Decoy state-based time synchronization

Time synchronization is a crucial requirement in quantum key distribution (QKD) protocols, ensuring accurate key generation via the correct assignment of bits of raw key and enabling eavesdropping detection via the precise recording of photon statistics. State-of-the-art experiments typically use an extra channel to synchronize the clocks of the transmitter and receiver via classical signals. In this work, we study the possibility of performing clock synchronization via the signals used for the key generation, which are already present in decoy-state-based BB84 protocols. Without altering the protocol in any way, we use the different mean photon numbers of the signal and decoy states for time synchronization without a dedicated physical channel capable of clock synchronization. The proposed method relies only on the photons sent and received for key generation and does not require any change to the QKD protocol. The only change in the experiment is on the software level, thus making it very simple to implement. We demonstrate a clock synchronization method in a simulation of a specific fiber-based QKD experiment. Like other decoy-state-based BB84 protocols, it is based on weak coherent pulses. In this simulation, we investigate the parameter space to find limits and optimal choices of our proposed method. In addition to the non-protocol-altering clock synchronization method, we also discuss an approach that significantly improves performance in lossy channels by introducing an additional decoy state with a very high mean photon number. By eliminating the need for an extra channel capable of clock synchronization, both methods proposed potentially reduce the complexity and cost of QKD systems and improve their agility.