Trapping cold atoms in microwave webs - microwave radiation used to trap neutral cesium atoms - Brief Article

Science News,  May 21, 1994  by Ivars Peterson

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Dramatic progress in the use of laser light along with various combinations of magnetic and electric fields to cool and trap neutral atoms has opened the possibility of studying exotic forms of matter.

For example, theorists have postulated that a gas of hydrogen atoms at high enough densities and low enough temperatures may undergo a transition known as Bose-Einstein condensation. The result would be a peculiar substance -- unlike any known form of matter -- in which a large fraction of the atoms are in the same quantum state.

Now, researchers have demonstrated that microwave radiation can trap neutral cesium atoms. The success of this particular type of atomic trap offers the possibility of capturing sufficiently large numbers of hydrogen atoms at temperatures near 1 microkelvin to achieve Bose-Einstein condensation.

Physicists Isaac F. Silvera and M.W. Reynolds of Harvard University and their collaborators at the National Institute of Standards and Technology in Gaithersburg, Md., report their results in the May 16 PHYSICAL REVIEW LETTERS.

According to quantum mechanics, particles come in two varieties. Fermions -- which include electrons, protons, and neutrons -- have spins measured in fractions of a quantum unit. Bosons, which include photons and certain atoms such as hydrogen, have whole-number spins.

Whereas no two interacting fermions can occupy the same quantum state, no such restriction limits bosons. Therefore, a sufficiently cold, dense collection of bosons, losing their individual identities, should undergo Bose-Einstein condensation to a single quantum state.

For more than a decade, physicists have struggled to create this state, and various groups throughout the world have tried different methods of chilling and capturing hydrogen atoms. These efforts have so far proved inadequate for achieving Bose-Einstein condensation.

In a test of a new approach, Silvera and his coworkers load neutral cesium atoms cooled to approximately 4 microkelvins into a small spherical cavity machined from steel. The slowly moving cesium atoms fill a trap defined by a web of laser light and magnetic fields at the cavity's center. This trap is then switched off and replaced by a new magnetic field and microwaves of a certain frequency, which keep the atoms in their lowest-energy spin state.

"You find that the atoms don't escape," Silvera says. "We've demonstrated that the microwaves can actually confine atoms and work as a new type of trap. We want to do it for atomic hydrogen now."

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