Technology Research News
Controlling interactions among individual
particles of light and matter could give rise to phenomenally powerful
quantum computers and devices that provide perfectly secure communications.
will need to transfer information stored in photons, which are easy to
transmit, and atoms, which is easier to use for calculations.
Researchers at the Harvard-Smithsonian Center for Astrophysics have taken
their second step this year toward this goal. In January, they brought
a light pulse to a halt inside a chamber of gas atoms, stored an imprint
of the pulse in the atoms and then reconstituted the pulse. Now they have
figured out how to alter the light information as it is stored in the
group of atoms.
This is possible because the process preserves the phase of the stored
light, said Phillips. "The phase of the light is transferred onto the
phase of the atoms and back to the light during the light storage process,"
This phase information can represent the ones and zeros of computing.
The phase of a lightwave corresponds to its position in the cycle between
the crest and trough. Individual photons also contain wave phase information.
An atom's phase is different. It is "related mathematically to the phases
of a child's top or a gyroscope as it rotates on its axis and precesses,"
said Phillips. If you set a top spinning on a post, then tip the top onto
its side, instead of falling off the post it will hang there sideways,
rotating, or precessing, around the post. The phase of a precessing top
is its position in the circle it makes as it travels around the top of
The researchers found that the phase information of the light pulse remains
stable and accessible when it is imprinted in the atoms: if the light
pulse is in one phase when it is stored in the atoms, the pulse remains
in that phase when it is restored.
This makes it possible to change the phase while the pulse information
is stored. "We can apply a magnetic field to our atoms during the storage
process to shift the phase of the atoms," which in turn changes that phase
of the reconstituted light, said Phillips.
So far the researchers have only stored ordinary light beams using the
technique. However, demonstrating control over the phase of the light
opens the door for using the technique to coax the quantum properties
of particles to do computing.
Being able to store and manipulate particle properties like phase paves
the way for building devices that store and transmit this quantum information.
Quantum repeaters, for example, could restore the quantum information
in photons, which begins to destabilize after traveling 10 kilometers
or so through fiber-optic communications lines. Like repeaters in conventional
computer networks, quantum repeaters would make it possible to send quantum
information over much longer distances. Phillips' Harvard colleague Mikhail
Lukin and researchers at the University of Innsbruck in Austria have designed
a quantum repeater based on the light storage technique.
Many researchers say it is likely to take decades for full-blown quantum
computers to become practical. It may be possible to use quantum information
for cryptography sooner, however, said Phillips. "The light storage technique
could prove useful as part of a quantum repeater in such a system. I would
be surprised if the techniques involved in stored light moved out of the
academic lab and into the development lab in less than five years, though,"
The researchers' next step is using the technique to store the quantum
information from a single photon, said Phillips.
Phillips' research colleagues were Lukin, Alois Mair, Jean Hager and Ronald
L. Walsworth of the Harvard-Smithsonian Center for Astrophysics. The research
was funded by the National Science Foundation (NSF), the Office of Naval
Research (ONR) and NASA.
Timeline: >20 years
TRN Categories: Quantum Computing
Story Type: News
Related Elements: Technical paper, "Phase Coherence and
Control of Stored Photonic Information," posted on the arXiv physics archive
Technical paper, "Long-Distance Quantum Communication with Atomic Ensembles
and Linear Optics," posted on the arXiv physics archive at http://arXiv.org/abs/quant-ph/0105105
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