Chu92
Chu92
Chu92
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Going beyond optical molasses, Co-<br />
hen-Taimouai, Alain Aspect, Ennip Ari-<br />
mondo, Robin Kaiser sod., Nathalie Van-<br />
steoddste, then all at the Ecole Normale,<br />
invented an ingenious scheme capable<br />
of cooling helium atoms below the re-<br />
coil velocity of a single scattered pho-<br />
ton. Helium atoms have been cooled to<br />
two microkelvins along one dimension,<br />
and work is under way to extend this<br />
technique to two and three dimensions.<br />
This cooling method captures an<br />
atom in a well-defined velocity state in<br />
much the same way atoms were trapped<br />
in space in our fast optical trap. As the<br />
atom scatters photons, its velocity ran-<br />
domly changes. The French experiment<br />
establishes conditions that allow an<br />
atom to recoil and land in a particular<br />
quantum state, which is a combination<br />
of two states with two distinct velod-<br />
ties close to zero. Once in this state,<br />
the chance of scattering more photons<br />
is greatly reduced, meaning that addi-<br />
tional photons cannot scatter and in-<br />
crease the velocity. If the atom does<br />
not happen to land in this quantum<br />
state, it continues to scatter photons<br />
and has more opportunities to seek out<br />
the desired low-velocity state. Thus, the<br />
atoms are cooled by lett&lg tiasai ran-<br />
ddy walk into a "velocity trapped"<br />
quantum state.<br />
Besides the cooling and trapping of<br />
atoms, investigators have demonstrat-<br />
ed various atomic lenses, mirrors and<br />
diffraction gratings for manipulating<br />
atoms. They have also fashioned de-<br />
vices that have no counterpart In light<br />
optics. Researchers at Stanford and the<br />
University of Bonn have made "atomic<br />
funnels" that transform a collection of<br />
hot atoms into a well-controlled stream<br />
of cold atoms. The Stanford group has<br />
also made an "atomic trampoline" in<br />
which atoms bounce off a sheet of light<br />
extending out from a glass surface. With<br />
a curved glass surface, an atom trap<br />
based on gravity and light can be made.<br />
Clearly, we have learned to push<br />
atoms around with amazing facility, but<br />
what do all these tricks enable us to do?<br />
With very cold atoms in vapor form,<br />
physicists are in a position to study how<br />
the atoms interact with one another at<br />
extremely low temperatures. According<br />
to quantum theory, an atom behaves<br />
like a wave whose length is equal to<br />
Planck's constant divided by the parti-<br />
cle's momentum. As the atom is cooled,<br />
COILS GENERATE MAGNETIC RELD<br />
its momentum decreases,<br />
creasing its wavelength. At sufficiently<br />
low temperatures, the average wave-<br />
length becomes comparable to the av-<br />
erage distance between the atoms. At,<br />
these low temperatures and high densi-<br />
ties, quantum theory says that a sig-<br />
nificant fraction of all the atoms will<br />
condense into a single quantum ground<br />
state. This unusual form of matter,<br />
called a Bose-Einstein condensation, has<br />
been predicted but never observed in a<br />
vapor of atoms. Thomas J. Greytak and<br />
Daniel Kleppner of M.LT. and Jook T. M.<br />
Walraven of the University of Amster-<br />
dam are trying to achieve such a con-<br />
densation with a collection of hydrogen<br />
atoms in a magnetic trap. Meanwhile<br />
other groups are attempting the same<br />
feat in a laser-cooled sample of alkali<br />
atoms such as cesium or lithium.<br />
Atom-manipulation techniques are<br />
also offering new opportunities in high-<br />
resolution spectroscopy. By combining<br />
several such techniques, the Stanford<br />
group has created a device that will al-<br />
low the spectral features of atoms to<br />
be measured with exquisite accuracy.<br />
We have devised an atomic fountain<br />
that launches ultra-cold atoms upward<br />
ATOMIC FOUNTAIN allows preelse measurements of the en- several light beams. After about 10 million atoms have accu-<br />
ergy states of atoms. Atoms are injected into the apparatus undated in die trap, the atoms are launched upward. At the<br />
and slowed by a laser beam. The atoms are then captured top of the tratectory, microwave pulses excite the atom from<br />
and cooled by the combined effects of a mgiwtic field and one enemy state to another.<br />
74 SCIENTIFIC AMERICAN February 1992