Fig. 1. Schematic diagram of a satellite-ground quantum network^[26]

Fig. 2. Optical layout of long-distance continuous-variable QKD system based on GG02 proposal^[45]

Fig. 3. Physical diagram of quantum direct communication system^[56]

Fig. 4. Security of quantum cryptography over various electromagnetic wave frequencies (at room temperature) as a function of channel transmission^[63]

Fig. 5. Practical implementation of the THz QKD system^[70]

Fig. 6. Secret-key rates versus distance assuming a detection efficiency η = 10%^[65]. The left panel refers to direct reconciliation. The curves are as follows: (a) 30 THz, (b) 20 THz, (c) 15 THz, (d) 100 GHz, (e) 200 GHz, and (f) 40 THz. For curves (a), (b), and (c), δ = 50 dB/km for 15-34 THz. In the range 40-55 THz, δ = 1.77 × 10³ dB/km. From 100 GHz to 10 THz, there are several transmission windows, within a generally rising atmospheric absorption: At 100 GHz δ = 0.6 dB/km, at 200 GHz δ = 1.2 dB/km, at 1 THz δ = 10² dB/km, while at 10 THz δ = 10³ dB/km. The right panel refers to reverse reconciliation. The curves are as follows: (a) 30 THz, (b) 20 THz, (c) 15 THz, (d) 50 THz (purple line) and 40 THz (blue line), (e) 10 THz

Fig. 7. Attenuation in clear atmosphere (blue dashed line), attenuation in water vapor (black dotted line), and the total path loss(red solid line) in the frequency range of 100 GHz-1 THz at a distance of 1 km^[70]

Fig. 8. The secret key rate of reverse reconciliation-based inter-satellite THz QKD protocol against distance for different THz frequencies. The solid lines refer to the temperature of 30 K, and the dash lines refer to the temperature of 173 K^[73]

Fig. 9. Structure diagram of THz-MCM-CVQKD system^[74]