Photons heft more data

By Eric Smalley, Technology Research News

Light doesn't weigh anything, but it does have momentum. In fact, a strong light beam can move microscopic objects. A lightwave that spirals also has orbital angular momentum, which is the same type of momentum the moon carries in its orbit around the earth.

Researchers from the Universities of Glasgow and Strathclyde in Scotland have found a way to measure the orbital angular momentum of individual photons, which allows them to distinguish photons that have incrementally different amounts of momentum.

The ability to measure these fine differences means the different states could be used to carry much more information than today's optical communications technologies, which generally use only the presence and absence of light pulses to represent the ones and zeros of digital information.

There are 32 unique combinations of five binary digits starting with 00000, 00001, 00010 and ending with 11111. In contrast, a single photon with 32 possible orbital angular momentum values could carry as much information as the whole group of five on/off pulses, and could therefore transmit data five times as fast.

Under the researchers' scheme, each photon "could, for example, represent a whole letter of the alphabet," said Johannes Courtial, a research fellow at the University of Glasgow in Scotland.

The technique could further increase information capacity if it is combined with measurements of other variables of light, including its two polarization states and however many colors can be distinguished, said Courtial. Polarized light vibrates in only one of two directions rather than in all directions at once. Combining these techniques "could literally multiply the information-carrying capacity of each photon," he said.

The researchers' method could also boost the capacity of quantum communications systems and quantum computer schemes that use attributes of photons to process and transmit information, said Courtial. Quantum communications can be used to send perfectly secure messages. Quantum computers have the potential to solve certain problems faster than the fastest possible classical computer because they can theoretically process all possible answers to a problem at once rather than looking at the possibilities one by one.

The researchers measured orbital angular momentum using a series of interferometers. An interferometer splits a light beam into two beams, changes the positions of one of the beam's waves, then passes the beams through each other to create an interference pattern. When light waves meet they mix; where wave crests or troughs meet they reinforce each other, and where a crest and trough meet they cancel out.

The researchers send a light beam whose photons have a mix of orbital angular momentums into an interferometer and rotate one of the split beams 180 degrees, said Courtial. When the beams are put back together, the resulting interference pattern sorts the photons into two groups with different orbital angular momentums. "All photons with an orbital angular momentum that is an even multiple... interfere constructively in one output port of the interferometer while all those with an odd orbital angular momentum interfere constructively in the other output port," he said.

"After this first sorting stage, the photons are... passed through further interferometers," Courtial said. As more interferometers are added, more states can be distinguished, and therefore each photon can represent one of a larger range of numbers, he said.

The researchers tested the method with individual photons by transmitting so little light that most of the time no photons were transmitted, occasionally single photons were transmitted and only rarely was more than one photon transmitted at once.

To encode data in a photon, the researchers would reverse the sorting process in order to make the interferometer emit a photon with a specific orbital angular momentum, said Courtial. "At the other end, it's [orbital angular momentum] could be measured to decode the data," he said.

Using the orbital angular momentum of photons to transmit real data means getting over several hurdles. In the short term, lining up multiple interferometers has proved challenging, said Courtial.

A longer-term problem is figuring out how to preserve photons' orbital angular momentum as they traverse fiber-optic cables. "The trouble is that optical fibers -- at least most fibers in use today -- alter the light's" orbital angular momentum, he said. Several years ago the researchers demonstrated the problem by adding a weight to a fiber-optic cable, which causes the cable to convert light with no orbital angular momentum into light with orbital angular momentum, said Courtial. "We're currently working on fixes... for both problems," he said.

The researchers have shown that it is possible in principle to encode single photons with many different states, said Kang Wang, a professor of electrical engineering at the University of California at Los Angeles.

The principle could also be extended to other types of particles, Wang said. "For computation, the number of states of electrons could be increased using similar waves interference techniques to increase information processing volume," he said.

A lot of work needs to be done before it is possible to use orbital angular momentum to transmit data, however, said Wang. "Practical realization for commercial applications remains... daunting."

The researchers are working on making their photon sorter more compact and more stable in order to commercialize the device, said Courtial. "We are also trying out different designs," he said.

The orbital angular momentum of photons could be put to use transmitting data in five to ten years, he said.

Courtial's research colleagues were Jonathan Leach and Miles Padgett of the University of Glasgow and Stephen Barnett and Sonja Franke-Arnold of the University of Strathclyde. They published the research in the June 24, 2002 issue of the journal Physical Review Letters. The research was funded by Glasgow and Strathclyde universities, the Royal Society, the Leverhulme Trust, the Royal Society of Edinburgh, the Scottish Executive Education and Lifelong Learning Department and the UK Engineering and Physical Sciences Research Counsel (EPSRC).

Timeline:   5-10 years
Funding:   Government, Private
TRN Categories:   Optical Computing, Optoelectronics and Photonics; Physics; Quantum Computing and Communications; Telecommunications
Story Type:   News
Related Elements:  Technical paper, "Measuring the Orbital Angular Momentum of a Single Photon," Physical Review Letters, June 24, 2002


July 10/17, 2002

Page One

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