The twentieth century has seen a long succession
of Nobel Prizes for fundamental work that has explained the nature
of matter, radiation, and the interaction between the two.
Planck's discovery of energy quanta, Einstein's exposition of its
nature, and the understanding of the structure of the atom by a
legendary group of physicists that includes Bohr, Schrodinger,
Heisenberg, Pauli, and Dirac have all paved the way for the
accomplishments of modern phvsics. Countless others have stood on
their shoulders to develop entirely new fields of science,
spawning new technologies at ever higher rates. About 25,000
patents were granted by the US Patent and Trademark Office in the
year 1900; more than 120,000 were granted in 1996. <>The 1997
Nobel Prize for the development of laser cooling techniques is a
direct descendent of this historical lineage and of one of its
most remarkable products - the laser (see table on p. 104). Since
the discovery of maser and laser theories and techniques by
Townes, Prokorov, Basov, Schawlow, Gould, Maiman, and others,
scientists have had an unprecedented tool with which to understand
light and to probe matter. The discover that coherent, intense
beams of light used in one arrangement to cut metal can be used in
another to cool matter is another of its many delights and
surprises. "In terms of the impact on science," says Harold
Metcalf, professor of physics at the State University of New York
at Stony Brook, "these developments have been enormous."
Work in the modern era on the effects of photons on neutral atoms
took place in the early 1970s, by Arthur Ashkin at Bell
Laboratories (Holmdel, NJ) and by V. S. Letokhov and other
physicists in the former USSR. They realized that the radiation
pressure of a laser beam could be used to manipulate dielectric
particles, and some of their earliest applications involved
levitating such particles against an external force such as
gravity. This early work in "optical trapping" led to the
development of "optical tweezers" to manipulate small objects such
as cells.
In 1975 Arthur Schawlow and Theodor Hansch proposed to use
counter-propagating laser beams to cool neutral atoms, with a
related proposal from David Wineland and Hans Dehmelt for ions in
ion traps. The laser beams in their proposals had to account for
the movement of the atoms under study, requiring a "detuning" of
the beam, so that, when the Doppler effect was taken into account,
the photons would have the resonant energy to be absorbed by the
atom, slowing it with the momentum transfer and thus cooling it.
But as the atoms slow down, the laser photons must follow the
change in the required Doppler shift.
One technique, frequency-chirping, was proposed by Letokhov;
another was the "Zeeman slower" built by William D. Phillips and
Harold Metcalf, which used a scheme in which the atomic beam
propagated along the axis of a varying solenoidal magnetic field
so the Doppler and Zeeman shifts compensate and the resonant
transition frequency remains constant. In 1985 Phillips and his
coworkers at what was then the National Bureau of Standards (now
the National Institute of Standards and Technology, or NIST) used
an adaptation of his apparatus - which he had built as a graduate
student under Daniel Kleppner at Massachusetts Institute of
Technology (MIT; Cambridge, MA) and inherited when he left - in a
technique to stop an atomic beam and trap the atoms in a magnetic
trap. "It was important that I had that apparatus, said Phillips,
"because it gave me a base from which to start." |
