Patent ID: 8175273

Claim:
A method of performing quantum cryptography communication by performing a communication process based on quantum cryptography, the method comprising: transmitting data from a data transmitting apparatus, the data including a sequence of signals in the form of quantum states randomly selected from a plurality of quantum states each having a different phase modulation angle; detecting the sequence of signals in a data receiving apparatus, the detection step including receiving the sequence of signals, randomly selecting from a plurality of bases corresponding to a plurality of different phase modulation angles, and performing a homodyne detection process using the selected bases; notifying the data transmitting apparatus of applied phase modulation mode information indicating the bases used in the detection step; and sharing information in the data transmitting apparatus, the information sharing step including (i) assigning, by the data transmitting apparatus, bit values to the plurality of different quantum states selected by the data transmitting apparatus in accordance with the applied phase modulation mode information notified by the data receiving apparatus, wherein the assigning bit values includes determining, for each of the signals received in the data receiving apparatus, a difference between the phase modulation angle corresponding to the quantum state of the received signal and the phase modulation angle corresponding to the selected base used in performing the homodyne detection process on the received signal and using an absolute value of the difference, in accordance with the plurality of quantum states randomly selected from and the plurality of bases randomly selected from, to assign the bit values; and (ii) notifying the data receiving apparatus, by the data transmitting apparatus, of the assigned bit values.