Number 449 (Story #1), September 23, 1999 by Phillip F. Schewe and Ben Stein|
ARE BOSE-EINSTEIN CONDENSATES SUPERFLUID? Previously physicists have demonstrated that Bose Einstein condensates (BEC is created when trapped atoms are chilled so low that they begin to overlap) constitute a single macroscopic quantum state, which implies superfluidity. However, physicists would like to see frictionless flow more directly. Two new experiments pave the way toward this goal. A NIST/Colorado group has observed quantized vortices in a condensate of rubidium atoms, while an MIT group has observed that objects can move through a condensate of sodium atoms and lose little or no energy if the velocity is below a certain critical value. In the Colorado/NIST work (Carl Wieman, 303-492-6963, email@example.com) the BEC state consists of atoms residing in two separate spin states (referred to as 1 and 2). Using microwaves and a separate probe laser beam working at the fringe of the condensate, the spins of 1-state atoms are flipped, turning them into 2-state atoms in one sector of the condensate after another. This sets a vortex of 2-state atoms into motion around the outer part of the condensate while 1-state atoms remain at rest at the core of the vortex (see the figures at www.aip.org/png). Thus the vortex is like a smoke-ring of 2-state atoms (with a filling of 1-state atoms) rotating about every 3 seconds. Furthermore, it has exactly one unit of angular momentum. Meanwhile the MIT group (Wolfgang Ketterle, firstname.lastname@example.org, 617-253-4876) uses a focused laser beam to punch a hole in the BEC blob (the light repels atoms from its focus) and then scans the hole along at various speeds. The moving hole is equivalent to a moving object. Below a scan velocity of about 2 mm/sec, no energy dissipation was observed. The existence of such a critical velocity for frictionless motion is an attribute of superfluidity. One reason for this kind of BEC research, other than for studying fundamental aspects of a novel form of atomic matter, is that it might afford a new way of learning about superfluidity and superconductivity (both reports appear in the 27 Sep issue of Physical Review Letters: Colorado/NIST in M.R. Matthews et al. and the MIT work in C. Raman et al).