heft more data
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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,
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