Taking the Temperature of Dark-State Atoms

In the October 27, 1997, issue of Physical Review Letters, Claude Cohen-Tannoudji and his colleagues at the Ecole Normale Superieure in Paris present a new way to explore the coldest realm in the universe. One problem in this line of research is that traditional methods of thermometry have failed to measure how truly low the temperatures are for the large clumps of ultracold atoms in the "dark state." Traditionally, physicists have used "time-of-flight" methods: by allowing a cloud of ultracold atoms to expand freely and measuring how quickly the cloud expands, the researchers estimate the range of velocities in the gas atoms; a narrower range corresponds to a colder temperature. But for large clouds of ultracold atoms, including those in the dark state, the clouds expand too imperceptibly for researchers to make good temperature measurements.

To create the dark state, researchers first trap and cool helium atoms using a combination of laser beams and magnetic fields. Then, two laser beams traveling in opposite directions put each atom into a combination or "superposition" of two low-energy states that interfere with each other so as to prevent the atoms from absorbing or emitting laser light. This is important since a helium atom absorbing or emitting a single photon recoils by 9.2 cm/second, corresponding to a temperature of 4 microkelvins. Oblivious to photons, atoms in dark states can have temperatures well below this "single photon recoil limit." To determine these "subrecoil" temperatures more precisely, Cohen-Tannoudji's group probes the wavelike properties in the group of atoms. Each dark-state atom can be thought of as a superposition of two "wavepackets," corresponding to the two low-energy states which interfere to prevent light absorption.

Associated with the two wavepackets are two equal and opposite momentum states characterizing the movement of the atom as a whole; in effect the atom is moving in two opposite directions at the same time. As long as the dark state lasers are on, these two wavepackets are constantly superimposed. But when the researchers turn off the lasers in their experiment, the two wavepackets fly apart. A subsequent laser pulse applied after a certain time measures the various degrees of overlap in the wavepacket pairs that make up the cloud of atoms, allowing the researchers to measure the momentum (and therefore velocity) distribution of the atoms and thereby the temperature as well.

Applying this technique to subrecoil helium atoms, the researchers have measured a temperature (at least in the one dimension probed by their laser) of 5 nanokelvins, 1/800 of the recoil limit. This is the lowest fraction of the recoil temperature ever measured for an atom; the lowest absolute temperature, 3 nanokelvins for much heavier cesium atoms, was measured by the same group in 1995.

[Item courtesy of Philip F. Schewe and Ben Stein of AIP Public Information.]