OKI Develops the World's Purest Quantum Entangled Light Source and Establishes Practical, Next-Generation Quantum Cryptography Technologies

Drawing on proprietary technologies, OKI has developed a quantum entangled light source that offers the highest level of purity achieved to date. It has successfully generated the high-purity, entangled photon pairs for the first time anywhere in the world operating at room temperature and for conventional optical communication bands that are currently in use. Joint demonstrations performed with the research group conducted by Professor Shuichiro Inoue at Institute of Quantum Science, Nihon University confirm a signal-to-noise ratio more than 100-fold better than optical fiber light sources. Transmission tests using this light source with standard optical fibers have successfully transmitted quantum entangled photon-pairs over a distance of 140km, demonstrating the feasibility of next-generation quantum cryptography communication systems(*1) covering metropolitan area.

"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.

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