Polymer serves up single photons

R. A. Hettinga rah at shipwright.com
Wed Sep 8 13:55:24 EDT 2004


TRN 090804

 Polymer serves up single photons

September 8/15, 2004

By Eric Smalley, Technology Research News

Quantum cryptography in theory allows someone to send a secret key and know
for sure that the key has not been seen by anyone but the intended
recipient. Each bit of information in the key is represented by attributes
of a photon, which makes it impossible for an eavesdropper to both read and
replace all the photons in a string.

 Although commercial quantum cryptography systems exist, they use photon
sources that sometimes represent information using a single photon and
sometimes a few photons. Sources that reliably emit just a single photon at
a time are needed to ensure perfect security. The goal is a reliable,
inexpensive device that emits single photons on demand at room temperature.

 Researchers from the Georgia Institute of Technology, the University of
Tennessee, and Oak Ridge National Laboratory have made a room-temperature,
single-photon source using polymer molecules.

 The device consists of precisely oriented single molecules of a
semiconducting polymer that are applied to a surface using a simple
spray-on technique, said Tae-Hee Lee, now a researcher at Stanford
University. "We proved that [a] simple, one-step polymer processing
technique can generate strong and stable single-photon sources," said Lee.
The method is an inexpensive way of generating single photons, he said.

 The single-photon source could be used in quantum cryptography devices and
eventually for quantum computing, said Lee.

 The polymer is similar to materials used to make new types of organic
light-emitting diodes. The researchers' prototype contains
10-nanometer-long single-chain molecules oriented perpendicular to a glass
surface. This orientation allows them to emit photons, and because they are
individual molecules, they emit one photon at a time.

 The researchers produced the device by making a solution of the polymer
polyethylene vinylene in toluene and spraying it through a
5-micron-diameter nozzle. This resulted in an aerosol of 5- to 10-micron
droplets, which rapidly evaporated, leaving the polymer molecules on the
glass surface.

 Researchers have made single-photon sources from fluorescent molecules,
but these tend to fade in a matter of minutes. The researchers' polymer
molecules last several hours.

 The most important application for the researchers device is quantum
cryptography. The ability to cheaply produce room-temperature single-photon
sources is also step forward for quantum computers. Quantum computers,
which manipulate information stored in the attributes of particles like
atoms and photons, have the potential to solve certain very large problems
much more quickly than conventional computers.

 Researchers generally agree that practical quantum computers are one to
two decades from realization.

 The researchers are working to make the material more stable, said Lee.
The researchers' prototype emits photons after being energized with lasers.
For practical applications, single-photon sources will need to be
stimulated electrically.

 Lee's research colleagues were Pradeep Kumar from the University of
Tennessee, Adosh Mehta and Michael D. Barnes from Oak Ridge National
Laboratory, and Kewei Xu and Robert M. Dixon from the Georgia Institute of
Technology. The work appeared in the July 5, 2004 issue of Applied Physics
Letters. The research was funded by the Advanced Research and Development
Activity (ARDA), the National Science Foundation (NSF), the Alfred P. Sloan
Foundation and Henry and Camilla Dreyfus Foundation.

Timeline:   unknown; 10-20 years
 Funding:   Government, Private
 TRN Categories:  Materials Science and Engineering; Quantum Computing and
 Story Type:   News
 Related Elements:  Technical paper, "Oriented Semiconducting Polymer
Nanostructures as on-Demand Room-Temperature Single-Photon Sources,"
Applied Physics Letters, July 5, 2004

R. A. Hettinga <mailto: rah at ibuc.com>
The Internet Bearer Underwriting Corporation <http://www.ibuc.com/>
44 Farquhar Street, Boston, MA 02131 USA
"... however it may deserve respect for its usefulness and antiquity,
[predicting the end of the world] has not been found agreeable to
experience." -- Edward Gibbon, 'Decline and Fall of the Roman Empire'

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