Polymer serves up single photons

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 Communications
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


September 8/15, 2004

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