Researchers Report on New Results for BEC Experiments
Since 1995, scientists have been creating a new state of matter, Bose-Einstein condensates of atoms, in which a sufficiently densely packed collection of gas atoms is cooled to such low temperatures that they enter a single quantum state and effectively act as a single entity, or "superatom." During a Sunday morning session at the APS/AAPT Spring Meeting in Washington, DC, three researchers spoke on the latest results from recent experiments using BEC condensates, which were first predicted by Albert Einstein in 1925.
Randy Hulet of Rice University reported that BECs of lithium atoms are different from other BECs in that the lithium atoms attract rather than repel each other, and are limited to a size of approximately 1500 atoms. According to some predictions, adding more atoms than this would cause the BEC to undergo a "macroscopic quantum tunneling," in which the condensate would collectively transform from a low-density to a high-density state, forming molecules which would then release excess heat and cause the BEC to blow apart.
Hulet said that the observation of a limited condensate number is already an indication that this phenomenon is happening, but the Rice team is attempting to directly observe the collapse and its aftermath. "Physicists are excited because of the opportunity to study quantum mechanics on a macroscopic scale," he said, adding that this tunneling effect usually applies only to single microscopic particles, not to collections of many atoms, like a BEC.
Studying BECs of rubidium atoms, Eric Cornell of NIST and the University of Colorado discussed experiments confirming that BECs are significantly more uniform in density than comparable clouds of cold atoms in a non-BEC state. He also described how the frequencies of sound waves in BECs unexpectedly depend upon temperature, something which current theory does not explain. The results were obtained using a double-magneto-optic trap (MOT) system, which increases the number of atoms in the condensate by multiple transfers between two connected MOTs. Cornell's team also condensed both spin states of rubidium using a sympathetic cooling technique, and studies the interactions between them.
Using laser light to excite a specific spot on his cigar-shaped BEC of sodium atoms, Wolfgang Ketterle of MIT described how the resulting disturbance in a typical condensate propagates at about 5 millimeters per second, roughly 70,000 times slower than the speed of sound in air. In one experiment, two condensates were created in a double-well potential formed by magnetic and optical forces. After switching off the potential and letting the condensates expand and overlap, the MIT group observed high contrast matter-wave interference fringes, demonstrating that Bose condensed atoms are coherent and show long-range correlations. Ketterle presented videos of the world's first "atom laser," first announced in January 1997 - a device that produces coherent beams of atoms highly analogous to laser light beams.
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