Figure 4

Contour plots of signal-to-noise ratio (\(\mathit {SNR}_{\mathit {E}91}\), left) and secure key rate (\(R^{S}_{\mathit {E}91}\), right) for an E91 scheme with a typical Fried parameter \(r_{0}=20\mbox{ cm}\) and an elevation of 60°, showing the trade-off between CubeSat detection efficiency \(\eta_{B}\) and timing resolution \(\tau\approx 2\sqrt{t_{A}^{2}+t_{B}^{2}}\) where we assume \(t_{A}=16\mbox{ ps}\) to be fixed. For our study, since we want to show principal feasibility, we chose the PDM020 detector [43] (shown with a cross) which provides a good compromise between SNR and key rate. Other detectors are shown for comparison.