Control of distributed interference in the one-way quantum cryptography system

The possibility of controlling interference in two spaced fiber Mach–Zehnder interferometers and maintaining a nearly ideal visibility has been demonstrated for the one-way quantum cryptography system directly in the key distribution process through a communication channel with a length of 50 km. It has been shown that the deviation of the visibility from ideal is certainly due to the detected difference between the numbers of 0’s and 1’s in the raw (sifted) key. For this reason, an interferometer can be balanced only in the quasi-singlephoton mode without the interruption of the process of key distribution by using the difference between the numbers of 0’s and 1’s in the raw key as an indicator of an error. The proposed approach reduces the balancing time and, furthermore, does not require additional exchanges through an open communication channel.