Fig. 1. Model of QPSK-QNRC system

Fig. 2. Constellation diagram of encryption code. (a) PSK-QNRC; (b) QPSK-QNRC

Fig. 3. Variation curves of the secrecy capacity with mechanism M. (a) C_Su0; (b) C_Sx0

Fig. 4. Variation curves of the secrecy capacity with internal optical amplifier gain. (a) C_Su0; (b) C_Sx0

Fig. 5. Variation curves of the secrecy capacity with mesoscopic coherent state power. (a) C_Su0; (b) C_Sx0

Fig. 6. Maximum achievable safety rates of the system in different situations. (a) QPSK-QNRC, M=32; (b) PSK-QNRC, M=32; (c) QPSK-QNRC, M=64; (d) PSK-QNRC, M=64