Figure 1

Simplified schematic of QKD system for high loss link. Alice's up-conversion photon source produces photons at 532 nm which are sent through the controllable-loss channel to Bob's receiver.Reuse & Permissions

Figure 2

Typically observed drift between Alice's and Bob's clocks. Alice's clock is determined by the repetition rate of the mode-locked laser and Bob's comes from his time tagger. Drifts in the clock are large compared to the nominal laser clock period of 13 ns, making timing synchronization a necessity. Our synchronization scheme correctly aligns Bob's detection events independent of which device is drifting.Reuse & Permissions

Figure 3

Secure key rate over high loss channel. (a) Simulation of total loss vs visible passage time for a satellite uplink. Here we show simulation data for the best overall passage in that year and, for comparison, the 142nd best passage, that is, 80th percentile of the 712 total passages for the year. The loss is minimum as the satellite is closest to the ground station (highest elevation angle) and increases as the satellite approaches the horizon. (b) Experimental results and simulation of secure key rate vs loss. Our data agree well with the theoretical curve, which uses a quantum optical simulation to predict key rates. Deviations from the theoretic curve are caused by imperfect alignment of the polarization inside the optical fibers. Treatment of error analysis is included in QKD security proofs, and is generally based on upper bounding the information given to an eavesdropper compatible with measurement results. (c) Expected secure key rate vs time for a satellite passage, based on simulations and experimental parameters. The secure key rate in bits/s on the right axis assumes the 76 MHz clock rate of our source. The loss vs time and secure key rate vs loss curves combine to produce the output key rate over one satellite passage.Reuse & Permissions

Figure 4

Stability of secure key rate (solid line) and QBER (dashed line). Data were taken at 30-dB total loss over 1000 s, and the mean secure key rate and QBER are $7560\pm 24$ bits/s and $1.85\pm 0.03$$\%$, respectively. The small drifts are likely due to temperature fluctuations which alter the polarization transformation in the connecting optical fibers.Reuse & Permissions

Figure 5

Raw key rate (all signal detection events), secure key rate, and quantum bit error rate (QBER) vs timing window from experimental data. (a) At 40-dB total loss. (b) At 54-dB total loss. Note that raw key rate and QBER both increase with timing window, as more dark and background counts are admitted. Secure key rate shows a maximum at 1.2 ns for the 40-dB case and 0.4 ns for the 54-dB case, as the benefit of increasing the raw key rate is offset by the detriment of increasing the QBER. (c) Secure key rate vs timing window for various loss points. The observed optimal timing window is marked on each curve with an X, and the dashed line is a guide to the eye for the optimal window width trend.Reuse & Permissions