Newswise — A
series of publications in the journal Nature highlights
the race among competing research groups toward the
long-anticipated goal of quantum networking.
In one of three papers published the journal's December
8 issue, a group of physicists from the Georgia Institute
of Technology led by Professors Alex Kuzmich and
Brian Kennedy describes the storage and retrieval
of single photons transmitted between remote quantum
memories composed of rubidium atoms. The work represents
a significant step toward quantum communication and
computation networks that would store and process
information using both photons and atoms.
But the researchers caution that even with their
rudimentary network operation, practical applications
for quantum networking remain a long way off.
“The controlled transfer of single quanta between
remote quantum memories is an important step toward
distributed quantum networks,” said Alex Kuzmich,
the Cullen-Peck Assistant Professor in Georgia Tech's
School of Physics. “But this is still a building
block. It will take a lot of steps and several more
years for this to happen in a practical way.”
Slightly more than a year ago in a paper published
in the journal Science, Kuzmich and collaborator
Dzmitry Matsukevich described transferring atomic
state information from two different clouds of rubidium
atoms onto a single photon. That work was the first
time that quantum information had been transferred
from matter to light.
the new paper in Nature, Kuzmich, Kennedy and collaborators
Thierry Chaneliere, Dzmitry Matsukevich, Stewart
Jenkins, Shau-Yu Lan carry the earlier operation
one step farther by storing and retrieving single
photons from clouds of ultra-cold rubidium atoms – demonstrating
the storage of light-based information in matter.
an applications perspective, the storage and retrieval
of a qubit state in an atomic quantum memory node
is an important step towards a “quantum repeater.” Such
a device would be necessary for transmitting quantum
information long distances through optical fibers.
Existing telecommunications networks use classical
light to transmit information through optical fibers.
To carry information long distances, such signals
must be periodically boosted by repeater stations
that cannot be used for quantum networking.
Georgia Tech researchers began their experiment
by exciting a cloud of rubidium atoms stored in
a magneto-optical trap at temperatures approaching
absolute zero. The excitation can generate a photon – but
only infrequently, perhaps once every five seconds.
Because it is in resonance with the atoms from which
it was created, the photon carries specific quantum
information about the excitation state of the atoms.
The photon was sent down approximately 100 meters
of optical fiber to a second very cold cloud of trapped
rubidium atoms. The researchers controlled the velocity
of the photon in the second cloud by an intense control
laser beam. Once the photon was inside the cloud,
the control beam was switched off, allowing the photon
to come to a halt inside the dense ensemble of atoms.
“The information from the photon is stored in the
state of excitation of many atoms of the second ensemble,” explained
Jenkins, a graduate student who specializes in quantum
optics theory. “Each atom in the ensemble is slightly
flipped, so the atomic ensemble is sharing this information – which
is really information about spin.”
After allowing the photon to be stored in the atomic
cloud for time periods that exceeded 10 microseconds,
the control beam was turned back on, allowing the
photon to re-emerge from the atomic cloud. The researchers
then compared the quantum information carried on
the photon to verify that it matched the information
carried into the cloud.
“When the single photon is generated, the first
atomic ensemble is in an excited state,” explained
Chaneliere, a postdoctoral fellow in the Kuzmich
lab. “When we read the information from the second
ensemble and find a coincidence between its excitation
and the excitation of the first ensemble, we have
demonstrated storage of the photon.”
To confirm the single photon character of the storage,
the researchers used anti-correlation measurements
involving three single photon detectors.
Storage of the photon for even a brief period of
time within the atomic ensemble depends on careful
control of potentially-interfering magnetic fields.
And it works only because the rubidium atoms are
so cold that their motion is limited.
“Quantum information is very fragile,” said Chaneliere. “If
you have a magnetic field, the atoms spin out of
phase, and you can lose the information. For the
moment, that is certainly a limitation on the use
of this for quantum networking.”
For the future, the team hopes to add additional
nodes to their rudimentary quantum network and encode
useful information onto their photons.
They must also increase the probability of creating
single photons from the first atomic cloud. While
gathering data, the researchers excited the first
cloud of atoms approximately 200 times a second.
A single photon was created about once every five
seconds, reported Matsukevich, a graduate student
in the Kuzmich lab.
Highlighting the speed at which progress is being
made toward quantum networking, Kuzmich, Kennedy
and their team have more recently demonstrated entanglement
between two atomic qubits separated by a distance
of 5.5 meters. The work is described in a paper submitted
to the journal Physical Review Letters .
“This entanglement would be important to a number
of applications, including quantum cryptography,” said
Kuzmich. “We have generated entanglement of atomic
qubits. We also showed that we can take this entanglement
and map it from atoms to photons.”
Research by Kuzmich, Kennedy and their colleagues
has been supported by NASA, the Office of Naval Research
Young Investigator Program, National Science Foundation,
Research Corporation, Alfred P. Sloan Foundation,
and Cullen-Peck Chair.