stores light pulse
Technology Research News
A year ago, two research teams independently
announced that they had stored light pulses in the atoms of gases and
then reconstituted the stored pulses. A third research team has accomplished
the same feat using a solid material, a crystal that could eventually
be used to make quantum computer memory chips.
Quantum computers would theoretically be much faster than today's classical
computers in solving certain problems like cracking secret codes, but
are difficult to build because quantum information is extremely fragile.
Being able to store quantum information for relatively long periods of
time would go a long way toward making practical quantum computers feasible.
"Most quantum processors require storage," said Philip Hemmer an associate
professor of physics at Texas A&M University. "For quantum storage, the
advantages of the crystal [over a gas] are a much larger storage capacity,
potentially much longer storage times, and the relative ease of incorporating
[it] into a system," he said.
Hemmer's research team from the U.S. and South Korea was able to store
light pulses in a crystal for a few tenths of a millisecond, which is
comparable to what the previous experiments accomplished using gases,
The researchers stored light in a yttrium silicate crystal with small
amounts of the rare earth metal praseodymium added to it. Light doesn't
travel through opaque matter because its photons are absorbed by the material's
The researchers fired a control laser beam into the crystal's atoms in
order to overload them with photons. At the same time, they sent a weaker
pulse of light of a different frequency into the crystal. The interaction
between this weaker light pulse and the crystal's overloaded atoms introduced
drag, which slowed the pulse to about 45 meters per second, or 100 miles
per hour. Light travels through a vacuum at 186,000 miles per second.
When the researchers turned the control laser beam off, the slowed light
pulse disappeared, but left an impression in the crystal's atoms. When
the researchers turned the control beam back on, the pulse was reconstituted
from the information stored in the atoms and it continued through the
The technique could eventually be used to store quantum information.
Quantum particles can be in one of two complementary states. For example,
photons can be polarized vertically or horizontally and atoms can be spinning
in one of two directions, up or down.
Using those states to represent the ones and zeros of digital information,
the particles can serve as quantum bits, or qubits. The qubits that represent
the output of a quantum processor could be transferred to photons that
could then be sent to a quantum memory chip where the qubits could be
transferred to atoms in the chip for storage.
The researchers have a ways to go before they produce a quantum memory
chip. In their experiment, they stored a light pulse consisting of many
photons. "The next steps will be to attempt storage of single photons,
and to improve the efficiency to be close to 100 percent," said Hemmer.
It could be 10 years before the light storage technique is used in practical
applications, he said.
"Slow light and light storage in solids are very exciting," said David
Phillips, a physicist at the Harvard-Smithsonian Center for Astrophysics.
"This experimental demonstration brings us a step closer to the era of
serious applications of the underlying physical concepts. While these
materials still require cryogenic temperatures to show the coherence times
necessary for light storage, perhaps more easily utilized materials will
be developed in the future," he said.
Hemmer's research colleagues were Alexey Turukhin and Sudi Sudarshanam
of the Massachusetts Institute of Technology, Selim Shahriar, now at Northwestern
University, Joe Musser of Texas A&M University, Byoung Ham of the Electronics
and Telecommunications Research Institute in South Korea. They published
the research in the January 14, 2002 issue of the journal Physical Review
Letters. The research was funded by the Air Force Research Laboratory,
the Army Research Office, the Air Force Office of Scientific Research,
and the Korean Ministry of Science and Technology.
Timeline: 10 years
TRN Categories: Quantum Computing
Story Type: News
Related Elements: Technical paper, "Observation of Ultraslow
and Stored Light Pulses in a Solid," Physical Review Letters, January
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