John Toon
Research News
A team of physicists at Georgia Tech has taken a significant
step toward the development of quantum communications
systems by successfully transferring quantum information
from two different groups of atoms onto a single photon.
The work, reported in the October 22 issue of the journal
Science, represents a “building
block” that could lead to development of large-scale
quantum networks. Sponsored by the Research Corporation
and NASA, the work is believed to be the first to demonstrate
transfer of quantum information from matter to light.
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Alex
Kuzmich (left) and Dzmitry Matsukevich operate
optical equipment used to transfer information
from two different groups of atoms onto a single
photon. |
The researchers, Assistant Professor Alex Kuzmich and
graduate student Dzmitry Matsukevich — both from
the School of
Physics — report transferring atomic state
information from two different clouds of rubidium atoms
to a single photon. In the photon, information about
the spatial states of the atom clouds was represented
as vertical or horizontal optical polarization.
“A really big issue in quantum information systems
today is distributed quantum networks. For that, you
must be able to convert quantum bits of information
based on matter into photons,” Kuzmich said. “This
is the first step, one building block. What we have
done is create a quantum network node, and now the next
step is to create a second quantum network node and
connect them.”
Quantum bits, or qubits, are very different from the
bits in conventional computing. Unlike conventional
bits that exist in either a 0 or 1 state, qubits can
simultaneously exist in both states. Qubits can also
interact with other qubits, their properties “entangled”
in ways unique to quantum systems. These odd properties
mean quantum computers could provide dramatic advantages
over conventional systems in certain types of computation
that are difficult for conventional computers.
The approach taken by Kuzmich and Matsukevich begins
with two clouds of rubidium atoms, each cloud with a
different state, forming a matter qubit. By passing
a split beam of light separately through each cloud
— also known as an ensemble — and then recombining
it, they were able to create a qubit that was entangled
with a single photon.
Other research teams have been working to map states
from a single atom onto a photon. Kuzmich says using
the atomic cloud of very cold atoms as a matter qubit
offers a simplicity advantage in creating the entanglement.
“The state of the qubit is the collective state
of the atomic ensemble,” he explained. “Conversion
from matter to light becomes efficient in one direction
because emission from all the atoms add together to
create a preferred forward direction, similar to how
radio frequency antennas are able to emit directionally.”
Using optically thick atomic ensembles for the interface
between matter and light in long-distance quantum communication
was proposed in 2001 by a team of researchers from the
University of Innsbruck in Austria. The Georgia Tech
researchers built on that work, which has become known
as the DLCZ protocol.
Conversion of quantum states from atomic-based systems
to photonic systems is necessary for long-distance communication.
While the matter-based systems can provide long-term
storage of information, efficient transfer of information
requires that it be converted into a photonic state
for transmission across optical fiber networks.
For their research, the Georgia Tech physicists used
light at a wavelength of 780 nanometers. For transmission
in conventional optical fiber networks, however, they
will have to switch to the 1550 nanometer wavelength
that has become standard in the telecommunications industry.
The Science paper reported on atom clouds containing
approximately a billion rubidium atoms. Kuzmich says
having 10 billion atoms compressed into the same space
would boost efficiency. “We should be able to
increase our efficiency by a factor of 10 at least,”
he said.
Practical applications are still at least 7 to 10 years
away, Kuzmich estimates.
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