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Zero-point motion in a Bose-Einstein condensate

Zero-point motion in a Bose-Einstein condensate has been quantitatively measured for the first time, allowing researchers, in effect, to study matter at a temperature of absolute zero. According to quantum mechanics, objects cooled to absolute zero do not freeze to a complete standstill; instead they jiggle around by some minimum amount. MIT researchers (Wolfgang Ketterle, 617-253-6815) measured such "zero-point motion" in a sodium BEC, a collection of gas atoms that are collectively in the lowest possible energy state (Update 233). According to Ketterle, "the condensate has no entropy and behaves like matter at absolute zero." The MIT physicists measured the motion (or lack thereof) by taking advantage of the fact that atoms absorb light at slightly lower (higher) frequencies if they are moving away from (towards) the light. To determine these Doppler shifts (100 billion times smaller than those of moving galaxies), the researchers used a technique known as Bragg scattering. In this technique, atoms absorb photons at one energy from a laser beam and are stimulated by a second laser to emit a photon at another energy which can be shifted upward or downward depending on the atoms' motion towards or away from the lasers. Measuring the range in energies of the emitted photons allowed the researchers to determine the range of momentum values in the condensate. Multiplying this measured momentum spread (delta p) by the size of the condensate (delta x) gave an answer of approximately h-bar (Planck's constant divided by 2 pi)-the minimum value allowed by Heisenberg's uncertainty relation and quantum physics. While earlier BECs surely harvested this zero-point motion, previous measurements of BEC momentum spreads were done with exploding condensates having energies hundreds of times larger than the zero-point energy. (J. Stenger et al., Physical Review Letters, 7 June 1999.)

Update 433
15 Jun 99


next up previous
Next: Quantum computers perform their Up: Atomic & Quantum Physics Previous: Helium atoms shoot down
Michael
2000-08-06