ITMO University researchers have developed a Quantum Key Distribution (QKD) system with coherent detection based on subcarrier wave quantum key distribution protocol. This is a continuous-variable quantum key distribution protocol using multimode coherent states generated on subcarrier frequencies of the optical spectrum
The main outstanding feature of the system is the method used to form states with an electro-optical phase modulator. If the radiation occurs at a certain optical frequency, phase modulation results in subcarrier frequencies being formed, which contain certain information about the signal transmitted. If the modulation index is low, light radiation at these subcarrier frequencies will be weak.
The main breakthrough of the researchers is a unique method of coherent detection for exactly this kind of system. At the base of the method is the use of a carrier optical frequency that doesn’t transmit any information by itself but can be used as a support to register the weak signal phase from subcarrier frequencies.
The researchers have created a mathematical model of the suggested scheme, which correlates well with experimental data. Their calculation shows that the current realistic system implementation would allow distributing secret keys over channels with losses up to 9 dB. They have also proposed an analysis of the developed protocol’s resilience to collective attacks. (Phys.org)
The paper was published in Scientific Reports.