Fig. 3: Secure key rate over time.

As long as the quantum bit error rate (QBER) stays below 11%, a quantum secure key can be created in principle (see Fig. 2). Our polarization alignment procedure allows to keep the QBER in the key creation regime for altogether 82 out of 110 h of total link operation. The red line gives the average secret key rate (1.4 bits/s) calculated from all coincidences over these 82 h. Thus, the total asymptotic secure key amounts to 403 kbit, calculated with the overall, i.e., average, QBER. In black, we show the secure key rate based on data acquired over 300 sec time windows. Its fluctuations in the order of ±0.3 bits/s originate from Poissonian photon statistics and polarization drifts. For consideration of finite-key effects, we refer the reader to the Methods section.