Speaker
Description
We implemented a simplified time-bin BB84 quantum key distribution protocol with the purpose of achieving the highest possible secret key rate at short distances. The sender Alice emits signals at a rate of 2.5 GHz. In the key-generating basis, we use a superconducting nanowire single photon detector (SNSPD) with a novel design optimized for fast count rates. The in-house designed and fabricated NbTiN detector consists of 14 nanowires which are arranged in an interleaved pattern. Together with the in-house made readout electronics, the detector shows a jitter below 60 ps and simultaneously an efficiency of 64\% at a count rate of 320 Mcps, which represents the operating point of the detector for our shortest-distance key exchange. We performed real-time error correction with a low-density parity check algorithm implemented on a dedicated field-programmable gate array. This algorithm has a leakage of 17% at the highest quantum bit error rate found in our experiment, which was 0.4%. The privacy amplification was performed in real time on a consumer-grade GPU. We achieved a secret key rate of 64 Mbps over a distance of 10.0 km of ultra-low-loss (ULL) single-mode fiber (0.16 dB/km) and 3.0 Mbps over 102.4 km of ULL single-mode fiber . Additionally, we monitored the secret key rate over a longer time over 10.0 km ULL SMF, showing that the secret key rate can be maintained at a similar value for more than 1000 consecutive privacy amplification blocks.
