A Y-00 transceiver for quantum encryption communication systems employing the Yuen-2000 protocol based on optical intensity modulation, which utilizes the quantum effect of coherent light has been developed and tested in field experiments. These experiments involved repeated transmission over a 192-km line on an existing commercial optical communication network with a bit-rate of 2.5 Gb/s based on STM-16 or OC-48. In addition, the affinity of the developed system for existing networks and communication protocols (Gigabit Ethernet and Fibre Channel) was examined. Subsequently, three optical fiber amplifiers were inserted as intermediate repeaters to give a non-repeated transmission distance of 48 km. The bit error rate after 192-km transmission was 10<sup>-12</sup> with an optical power of -19.4 dBm at the receiving end. In addition, it was confirmed that if an eavesdropper increased the received optical power, the bit error rate converged to about 5 x 10<sup>-1</sup> and the identification of the true signal levels was virtually impracticable. These experimental results confirmed the high affinity between the Y-00 system and existing optical commercial networks and protocols used in the fundamental experiments. It was also apparent that the realization of highly secure optical communication networks was feasible. The performance of the Y-00 transceiver can be expected to improve to meet the strictest security evaluation.
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