A quantum key distribution system is configured with a transmitter, a receiver, and an optical fiber link that connects the transmitter and the receiver. The transmitter transmits photons to the receiver via the optical fiber link (a quantum communication channel). After that, the transmitter and the receiver exchange control information with each other, and share cryptographic keys. This technology is implemented using the technology generally referred to as quantum key distribution (QKD).
In order to share cryptographic keys between a transmitter and a receiver using quantum key distribution, it is necessary to perform a key distillation operation in the transmitter as well as in the receiver. The key distillation operation includes a sifting operation, an error correction operation, and a privacy amplification operation. As a result of performing the key distillation operation, the transmitter and the receiver share cryptographic keys. The amount of generation per unit time of the shared cryptographic keys is called a secure key rate. Being able to use a number of cryptographic keys enables performing high-speed and safer cryptographic data communication. Hence, it can be said that, higher the secure key rate, the more enhanced is the performance of a quantum key distribution system.
In such quantum key distribution systems, there is a system in which an optical processing device, a high-speed signal processing unit, and a central processing unit (CPU) are arranged in series for the purpose of quantum key distribution and, depending on the details of communication to be done with another communication device, switching is done between whether the key distillation operation is to be assigned to the high-speed signal processing unit or to the CPU. In this way, the processing load and the communication load required in the key distillation operation is distributed to achieve a high speed during the operations.
However, in such a quantum key distribution system, the optical processing device, the high-speed signal processing unit, and the CPU are connected in series; and the operations include sequential transfer of data. Hence, in case some of the operation modules (for example, the high-speed signal processing unit) stop operating, then the operations at the prior stage and the subsequent stage also get terminated. Moreover, the key distillation operation includes a plurality of different algorithms intended for the sifting operation, the error correction operation, and the privacy amplification operation. Hence, if the configuration includes only a single high-speed signal processing module, then it is not possible to have a high-speed signal processing unit with the most suitable configuration for each operation constituting the key distillation operation.