[MURG] The American Institute of Physics Bulletin of Physics News

[Joseph M. Graham Jr.]

Hello, murg,

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Physics News Update
The American Institute of Physics Bulletin of Physics News
Number 472, February 24, 2000 by Phillip F. Schewe and Ben Stein
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LIGHT AT 1 MPH. A year ago Lene Verstergaard Hau used a Bose Einstein condensate (BEC) as a special nonlinear optical medium for slowing light from 3 x 108 m/sec to a mere 17 m/sec (38 mph; Update 415). This comes about when an incoming light pulse enters the BEC and experiences an extremely abrupt change in index of refraction (and as for absorption of the light, this is prevented by applying two laser beams which induce a transparency at the frequency of the incoming light). 

In a talk presented at this week's meeting of the American Association for the Advancement of Science (AAAS) in Washington, DC, Hau said that she and her Harvard colleagues had slowed the light further, to a speed of 1 mph. She said that if the velocity could be slowed still more, to a value of 1 cm/sec, then this would be comparable to the speed of sound in the condensate and it might be possible to get atoms to surf on the front of the light pulse. Hau believes that this approach to slowing light, if it can be simplified, would lead to highly sensitive light switches and to low-power nonlinear optics (right now high-power laser light is required to produce nonlinear effects). 

ALL-OPTICAL NMR has been achieved by David Awschalom's research group at the University of California, Santa Barbara. Previous versions of nuclear magnetic resonance (NMR) have relied on radio-frequency electromagnetic fields to tip nuclear magnetic moments. This approach (nearing its 50th anniversary with proven success in medical imaging and chemistry), is modified by the UCSB scientists in the following way. They use a laser to excite a bath of electron spins, which then do all the work. As these electron magnetic moments swarm about the nuclei, their number and direction are controlled by the laser in a way that tips nuclear spins. Nuclei are monitored during the process by a second laser beam, making this an entirely optical approach. 

Demonstrated in the semiconductor GaAs, this fundamentally different alternative to ordinary NMR offers potentially increased resolution because light can be focused more tightly than RF fields. Moreover, because the UCSB strategy exploits electrons as an intermediary, individual electron orbits themselves might be used to obtain atomic-scale focusing. (For background, see Kikkawa and Awschalom, Science, Jan. 21, 2000). One aim of this research is to "imprint" electron spin on the nuclear system within integrated "spintronic" devices, where electron spin supplants charge as a source of information. 

CANDIDATE DARK MATTER PARTICLES, specifically thought to be examples of "weakly interacting massive particles" (WIMPs), have been indirectly detected by a group operating in the Gran Sasso National Lab (INFN) in Italy, according to a paper to be delivered by Pierluigi Belli of the University of Rome (DAMA collaboration) at a dark matter detection meeting in Marina del Rey, California this week (a meeting sponsored by UCLA: information from dm20 at physics.ucla.edu). 

Dark matter is a hypothetical non-luminous substance thought to be lurking in and around galaxies, influencing the way the galaxies rotate and interact. The dark matter might consist in part of baryons (particles such as the protons found in common atoms) or more novel forms such as WIMPs. Because of the way the Earth orbits the sun and the way our solar system moves through the galaxy (buffeting the presumed dark matter halo as it goes) there is reason to think that the flux of WIMP wind we encounter (and the rate at which WIMPs feebly interact in terrestrial detectors) would be higher in June than in December. 

The DAMA experiment reports having discovered just such an seasonal effect in the frequency of events in which a presumed incoming WIMP (with masses about 50 times that of the proton) strikes a shielded sodium-iodine scintillation material, causing tiny flashes of light deep within the detector (INFN preprint AE-00/01; www.lngs.infn.it). Dark matter interactions in detectors are expected to be rare and analysis difficult, so the DAMA interpretation will be subject to great scrutiny at the California meeting, where other groups searching for WIMPs will be reporting as well. 

CORRECTION: In work at the University of Texas (Update 471), molecules formed in an atomic Rb Bose-Einstein condensate (BEC) did not themselves constitute a molecular BEC. Instead, the molecules formed an ultracold gas which exhibited observable interactions with the atomic BEC. 
 http://www.aip.org/enews/physnews/2000/physnews.472.htm#2

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Best regards. 

Joseph M. Graham Jr.
jmgj2 at netzero.com
2003-09-04