Fumio Futami at Tamagawa University, Quantum ICT Research Institute, (Director: Osamu Hirota, 6-1-1 Tamagawa Gakuen, Machida, Tokyo, Japan), announced the world's first transmission of the stream cipher by Yuen 2000 protocol (Y-00) at the bit rate of 40 Gbit/sec over 120 km. Y-00 is a physical cipher that has a possibility to avoid the decipher and hence it is a promising candidate to realize secure networks. The key to success of high capacity transmission was the use of wavelength division multiplexing (WDM) scheme. Four lights with different wavelengths each carrying 10-Gbit/sec Y-00 encrypted optical signals were multiplexed into an optical fiber to attain the aggregate capacity of 40 Gbit/sec. In the experiment, it has been proved that the transmission capacity of Y-00 signals was increased by employing the WDM scheme. The capacity can be further increased with use of more number of lights with different wavelengths. The result made a step closer to practical use of Y-00 stream cipher in the real network services.