Fig. 3: Open quantum channel stabilisation.

Alice and Bobʼs lasers are fully independent, and their frequency difference is adjustable by offsetting Aliceʼs laser to a high-fineness cavity. Except a, all data in this figure were measured with the 615.6 km quantum fibre. a Histograms of phase compensation signals integrated over 10 ms time intervals for a 10 m quantum link; b same as a but with 615.6 km quantum link; c compensation signal angular frequency as a function of the laser frequency offset (Δν). Error bars represent standard deviation. The blue line is a guide to the eye and has a slope of 1. d Optical output power as a function of time measured at one output of Charlieʼs 50/50 interfering beam-splitter; Purple (green): channel reference or the λ_c signal when the FPGA PID controller is turned off (on); Orange: slowed drift of the λ_q signal. e–g The phase angle distributions of the channel reference before (purple) and after (green) the simplex channel stabilisation, measured for laser frequency offsets of −2, 0 and 2 kHz, respectively. Symbol σ represents standard deviation. Data in panels d–g were measured with a power metre.