IN TINY SUPERCOOLED CLOUDS, PHYSICISTS EXCHANGE LIGHT AND MATTER
Technique may give scientists a new degree of control over fiber-optic
communication and quantum information processing
CAMBRIDGE, Mass. -- Physicists have for the first time stopped and
extinguished a light pulse in one part of space and then revived it in a
completely separate location. They accomplished this feat by completely
converting the light pulse into matter that travels between the two
locations and is subsequently changed back to light.
Matter, unlike light, can easily be manipulated, and the experiments
provide a powerful means to control optical information. The findings,
published this week by Harvard University researchers in the journal
Nature, could present an entirely new way for scientists and engineers
to manipulate the light pulses used in fiber-optic communications, the
technology at the heart of our highly networked society.
"We demonstrate that we can stop a light pulse in a supercooled sodium
cloud, store the data contained within it, and totally extinguish it,
only to reincarnate the pulse in another cloud two-tenths of a
millimeter away," says Lene Vestergaard Hau, Mallinckrodt Professor of
Physics and of Applied Physics in Harvard's Faculty of Arts and Sciences
and School of Engineering and Applied Sciences.
Hau and her co-authors, Naomi S. Ginsberg and Sean R. Garner, found that
the light pulse can be revived, and its information transferred between
the two clouds of sodium atoms, by converting the original optical pulse
into a traveling matter wave which is an exact matter copy of the
original pulse, traveling at a leisurely 200 meters per hour. The matter
pulse is readily converted back into light when it enters the second of
the supercooled clouds -- known as Bose-Einstein condensates -- and is
illuminated with a control laser.
"The Bose-Einstein condensates are very important to this work because
within these clouds atoms become phase-locked, losing their
individuality and independence," Hau says. "The lock-step nature of
atoms in a Bose-Einstein condensate makes it possible for the
information in the initial light pulse to be replicated exactly within
the second cloud of sodium atoms, where the atoms collaborate to revive
the light pulse."
Within a Bose-Einstein condensate -- a cloud of sodium atoms cooled to
just billionths of a degree above absolute zero -- a light pulse is
spatially compressed by a factor of 50 million. The light drives a
controllable number of the condensate's roughly 1.8 million sodium atoms
to enter into quantum superposition states with a lower-energy component
that stays put and a higher-energy component that travels between the
two Bose-Einstein condensates. The amplitude and phase of the light
pulse stopped and extinguished in the first cloud are imprinted in this
traveling component and transferred to the second cloud, where the
recaptured information can recreate the original light pulse.
The period of time when the light pulse becomes matter, and the matter
pulse is isolated in space between the condensate clouds, could offer
scientists and engineers a tantalizing new window for controlling and
manipulating optical information; researchers cannot now readily control
optical information during its journey, except to amplify the signal to
avoid fading. The new work by Hau and her colleagues marks the first
successful manipulation of coherent optical information.
"This work could provide a missing link in the control of optical
information," Hau says. "While the matter is traveling between the two
Bose-Einstein condensates, we can trap it, potentially for minutes, and
reshape it -- change it -- in whatever way we want. This novel form of
quantum control could also have applications in the developing fields of
quantum information processing and quantum cryptography."
Ginsberg, Garner, and Hau's work was supported by the Air Force Office
of Sponsored Research, the National Science Foundation, and the National
Aeronautics and Space Administration.