Chu92
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'gently enough to have gravity turn them<br />
around. Atoms for the fountain are col-<br />
lected by a magneto-optic trap fck 0.5<br />
second. After that amount of time,<br />
about 10 million atoms are launched<br />
upward at a velocity of roughly two<br />
meters per second. At the top of the<br />
trajectory, an atom is probed with two<br />
pulses of microwave radiation separat-<br />
ed in time. If the frequency of the radi-<br />
ation is properly tuned, the two pulses<br />
cause the atom to change from one<br />
quantum state to another. (Norman<br />
Ramsey shared the Nobel Prize in Phys-<br />
ics in 1989 for inventing and applying<br />
this technique.) In our first experiment<br />
we measured the energy difference be-<br />
tween two states of an atom with a res-<br />
ohrtion of two parts in 100 billion.<br />
How does the fountain make such<br />
precise measurements possible? First,<br />
the atoms fall freely and are easy to<br />
shield from any perturbation that might<br />
alter their energy levels. Second, such<br />
measurements are limited in precision<br />
by the Heisenberg uncertainty principle.<br />
This principle states that the resolution<br />
of an energy measurement will be limit-<br />
ed to Planck's constant divided by the<br />
time of the "measurement." to our case,<br />
this time corresponds to the time be-<br />
tween the two microwave pulses. With<br />
an atomic fountain the measurement<br />
time for unperturbed atoms can be as<br />
long as one second, a period impossible<br />
with atoms at room temperature.<br />
Because the atomic fountain allows<br />
extremely precise measurements of the<br />
energy levels of atoms, it may be possi-<br />
ble to adapt the device to make an im-<br />
proved atomic clock. At present, the<br />
world time standard is defined by the<br />
energy difference between two particu-<br />
lar energy levels in ground states of the<br />
cesium atom. Two years after the first<br />
atomic fountain, the group at the ficole<br />
Normale used a fountain to measure<br />
the "clock transition" in the cesium<br />
atom with high precision. These two<br />
experiments suggested that a properly<br />
engineered instrument might be able to<br />
measure the absolute frequency of this<br />
transition to one part in 1016, 1,000<br />
times better than the accuracy of our<br />
best clocks. Lured by this potential,<br />
more than eight groups around the<br />
world are now trying to improve the<br />
cesium time standard with an atomic<br />
fountain.<br />
A<br />
nother application being intensively<br />
studied is atom interferometry.<br />
The first atom interferometers<br />
were built in 1991 by investigators<br />
at the University of Konstanz,<br />
M.I.T., the Physikallsch-Technische Bundesanstalt<br />
and Stanford.<br />
An atom interferometer splits an<br />
SAMPLE<br />
OPTICAL TWEEZERS can manipulate microscopic objects such as cells. A ample is<br />
placed on the stage of a microscope, which has been adapted to admit green laser<br />
light and infrared laser radiation. The green light illuminates the sample while the<br />
infrared radiation traps and holds it.<br />
atom into two waves separated in space.<br />
The two parts of the atom are then re-<br />
combined and allowed to interfere with<br />
each other. The simplest example of<br />
such a splitting occurs when the atom<br />
is made to go through two separated<br />
mechanical slits. If the atom is recom-<br />
bined after passing through the slits,<br />
wavelike interference fringes can be<br />
observed. The interference effects from<br />
atoms dramatically demonstrate the<br />
fact that their behavior needs both a<br />
wave and a particle description.<br />
More important, atom interferometers<br />
offer the possibility of measuring phys-<br />
ical phenomena with high sensitivity.<br />
In the first demonstration of the poten-<br />
tial sensitivity, Mark Kasevich and I have<br />
created an interferometer that uses slow<br />
atoms. The atoms were split apart and<br />
recombined in a fountain. With this in-<br />
strument we have already shown that<br />
the acceleration of gravity can be mea-<br />
sured with a resolution of at least three<br />
parts in 100 million, and we expect an-<br />
other 100-fold improvement shortly.<br />
Previously, the effects of gravity on an<br />
atom have been measured at a level of<br />
roughly one part in 100.<br />
In recent years the work on atom<br />
trapping has stimulated renewed inter-<br />
est in manipulating other neutral parti-<br />
cles. The basic principles of atom trap-<br />
ping can be applied to micron-size par-<br />
ticles, such as polystyrene spheres. The<br />
intense electric field at the center of a<br />
focused laser beam polarizes the parti-<br />
cle, just as it would polarize an atom.<br />
The particle, like an atom, will also ab-<br />
sorb light of certain frequencies. Glass,<br />
for example, strongly absorbs ultravio-<br />
let radiation. But as long as the light is<br />
tuned below absorption frequency, the<br />
particle will be drawn into the region<br />
of highest laser intensiq,<br />
to 1986 Ashkin, Bjorkholrn, J. B.<br />
Dziedzic and I showed that particles<br />
that range in size between 0.02 and 10<br />
microns can be trapped in a single<br />
focused laser beam. In 1970 Ashkin<br />
trapped micron-size latex spheres sus-<br />
pended in water in between two fo-