Catching the ‘Atom Wave’
Rice University physicists have shown for the first time that ultracold
atoms can form bright “solitons,” localized bundles
of waves that maintain a constant shape as they propagate. Solitons
of light are used in ultrahigh-speed optical communication networks
because they can carry data over great distances without the use
of signal boosters. At the atomic level, solitons could further
the development of new forms of atom lasers.
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The experiments involve a Bose–Einstein condensate (BEC),
a collection of atoms that is cooled to the point where the mysterious
and counterintuitive forces of quantum mechanics take over, causing
the atoms to lose their individual identities and behave as a single
collective wave. To create a BEC, physicists tightly confine atoms
in a magnetic field and cool them using lasers and evaporation until
they reach a temperature that is about one billion times colder
than room temperature.
In 1995, a research group led by Randall Hulet, the Fayez Sarofim
Professor of Physics and Astronomy, created the first BEC from lithium
atoms, something some theorists had predicted could not be done
because of the attractive nature of the atoms. Further study of
this novel BEC system led to direct observations of the growth and
collapse of the condensate. This provided new insights into weakly
interacting Bose gases and laid the groundwork for the soliton experiments
just completed. The new research is described in the May 9 issue
of Nature.
Like any confined wave, BECs are fragile and tend to disperse quickly
when released from confinement. In the latest experiments, Rice
scientists trapped atoms from a BEC in a narrow beam of light that
only allowed the atoms to move in a single-file line. By causing
the atoms to attract each other, the physicists were able to create
atomic solitons, atom waves whose self-attraction balances perfectly
with their tendency to disperse.
In the world of optics, solitons of light have been created by sending
light pulses down specially designed optical fibers. Unlike typical
data in telecommunications networks, which must be reinforced with
“repeaters” that boost the signal at regular intervals,
soliton signals don’t disperse and become weaker as they travel
down the fiber.
In the latest experiments, Rice’s BEC researchers observed
atomic “soliton trains,” which are groups of as many
as 15 solitons lined up end-to-end. These solitons were observed
to propagate for several seconds—an eternity for a localized
wave bundle—without spreading.
The techniques that are being developed to control matter in BEC
experiments could eventually be used to perform extremely precise
measurements. For example, the same principle that makes lasers
useful in interferometric fiber-optic gyroscopes could be applied
with atom lasers to form instruments that are millions or perhaps
billions of times more sensitive.
“Forty years ago, no one imagined that lasers would be used
to play music in our cars or scan our food at the grocery store
checkout,” Hulet says. “BEC researchers are in a similar
situation. We’re getting our first glimpse of a wondrous and
sometimes surprising set of dynamic quantum phenomena, and there’s
no way to know exactly what may come of it.”
—Jade Boyd
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