OKI develops the world's purest quantum entangled light source and establishes practical, next-generation quantum cryptography technologies

Quantum cryptography technologies apply the principles of quantum mechanics for eavesdropping detection. They have attracted attention as an exceedingly high-security service for a smart community due to their potential to achieve indecipherable encoding. A number of major hurdles confront research and development teams working to achieve practical applications, including the need for ultra-low-temperature cooling for light sources and the generation of light at wavelengths beyond the optical communications band, as well as difficulties achieving the photon purity needed, says Takeshi Kamijoh, General Manager of Research and Development Center at OKI. In response, OKI has developed a quantum entangled light source based on cascaded nonlinear optical effects using a proprietary periodically poled lithium niobate (PPLN) ridge-waveguide device(*2). Operating at room temperature and configurable at optical fiber communications wavelengths alone, the device represents a practical next-generation quantum cryptography technology.

The performance of this quantum entangled light source has been tested using a semiconductor-based single-photon detector developed by Institute of Quantum Science, Nihon University, which can detect photons with low noise and high efficiency at high repetition rates of 1 GHz. These tests show that the signal-to-noise ratio for the photon pairs generated is one to two orders of magnitude greater than for conventional light source and detector combinations and demonstrate the feasibility of quantum cryptography communications at low signal error rates, using the quantum entangled light source developed in this research program and the single-photon detector.

Other tests performed to transmit generated quantum entangled photon pairs confirm that the quantum entanglement state can be sufficiently maintained even when transmitted over a distance of 140 km over standard optical fibers. This performance is sufficient for quantum cryptography communications over metropolitan area.

Part of this research was published in Optics Express, Vol. 19, No. 17, pp. 16032-16043 (2011). Optics Express is the journal of the Optical Society of America (OSA).

OKI will continue to refine this new quantum entangled light source while working to reduce size and cost to achieve a practical quantum cryptography communications system.

Quantum key distribution system

##IMAGE_2_C##

Features of quantum entanglement light source developed by OKI
Generates polarization entangled photon-pairs with the highest purity achieved to date
Simple optical circuits (can be configured using optical telecommunication wavelength-band)
Capable of operating at room temperature (eliminates need for huge cooling systems)
Easily integrated into communications systems

Explanation of quantum entanglement light source

The quantum entangled light source is achieved using a nonlinear optical device. One or more pump photons are lost during the nonlinear optical process; at the same time, a new photon-pair (a correlated photon-pair) is generated. This photon-pair is always generated as a pair. Physical measures (such as wavelength and polarization) have a correlation independent of distance (nonlocality) and are used to achieve the quantum entangled light source.

Explanation of cascaded nonlinear systems

Generating quantum entangled photon pairs requires a laser light source, but photon-pair generating systems based on conventional PPLN ridge-waveguide devices require a laser light source to oscillate at a special wavelength. And while studies have been carried out into entangled photon pair generation systems based on nonlinear optical effects in optical fibers, those systems require cooling systems to keep the optical fiber at extremely low temperatures, necessary because of the noise photons generated at room temperature and the difficulty in achieving the purity required. This, in turn, results in impractical cost and performance.

The cascaded nonlinear system developed by OKI (see Figure 1) makes it possible to devise a light source system using only the typical resources used in standard optical communications, including lasers and wavelength filters. It also achieves high signal-to-noise ratios, since the PPLN ridge-waveguide device has high optical nonlinearity, reducing the effect of noise photons due to Raman scattering and other phenomenon.

Diagram to illustrate generating photon pairs by cascaded nonlinear optical effect

##IMAGE_3_C##

Glossary
*1 :Next-generation quantum cryptography communication systems
Next-generation quantum cryptography communication systems are quantum cryptography systems based on a quantum entangled light source. They are expected to offer longer distance capabilities and greater security than systems based on single-photon light sources.

*2 :PPLN ridge-waveguide device
A ridge-waveguide device using LiNbO3 (lithium niobate)crystals with a structure exhibiting periodically reversed spontaneous polarization. Phase-matching makes it possible to achieve high nonlinear optical effects. High optical confinement of the ridge-waveguide structure increases the efficiency of nonlinear optical effects.