|
Number 449, 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, cwieman@jila.colorado.edu) 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, ketterle@mit.edu, 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).
SEPARATING CHEMICAL ISOTOPES WITH A TABLETOP TERAWATT LASER has been demonstrated by researchers at the University of Michigan, providing a more compact alternative to the bulky techniques for extracting isotopes, and introducing a new method for making ultrapure thin films which can be used in electronic devices. Using a technique known as chirped pulse amplification (Update 154), University of Michigan researchers (Peter Pronko, 734-763-6008) produced laser pulses that deliver between 10 trillion and 1 quadrillion watts (10-1000 terawatts) of power per square centimeter for an extremely short instant--between 150 and 200 quadrillionths (10-15) of a second. Aimed at a target inside a vacuum chamber containing the isotopes of interest, the pulse vaporized some of the isotopes, which escaped in the form of ions (charged atoms). Intense magnetic fields associated with the pulses exerted forces on the ions which deposited them at different locations on a nearby silicon disk depending on the isotope's weight. With their technique, the researchers separated boron-10 from boron-11 and gallium-69 from gallium-71. It's an open question if their technique will be feasible on the large scales required for separating isotopes at nuclear facilities, but the researchers are initially setting their sights on other applications, such as depositing pure thin films of isotopes directly onto microelectronic devices. (Pronko et al., Physical Review Letters, 27 September 1999; figure at www.aip.org/png)
|
