switch promises powerful computers
Technology Research News
At first glance, a switch is a simple concept.
It is either on or off.
Today's computer chips harbor millions of microscopic electrical switches.
These transistors turn on when an electromagnetic field generated by a
control electrode lowers the transistor's resistance to the flow of electrons,
which allows electrical current to flow from one end of the device to
the other. The presence or absence of this flow represents a 1 or a 0
of digital computing.
Circuits that switch light rather than electricity would make for faster
computers, but it's difficult to use a beam of light to turn another light
beam on and off. Light beams usually just pass through each other, especially
if they are relatively weak.
Researchers from the University of Toronto in Canada have figured out
a way to allow beams of individual photons to affect each other, and have
made a device that switches light in a manner similar to the way electrical
transistors switch electrical current.
Photon transistors could pave the way for fast, low-power, all-optical
Extremely low-power switches are also a necessary component of quantum
computers, which use the delicate differences in the states of atoms
and subatomic particles to compute.
The researchers demonstrated the photon switch by shooting two weak beams
of light into a crystal that was simultaneously bombarded by intense laser
light of another wavelength. "The switch allows two beams of light so
weak that they contain at most a single photon, and most often none at
all, to meet up inside a thin optical crystal," said Aephraim Steinberg,
an associate professor of physics at the University of Toronto in Canada.
One of the weird quantum traits of light is that it is simultaneously
a continuous wave and a stream of tiny particles, or photons. Different
colors of light are different wavelengths. Red light, for example, is
around 650 nanometers, or millionths of a millimeter, from crest to trough,
while higher-frequency blue light measures around 450 nanometers.
Lit up by an intense laser beam of blue light that measures half the wavelength
of the weak red beams, the researchers' crystal allows weak beams of red
light to pass through unless they both contain a photon. "The crystal
is transparent to the two weak signal beams except when both beams contain
a photon, in which case the two photons annihilate [each other], and are
prevented from passing. This is the switch effect," said Steinberg.
The red color of the weak beams disappears, turning the switch off, when
each contains a photon because the two photons essentially merge into
one higher-energy photon of blue light, a process known as upconversion,
according to Steinberg. "A single red photon doesn't possess enough energy
to "turn blue" and will therefore be transmitted undisturbed," he said.
"But since any pair of red photons will upconvert, it's as though a single
photon is enough to switch off the path for the other photon."
The switching interaction occurs in a region of the crystal that is about
one tenth of a millimeter across, but the equipment required for the researchers'
prototype includes an inch-long crystal and a six-foot-wide table containing
lasers and detectors. Because the actual switching is purely optical,
it could in theory be miniaturized using techniques that exist today,
The researchers' prototype works about 60 percent of the time, but the
concept could lead to a reliable switch, according to Steinberg.
The researchers' eventual aim is to use the switch in quantum computers,
Steinberg said. "Our hope is that this could be used as a fundamental
logic gate inside quantum computers, whose [potential] uses are still...
being discovered," said Steinberg.
Quantum computers could be much faster than the fastest possible electronic
computers, because they have the potential to examine every possible answer
to a problem at once. "If you know how to ask the computer the right question,
instead of getting the results of just a single calculation, you may find
out something about the results of all possible calculations, something
the classical computer would've had to run exponentially many times to
determine," Steinberg said.
The research is impressive, and "potentially very significant," said Robert
Boyd, a professor of optics at the University of Rochester. "It's been
well-established that a strong beam of light can be used to control another
beam of light. The novel feature of the present approach is that the two
weak beams interact in the presence of a strong beam, which allows the
interaction to be strong even though the control and signal beams are
both weak," he said.
This method has the potential to produce energy-efficient optical switches
that operate with very weak power levels, which would be useful for applications
like telecommunications and optical computing devices, said Boyd.
The switches are potentially useful for quantum computing for similar
reasons. "The signal levels must necessarily be very weak" for quantum
applications, he said.
Although there are many research efforts under way to bring quantum computing
to reality, it is hard to know if and when these fantastically fast computers
will materialize, said Steinberg. "Thousands of people around the world
are working towards the construction of quantum computers and algorithms
for use on them, but none of us knows if a full-scale device will ever
work," he said. "I'd say it's equally likely that we will never see a
quantum computer in our lifetimes, or that people will stumble across
the right architecture for one in the next ten years or so."
Steinberg's research colleagues were Kevin J. Resch and Jeff S. Lundeen.
They published the research in the November 15, 2001 issue of Physical
Review Letters. The research was funded by the Canadian Natural Sciences
and Engineering Research Council, Photonics Research Ontario, the Canada
Fund for Innovation, the Ontario Research and Development Challenge Fund,
and the U.S. Air Force.
Timeline: > 10 years
TRN Categories: Optical Computing, Optoelectronics and Photonics;
Story Type: News
Related Elements: Technical paper, "Nonlinear Optics with
Less Than One Photon," Physical Review Letters, September 17, 2001.
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