Pablo Arteaga-Díaz

Quantum Key Distribution (QKD) systems’ performance is highly affected by losses and background noise, decreasing the secure bit rate in long-distance and/or high noise links. This is one of the limitations of this technology, implying long times for sharing cryptographic keys. One of the solutions for this problem consists in simply sending more pulses in less time, that is to say, increasing the bit rate. However, this solution has a problem when it comes to discrete variable QKD systems using single photon detectors, especially with semiconductor Single Photon Avalanche Diodes (SPADs). Commercially available SPADs introduce high temporal jitter to the measurements, increasing the probability of inter-symbol interference. Temporal jitter can produce measurements of a bit in the temporal window of previous or next bits, which may generate measurement errors increasing the Quantum Bit Error Rate (QBER). This means that we achieve higher bit rates but we may also increase QBER, decreasing secure bit rate, imposing a limit in the bit rate. In this work, we model the effect of temporal jitter on the QBER, validate the model with actual measurements, and use the model to study the effect of temporal jitter on high-speed QKD systems. In the case of commercial SPADs, with standard deviation values for the temporal jitter near 150 ps, the optimum bit rate is about few GHz. In order to operate at higher speeds we need single photon detectors with less jitter, what we can achieve with superconducting nanowire single-photon detector (SNSPD).