One starts with a beam
of hot sodium atoms, glowing like a pale orange
street lamp. The atoms are collected in a vacuum
chamber and subjected to various cooling methods, one
of which involves directing laser light at the atoms
to slow their motions. In this bath of laser light
and radiating atoms, the system shines brilliantly
like a miniature Sun. In the chamber, about 10 billion
sodium atoms float almost mysteriously, suspended by
magnetic fields and lasers. At this point, the temperature of
the tiny, millimeter-sized cloud of atoms
is a millikelvin.6
Next, the laser beams are turned off, and
the dark, cigar-shaped (0.2-millimeter long
and 0.02-millimeter wide) shadow of the cloud
can be seen. Its temperature is
only 50 microkelvins.1 The
sample is so small that lenses must be used for
focusing, and microscopes are needed for
imaging.
The final cooling process, which
takes only about a minute, is
through evaporation.7
Only the coldest two million sodium atoms remain. At
a temperature
below 435 nanokelvins,2 a
dramatic change takes place: A Bose-Einstein condensate
forms. The individual atoms loose
their identity and dissolve into a kind of
global quantum macromolecule.
At temperatures below a microkelvin, the
suspended sodium atoms
are opaque. Light is unable to penetrate them. One
is unable to "see" through them, much like the way
that one cannot see through a block of lead. But
then the "miracle" of electromagnetically induced
transparency is performed. Laser light of a
particular color is directed onto the cloud of sodium atoms. Lo
and behold, the cloud becomes
transparent. It is an alchemistic feat. It is
like turning lead into plastic. When a pulse of
light of a certain color is sent through the
sodium atoms, it is no longer completely absorbed. A
fraction (typically 25%) of it is able to pass through.
How does this miracle
work? In general, light is absorbed by a substance
when an electron in an atom of the substance
absorbs a photon4
of the light. As a
consequence, the electron gains the photon's
energy and jumps to a higher energy level in the
atom. When the sodium atoms are
subjected to laser light of a particular color, electrons
cannot easily make the transition to the higher
energy levels due to a quantum interference effect: A
cancellation occurs in the interacting coupled
laser-atom system to render the probability of
a transition quite low. Since absorption by
electrons of light is inhibited, light is able
to pass through the collection of sodium atoms. Without this
effect, the sodium atoms
would be as optically impenetrable as
a block of lead.
A brief
pulse of light of a certain color is shot
at the system. It travels
at 186,000 miles per second in the vacuum
chamber. When it strikes the medium of sodium atoms, it
comes to a grinding halt. The length of the pulse
collapses 20-million fold. The pulse struggles
to pass through the unusual medium, moving at
only 38 miles per hour when the system is at 50
nanokelvins. But in a small fraction
of a second later, it reaches the end of the
tiny collection of atoms. Liberated, it speeds
off at 186,000 miles per second again.
Copyright ©1999
by Jupiter Scientific,
the publishers
of
The Bible According to Einstein
and
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