Device Produces Light One Photon At A Time
Applied physicists at Stanford University (Stanford, CA; yamamoto@loki.stanford.edu), have produced the first device that can create a beam of light made up of a steady stream of photons. The single-photon turnstile device may increase the transmission rates in quantum cryptography systems 200-fold and is likely to impact other areas of quantum information technology. The Stanford team also developed a visible-light photon counter that can detect individual photons almost nine times out of 10 without any noise.
How It Works
Why It's Important
Applications
The photon turnstile was fabricated 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 has a diameter of 100 nm and 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 to two dimensions. The holes are localized areas of positive charge that act much like particles. Because the well is extremely small, the diameter of the post, the lateral motion of the electrons, and the holes are also limited.
This restriction magnifies the force that acts between charged particles, allowing 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 single-photon turnstile device 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.
The next-generation turnstile that Yamamoto's group is now developing will replace 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.
Because there is no benefit in generating regulated beams of photons without a means of detecting them, the Stanford group also developed a "visible-light photon counter." This device can detect individual photons almost nine times out of 10 without any noise.
Why It's Important (Back to Top)
Normal light sources such as 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. For a sense of the difficulty of the Stanford team's 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/sec.
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 quantum information technology. This field 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/sec. This is very slow compared to current optical communication rates of billions to trillions of bits/sec.
By contrast, the new turnstile device can produce a stream of a million to 10 million photons/sec. Yamamoto says that an improved version in the works has the capability of increasing the transmission rate 200-fold.
Quantum information technology is an active new research area that may be impacted by the single-photon turnstile device. Last spring, a group of researchers from IBM Corp., 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 some 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 the DOE's Los Alamos National Laboratory demonstrated that they could use the technique to transmit an encrypted key to a point 1-km away using a beam of individual photons. The sender switches the polarization of individual photons back and forth between two states. The beam is kept so faint that the receiver can only read the polarization of about 25% of the photons.
If the receiver measures the polarization of photons numbering 1, 5, 6, and 16, for example, the sender and receiver can construct an encryption key from the polarization sequence. Even if their phone line is bugged, an eavesdropper cannot steal the key without knowing the polarization values. 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. Because of this, traditional lasers cannot be used in the process, as their random generation of photons will interfere with the signal (no information can be transmitted if a photon is not created during a time slice). 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 successful creation of a "single-photon turnstile device" was reported in the Feb. 11, 1999, issue of Nature. Articles on the counter and the single-photon counting system are scheduled for publication in the Feb. 15, 1999, and Feb. 22, 1999, issues of Applied Physics Letters.
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 U.S. Office of Naval Research.
For more information, call 650-725-3327 or e-mail yamamoto@loki.stanford.edu.