A measurement over 6.5 h demonstrates excellent passive stability of the system. a QBER, estimated from the visibility measurements. The QBER shown were calculated from eight different combinations of measurement bases each integrated over 100 s (The third and fourth data points correspond to a measurement duration of \(84\) s and \(80\) s long, respectively). b The red points correspond to the estimated secure key rate, based on the observed count rate and fidelity in the asymptotic time limit. c Position of the coincidence peak, as observed from histograms similar to Fig. 2 over an interval of several hours during which no compensation of the fibre birefringence was carried out. The error bars correspond to one standard deviation as derived from the fitting parameters.