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Condensate divided? Quantum unity stands.

Atoms trapped in the remarkable form of matter known as a Bose-Einstein condensate share one quantum mechanical state, behaving collectively as a single superatom (SN: 7/25/98, p. 54). A new experiment demonstrates how robust, and possibly useful, that coherence can be, even when the superatom is minced into many parts, physicists say.

Although the first condensates were created in 1995 (SN: 7/15/95, p. 36), not until last year did physicists at the Massachusetts Institute of Technology prove they could manipulate a condensate as a coherent entity (SN: 2/1/97, p. 71). Taking a stopwatch to the phenomenon, experimenters at the University of Colorado and the National Institute of Standards and Technology, both in Boulder, reported in the Aug. 24 Physical Review Letters that coherence can last surprisingly long--at least 100 milliseconds.

Now, Brian P. Anderson and Mark A. Kasevich at Yale University have split a condensate of ultracold rubidium atoms into roughly 30 parts without dashing their shared identity. "It's a beautiful and dramatic demonstration of what makes Bose-Einstein condensates so special--their coherence," says MIT's Wolfgang Ketterle.

In the Yale team's experiment, reported in the Nov. 27 Science, a vertical laser beam creates a standing wave, or series of peaks and troughs of light intensity, across the 15-micron-tall condensate. Acting as an "optical lattice," the laser wave nudges atoms to the nearest intensity peaks and suspends the condensate portions in layers "like a stack of pancakes," Anderson says.

Shutting off the magnetic fields that originally trapped the condensate, the researchers observe drips of about 1,000 atoms apiece that detach themselves all along the lattice and fall away. After about 10 such pulses, 1.1 milliseconds apart, the lattice runs out of atoms.

The formation of the regularly timed pulses shows that the layers within the lattice retain the original coherence of the condensate, the researchers maintain. Because of the wavelike behavior of coherent layers, the optical planes of the lattice can reflect them back when gravity pulls them downward. As the atoms rebound, a fraction of them are forced into a higher energy not allowed in the lattice, break free, and fall together as an atomic laser pulse.

The Yale researchers have also shown that they can precisely measure gravity by observing the rate at which the pulses fall.

The team interprets the oscillation within the lattice that creates the pulses as the first evidence in a condensate of a phenomenon called the Josephson effect. In the effect, previously seen in superconductors and superfluids and of wide use in electronics, an oscillating flow of particles passes through a barrier. In this instance, planes defined by the optical laser act as barriers to the motion of atoms.
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Title Annotation:Bose Einstein condensate is manipulated in experiment
Author:Weiss, P.
Publication:Science News
Article Type:Brief Article
Date:Nov 28, 1998
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