Applied physicists develop
first device that produces light, one photon at a time
BY DAVID F. SALISBURY
Applied physicists at
Stanford University have produced the first device that
can create a beam of light made up of a steady stream of
photons. Normal light sources, even lasers, generate
photons at random intervals. Finding a way to produce
photons one by one at a regular interval has been a
long-standing research goal.
The successful creation of
a "single-photon turnstile device" with this
capability was reported in the Feb. 11 issue of the
journal Nature. It was developed by a research
team headed by Yoshihisa Yamamoto, professor of applied
physics and electrical engineering at Stanford. Team
members included doctoral student Jungsang Kim and
post-doctoral student Oliver Benson from Stanford; and,
Hirofumi Kan from Hamamatsu Photonics Inc. in Japan.
For a sense of the
difficulty of this achievement, consider the
infinitesimal size of the light particle, or photon. The
beam of light produced by one of the tiny lasers in a
compact disc player produces a hundred million billion
photons per second.
Microscopic
posts – each about one thousandth of the width of a
human hair – act as single photon
turnstiles. They are the first devices that can emit
light one photon at a time. Beams of regularly spaced
photons can be used to improve new technologies like
quantum computing and quantum encryption.
Courtesy
Yoshihisa Yamamoto
The
microscopic fluctuations in ordinary light are
imperceptible to the naked eye and don't make a
difference in most cases. But they are a major source of
noise that has limited the development of a set of
cutting-edge applications, called collectively quantum
information technology. This term includes novel methods
of computation and encryption that ultimately may be
incorporated into mainstream computer and
telecommunications devices.
"This is a serious
problem when sending secret information over a quantum
cryptographic system," Yamamoto says. The noise
caused by the irregular flow of photons has kept
transmission rates in quantum cryptography systems down
to a few thousand bits of information per second. This is
very slow compared to current optical communication rates
of billions to trillions of bits per second.
By contrast, the new
turnstile device can produce a stream of a million to 10
million photons per second, and the scientist says that
an improved version in the works has the capability of
increasing the transmission rate 200-fold.
The photon turnstile was
fabricated on campus at Stanford's Center for Integrated
Systems and the Edward L. Ginzton Laboratory. It consists
of a chip made from gallium, aluminum and arsenic that is
covered with a regular array of microscopic posts. Each
post, which has a diameter of 100 nanometers (about one
thousandth the width of a human hair), is an individual
photon turnstile.
Single photons are
produced in a structure called a quantum well at the base
of each post. A quantum well is a layer of electrically
conducting material that is so thin that it restricts the
motion of the electrons and holes (localized areas of
positive charge that act much like particles) to two
dimensions. Because the well is extremely small, the
diameter of the post, the lateral motion of the electrons
and holes is also limited.
This restriction magnifies
the force that acts between charged particles, allowing
the scientists to alternately inject single electrons and
holes into the well. Once in the quantum well, the
electron and hole rapidly recombine to produce an
individual photon that travels up the post to the end
where it is emitted.
The next generation
turnstile on which Yamamoto's group is working replaces
the quantum well with a quantum dot. A quantum dot is a
volume of electrically conducting material so small that
it restricts an electron's movement even more tightly in
all three dimensions. The researchers calculate that
electrons and holes should recombine much quicker in a
quantum dot, allowing them to produce photons at a faster
rate.
Of course, there is no
benefit in generating regulated beams of photons without
a means of detecting them. The Stanford group also has
developed a special "visible light photon
counter" that can detect individual photons almost
nine times out of 10 without any noise. Articles on the
counter and a single photon counting system based on it
are scheduled for publication in the Feb. 15 and Feb. 22
issues of the journal Applied Physics Letters.
New research area
Quantum information
technology is an active new research area.
Last spring a group of
researchers from IBM Corporation, the Massachusetts
Institute of Technology, the University of
California-Berkeley, and the University of Oxford
reported building the first working computer based on the
principles of quantum mechanics. Quantum computers have
the potential for solving certain kinds of problems
hundreds or millions of times faster than today's most
powerful supercomputers.
Similarly, quantum
encryption took a step from the laboratory to the field
last fall when a team of physicists from Los Alamos
National Laboratory demonstrated that they could use the
technique to transmit a secret key the distance of a
kilometer.
The system transmits the
encryption key by means of a beam of individual photons.
The sender switches the polarization of individual
photons back and forth between two states. But the beam
is kept so faint that the receiver can only read the
polarization of about 25 percent of the photons. If the
receiver measures the polarization of photons numbering
1, 5, 6 and 16, for example, then he calls up the sending
and gives him these numbers. Then they both can construct
the encryption key from the polarization sequence. But
even if their phone line is bugged, an eavesdropper
cannot steal the key because he doesn't know the
polarization values. On the other hand, an attempt to
eavesdrop on the original laser signal would be readily
detected by an increase in the error rate at the
receiver's end.
The quantum encryption
system works by dividing the laser beam into millisecond
time slices. The system only works when a single photon
is emitted during a time slice. So an ordinary laser's
random generation of photons interferes with the process.
For example, if a photon is not created during a time
slice then no information can be transmitted. On the
other hand, when two or more photons are emitted at the
same time, an eavesdropper can steal the information
without being detected. By producing a regulated stream
of photons, the photon turnstile will greatly reduce such
problems, Yamamoto says.
The research was supported
by the Exploratory Research for Advanced Technology
(ERATO) program of the Japanese government and the Joint
Services Electronics Program of the Office of Naval
Research. SR
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