PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 305 January 27, 1997
by Phillip F. Schewe and Ben Stein
A RUDIMENTARY ATOM LASER has been created at MIT,
promising significant improvements in high precision
measurements with atoms and offering the prospect of future
nanotechnology applications, such as atom lithography, in which
lines are drawn on integrated circuits (by directly depositing
atoms) with greater precision than ever before. In an atom laser
the output beam consists of a single coherent atom wave, just as
in a regular laser the beam consists of a coherent light wave.
The working substance for the atom laser is a Bose-Einstein
condensate (BEC) of sodium atoms, cooled and contained within
an atom trap by a shaped magnetic field. BEC itself was
achieved for the first time only as recently as 1995 (see Update
233). It is a condition in which atoms are chilled to such low
energies that, in a wavelike sense, the atoms begin to overlap and
enter into a single quantum state.
Wolfgang Ketterle and his colleagues at MIT make their claim
of producing the first atom laser on the basis of two experimental
developments, as reported in two journals this week. In the first
effort (M.-O.Mewes et al., Physical Review Letters, 27 January
1997) a portion of a sodium condensate was successfully
extracted under controlled conditions. They achieve "output
coupling" by applying radiofrequency radiation to the BEC; this
"tips" the atoms' spins by an adjustable amount, putting the
atoms in a superposition of quantum states. Thereafter some of
the atoms feel the effect of the surrounding magnetic field in a
different way and are able to leave the atom trap. It is these
departing atoms, still enjoying the coherent properties of the BEC
state, that constitute an atom laser beam. Pulled downward by
gravity, the beam was observed over a distance of millimeters,
although in principle it could travel further in an undisturbed
vacuum environment.
The second development was to verify that the atom waves are
indeed coherent (M.R. Andrews et al., Science, 31 January
1997). At the time of the original BEC discovery, many
physicists expected the atoms in the condensate to fall into a
single quantum state; some hypothesized that it could take a time
equal to the age of the universe for true coherence to come about.
The MIT group addressed this issue by creating two BEC clouds
in a special trap. Turning off the trap allows the clouds to
expand, overlap, and interfere, producing a pattern of light and
dark fringes. The observed patterns (viewed with an electronic
camera) could only exist if each BEC was an intense coherent
wave. The MIT team determined that the atom wave associated
with each BEC had a wavelength of 30 microns, a million times
larger than the wavelengths associated with room-temperature
atoms.
In addition to coherence, the atom laser waves are analogous to
the light waves in an optical laser in another respect as well. Just
as a laser beam is more intense than an equivalent stream of light
from the Sun, the MIT atom beam is also more intense (for a
given beam spotsize) than ordinary atom beams (whose atoms
possess a variety of energies) since it delivers a powerful,
directional stream of atoms in a single quantum state. In other
ways, the atom lasers and light lasers are different. According to
Ketterle, "Photons can be created but not atoms. The number of
atoms in an atom laser is not amplified. What is amplified is the
number of atoms in the lowest-energy quantum state, while the
number of atoms in other states decreases."
Graphics and more text can be viewed on the World Wide Web
at this address: www.aip.org/physnews/special.htm.
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