
which is sufficient for thermalization and the formation of
a Bose-Einstein condensate (BEC) of molecules. The extended
lifetime is apparently a quantum statistical effect related
to the suppression of s-wave interactions between identical
fermions. The phase space density of the bosons is estimated
to be greater than 0.5. I will discuss our attempts to
directly image the molecular condensate using an optical
transition in the molecule. This would provide direct
detection of the condensate and the means to observe its
formation. The ultimate objective of this work is to form a
Fermi superfluid by inducing Cooper pairing in the Fermi
gas. According to theory, the necessary temperature and
density conditions have been achieved. I will discuss our
attempts to detect superfluid properties.
[A2.002] Feshbach resonances in ultracold atomic gases
Georg Morten Bruun (Niels Bohr Institute, Blegdamsvej 17, 2100 Copenhagen, Denmark)
Dilute gases interacting via a Feshbach resonance can be
strongly correlated. Under certain resonant conditions, such
gases are predicted to show universal behaviour in the sense
that only the density and the temperature determine their
thermodynamics. To examine this effect, we develop a
low-energy effective theory in which the parameters that
enter are an atom-molecule coupling strength and the
magnetic moment of the molecular resonance. Using this
theory, we demonstrate under which conditions strongly
correlated dilute gases can exhibit universal behaviour and
we relate our results to the experimentally relevant
fermionic systems ^6Li and ^40K.
[A2.003] Correlated states of ultra cold atoms.
Ehud Altman (Department of Physics, Harvard University)
Experiments with ultra cold atoms highlight new questions in
strongly correlated systems. One problem is how to detect
many body correlations. We propose to utilize atomic noise
in the image of an expanding gas cloud to probe complex many
body states of trapped ultra cold atoms. In particular we
show how this technique can be used to detect superfluidity
of fermionic gases and spin ordered states of
multi-component bosons. We also discuss the phase diagram
and dynamics of two component bosons on an optical lattice.
Specifically, spin correlations are shown to modify the
nature of the superfluid to insulator transition.
[A2.004] Atomic quantum dots embedded in a Bose-Einstein-Condensate
Wilhelm Zwerger (Institute for Theoretical Physics, University of Innsbruck, Austria)
It is shown that spin-dependent optical lattices allow to realize an atomic quantum dot, where single atoms are stored in a tight trap and are coupled to a superfluid reservoir via laser transitions. Quantum interference between the collisional interactions with the condensate and the laser induced coupling to its phase fluctuations results in a tunable coupling of the dot to a dissipative reservoir, allowing an essentially perfect decoupling from the environment. In the case of a one-dimensional condensate, a dissipative phase transition occurs to a state with a frozen occupation number of the dot. In addition several dots may be coupled via strong, long range Casimir type forces, opening possibilities for quantum gate operations.