Timing Synchronization in an FPGA-based Real-time Quantum Key Distribution System

In Quantum Key Distribution (QKD), the time tagging (with a synchronized timing reference) of photon transmission/reception events is of utmost importance for the purposes of coincidence identification to obtain the sifted key between the communicating terminals (A: Aryabhata and B: Bhaskara). In entangled photon based QKD system, both communicating terminals receive the photons whereas in case of weak coherent photon based QKD system, one terminal transmits the photons, which are received at receiver. For the coincidence identification in QKD system, it is necessary to synchronize the time bases used for time tagging of photons between the communicating terminals operating at independent clocks. This paper presents timing synchronization and FPGA implementation of Time of Flight (ToF) estimation in a NavIC synchronized real time Quantum Key Distribution (QKD) system. The proposed time synchronization and Time of Flight estimation method has been implemented and successfully used in ISRO’s and India’s first successful demonstration of 300m free space entangled photon based real time quantum key distribution system and weak coherent photon based real time QKD system as well. The achieved accuracy for time of flight estimation is 10ns and timing synchronization within 20ns.