Accelerator for BECs

Two research groups have banged quantum gases together at record high velocities. Both groups begin by cooling clouds of rubidium atoms to ultralow temperatures. Next, through magnetic manipulation the clouds could be split into two separate clouds, each containing a native population with a characteristic spin value.

Physicists in the Netherlands (FOM Institute for Atomic and Molecular Physics and the University of Amsterdam) further cool the clouds to produce Bose-Einstein condensates (BEC) before using the same magnetic control over the atoms to urge the clouds back together again at an increasing speed. Earlier experiments had managed to "collide" separate BEC samples at slow speeds of mm/sec (slow in relation to the velocity of sound in the BEC---several mm/sec) in order to observe characteristic interference stripes, and affirm the intrinsic wavelike nature of BEC as a whole.

Now, the Dutch experiment is able to achieve speeds of 20 cm/sec; in effect their apparatus is a linear accelerator for BECs. The respective clouds are about 10 microns in size; the relative size of the clouds and their initial separation (up to record distances of 4 mm) is analogous to the separation of two tennis balls on opposite sides of a tennis court. When the two "tennis balls" collide, a spherical interference pattern shows up (see animation at staff.science.uva.nl/~walraven/walraven/Highlights.htm).

Why is the higher speed important? It’s because below sound speed, the superfluid BEC behaves like one giant matter wave, while above sound speed the BEC behaves like a collection of individual atoms. So in this experiment it is more accurate to think of 100,000 atoms (in the one cloud) scattering with 100,000 atoms (in the other cloud) rather then to think of two interacting clouds.

Furthermore, because the speeds are still slow, the atom-atom collision can still be thought of as being the collision of two waves (like separate ripples in a pond passing through each other). In other words, the experiment probes the interaction between atoms rather than between BECs.

In the BEC accelerator, matter waves of atom pairs are scattered out of the clouds at an energy of 10^-7 eV. (Compare this to Fermilab’s 10¹² eV energy scale.) These matter waves are a superposition of spherical-shaped "s" and dumbbell-shaped "d" waves and hence show quantum mechanical interference. This interference is being directly imaged (Buggle et al., Physical Review Letters, 22 October 2004; contact Jeremie Leonard, jleonard@science.uva.nl), and yields accurate measurement of the interaction properties between ultracold atoms.

Comparable observations are being reported by physicists from the University of Otago in New Zealand, although in this experiment the atoms were at 200-nanokelvin temperatures but did not constitute a BEC. (Thomas et al., Physical Review Letters, 22 October 2004; contact Niels Kjaergaard, nk@physics.otago.ac.nz; lab website at http://www.physics.otago.ac.nz/research/bec/Files/collisions/collisions.
html).