Very cool. Two laser images of a cigar-shaped BEC (below) and its reflection (above) off a silica plate (dashed line).

At BEC and Call

Imagine a force so weak that it's barely strong enough to nudge a single atom. Now imagine trying to detect it. Researchers have done just that using a clump of ultracold atoms called a Bose-Einstein condensate (BEC). The measurement is the first to use a BEC to detect anything at all, and it hints that BECs may soon find diverse uses, from gravity sensors to supremely accurate clocks.

When a group of identical atoms is chilled to just a few billionths of a degree above absolute zero, they all clump into the same quantum state, forming a BEC. The condensed atoms are indistinguishable and behave sort of like a single giant particle--albeit one with decidedly weird quantum characteristics. "The BEC is for atoms what the laser is for light," explains Wolfgang Ketterle, a physicist at the Massachusetts Institute of Technology in Cambridge. A laser is a light source in which all the photons have exactly the same energy; a BEC is a matter source in which all the atoms have exactly the same energy.

In the 15 September issue of Physical Review A, Eric Cornell and colleagues at the University of Colorado, Boulder, report how they used a BEC to detect the Casimir-Polder force. The force is caused by tiny electric fields that pop in and out of existence in a vacuum. These fields weaken near a surface--say, a silica plate. The stronger electric field out in the vacuum will push an atom toward the silica, similar to wind blowing a leaf on a gusty day--but much, much weaker.

The movement of a single atom is very hard to measure, so the team substituted a BEC superatom. The scientists suspended a BEC shaped like a cigar a few micrometers thick in a magnetic field, several micrometers away from a silica plate. As the cigar BEC oscillated in the magnetic field, the Casimir-Polder force pulled very slightly on the atoms nearest the plate, causing them to drag and throwing off the frequency of the quantum cigar's jiggling. By comparing it to the frequency of the BEC cigar when it was far away from the plate, the researchers figured out how much the Casimir-Polder force was dragging on the BEC.

"The paper is very nice; it's a very interesting application of BECs," says Ketterle, who was not involved in the research. He predicts that BECs might develop a whole repertoire of uses: for example, atom interferometers, which could measure matter waves similarly to the way laser interferometers measure light waves. Because matter waves could be detected extraordinarily precisely, this could lead to extremely accurate clocks and precision measurement devices.

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