Bose-Einstein condensation in a gas has made possible many
novel capabilities for the manipulation and study of the
behavior of quantum wavefunctions. One of these is the
recently demonstrated ability to adjust the self-interaction
of the wavefunction. In contrast to rubidium 87, for most
values of magnetic field rubidium 85 has a large negative
scattering length, which means its Bose-Einstein condensate
(BEC) will have a large attractive self-interaction. This
limits the size of a stable 85Rb condensate at zero Gauss to
less than 100 atoms. However, there is a Feshbach resonance
at 155 G that allows one to change the magnitude and sign of
the self-interaction by adjusting the magnetic field. We
have used this resonance to obtain a positive
self-interaction and created condensates of ten thousand
rubidium 85 atoms. We can then change the magnetic field to
produce attractive or repulsive interactions of almost any
size and study the resulting dynamics and loss. We see a
wide variety of behaviors including dramatic explosions and
implosions of the condensate, with much of it as yet
unexplained. Most striking is the "Bosenova" in which the
condensate collapses in and then explodes out in a manner
quite reminiscent of a supernova.
[E1.002] Vortex nucleation in a stirred gaseous Bose-Einstein condensate
Jean Dalibard (Laboratoire Kastler Brossel, Ecole normale superieure, 24 rue Lhomond, 75005 Paris, France)
As a result of the discovery of Bose-Einstein condensation (BEC) of atomic gases, there has been a new impulse in the study of quantum fluids. Of the many questions that can be studied, superfluidity is one of the most intriguing and fascinating. Originally discovered and studied with liquid Helium and later in the context of superconductors, superfluidity is a hallmark property of interacting quantum fluids and encompasses a whole class of fundamental phenomena. One striking consequence of superfluidity is the response of such a system to rotating peturbations. In contrast to a normal fluid, which at thermal equilibrium will rotate like a solid body with the peturbation, a superfluid will not circulate unless the rotation frequency of the perturbation is sufficiently large. Moreover, when the superfluid does circulate, it can only do so by forming vortices in which the condensate density vanishes and for which the velocity field flow evalutated around a closed contour is quantized.
In this talk we present experimental observations of the
nucleation of such quantized vortices in a stirred gaseous
condensate of atomic rubidium (K. Madison, F.
Chevy, W. Wohlleben, and J. Dalibard, Phys. Rev. Lett.
84), 806 (2000).. We superimpose a rotating,
non-axisymmetric potential created by a laser beam onto a
^87Rb condensate confined in a magnetic trap, and we
observe the formation of vortex filaments when the stirring
frequency is above a critical frequency Ømega_c. The
presence of the vortices is determined by optical absorption
imaging after a 20~ms period during which the atomic cloud
expands and falls in the absence of the magnetic trapping
potential. We also observe the formation of Abrikosov-like
lattices when there are several vortices
present.(K.~W.~Madison, F.~Chevy, W. Wohlleben,
J.~Dalibard., Proceedings of ICAP 2000 and cond/mat
0004037.) In addition, we present the results of a method to
measure the angular momentum of the vortex states. By
exciting a collective quadrupolar oscillation of the
condensate, the angular momentum is determined by the
condensate size and the precession frequency of the
oscillation axes.(F.~Chevy, K.~W.~Madison, and
J.~Dalibard., Phys. Rev. Lett.)~85, 2223 (2000).
Just above Ømega_c, the angular momentum per atom deduced
from the precession is \sim \hbar.
[E1.003] Molecules in a Bose-Einstein Condensate
Daniel Heinzen (University of Texas)
This abstract not available.
[E1.004] Phase-coherent amplification of matter waves.
Mikio Kozuma (Institute of Physics, University of Tokyo, Komaba)
Optical amplifiers, such as pulsed-dye amplifiers, are key
components in many laser systems and have been responsible
for many important scientific advances. In such devices the
optical field amplitude of a "seed" laser pulse is amplified
while its phase coherence is preserved. In the field of
coherent matter waves it has long been speculated, in direct
analogy with optical amplifiers, that it should be possible
to coherently amplify a matter wave by using an appropriate
gain medium. Here, we report phase coherent amplification of
matter waves using Bose-Einstein condensed Rb atoms. The
seed wave is a small matter wavepacket in a high momentum
state created using optical Bragg diffraction. The gain
medium is provided by the recently demonstrated
superradiance effect. Clear evidence of the increase in the
population of the seed wavepacket is observed. The phase of
the amplified matter wave is shown to be locked to that of
the seed wave using a new type of Mach-Zehnder
interferometer.
[E1.005] Soliton and vortex excitations of Bose-Einstein condensates
Charles W. Clark (National Institute of Standards and Technology)
Trapped-atom Bose-Einstein condensates are providing a platform for exploring the basic dynamics of superfluidity, in systems with well-understood interactions. In particular, the Gross-Pitaevski or nonlinear Schrödinger equation has been remarkably successful in first-principles modelling of the static structure and linear response of these systems. Its time-dependent form supports large-amplitude, nonlinear collective motions, which take the form of solitons in the limit of weak trapping potential; these are just beginning to be explored experimentally. We have shown that imprinting a strong initial gradient on the phase of a trapped condensate wavefunction, can give rise to periodic soliton motion in the simple one-dimensional case. For a three-dimensional condensate, on the other hand, such motions are unstable. The decay of these solitons often proceeds by the spinning off of vortex pairs. We have investigated issues of vortex formation and stability in stationary and rotating three-dimensional condensates, and find that the ``anomalous modes'' of the Bogoliubov-deGennes equations play a critical role.