Tokyo, Japan, July 20, 2004--The University of Tokyo's Nanoelectronics Collaborative Research Center and Fujitsu Laboratories Ltd. announced the joint development of technologies that generate and measure single photons, succeeding in observing the world's first emission of single-photons at data-transmission wavelengths. The technologies will speed up quantum-encryption data transmission (considered the ultimate method of encrypted transmission) to 400 times faster than the data-transmission speeds of conventional quantum encryption, which could only transfer data at a few hundred bits per second (bps). The technologies are a major step toward the practical application of quantum-encryption data transmission.

Details of the new technologies will be presented at The 27th International Conference on the Physics of Semiconductors (July 26 to 30, 2004; Flagstaff, AZ). Partial research and development of the new technologies were supported by the Nano-Photonic and Electron Devices Technology Project, one of several projects in an IT program of Japan's Ministry of Education, Culture, Sports, Science and Technology, entitled "Focused Research and Development Project for the Realization of the World's Most Advanced IT Nation." The National Institute for Materials Science (Ibaraki, Japan) also cooperated on the new developments.

Technological challenges

To enable data transmission using quantum encryption, a single-photon generator capable of limiting emission of photons to one photon per pulse is required. However, technology to generate single-photons has not previously existed for wavelengths used in practical optical-fiber transmission (1.3 to 1.55 micron), therefore a laser was used in place of single photons for conventional quantum-encryption experiments.

When using a laser for transmission involving quantum encryption, there is the risk of having two or more photons per single pulse, leaving the potential for eavesdropping. To lower the risk of emitting two or more photons, it is necessary to dramatically reduce light intensity. As such, laser-sourced quantum encryption for long-range transmission could only achieve extremely slow transmission speeds of a few hundred bps.

Newly developed technologies

The new technologies realize the emission and measurement of single-photons in practical transmission wavelengths of 1.3 to 1.55 microns.

A semiconductor device capable of efficiently emitting single photons from nanometer-sized quantum dots was designed. A process technology that does not impair the minutely structured quantum dots was also developed. The quantum dots that were used for this technology were jointly created by Yoshiki Sakuma's research group at the National Institute for Materials Science, and Fujitsu Laboratories Ltd. (Akashi, Japan).

A single-photon transmitter was designed and developed to efficiently collect light emitted from the semiconductor chip and transmit only light emitted from quantum dots to optical fibers. A single-photon receiver was also designed and developed, capable of splitting light that has passed through an optical fiber into two and accurately measuring the detection time of the split light. By confirming that detection times of the split light are not simultaneous, it is possible to prove that the emitted light is a single photon.

Results

Experiments confirmed that both parts of the light split into two were not detected simultaneously after noise correction, thereby verifying that single photons were successfully emitted from quantum dots at data-transmission wavelengths. Although verification of single photons for the new developments was for wavelengths of 1.3 microns, light emission from quantum dots for the more commonly used 1.55-micron wavelengths were also successfully observed.

With the confirmation of single-photon transmission over conventional data-transmission wavelengths, quantum-encryption transmission is now possible without lowering the sender's light-emission intensity. This enables transmission at speeds of 100 kbps (kilobits per second) across a distance of approximately 100 kilometers, 400 times faster than conventional laser-sourced quantum-encryption transmission. This dramatically increases the potential for the practical application of quantum-encryption-transmission technology at government, financial, and medical sites that require high-level data security.

Future developments

The University of Tokyo and Fujitsu Laboratories Ltd. will promote further R&D to verify single-photon transmissions at 1.55-micron wavelengths and aim to increase single-photon extraction efficiency, targeting realization of a practical single-photon generator by approximately 2007. The development of quantum-computation technology and quantum-relay technology will also be promoted for the realization of quantum networks.