Fig. 1. Experimental setup. (a) The detailed structure of the experimental system which consists of four main parts. AWG, arbitrary waveform generator; IQM, IQ modulator; VOA, variable optical attenuator; BS, beam splitter; BHD, balanced homodyne detector; OSC, oscilloscope. (b) The signal loaded into the two channels of the AWG, each of which is a combination of the classical signal and the quantum signal. The blue line represents the classical (QPSK) signal, and the orange line represents the quantum (GMCS-CVQKD) signal.

Fig. 2. Relation between frequency interval Δf and frequency division noise ε_F under different amplitude ratios n. Parameters are set as a = 9.432 × 10⁹⁹ and b = −12.79. It describes the change of frequency division noise at different frequency intervals and different amplitude ratios.

Fig. 3. The constellation diagram of the classical signal and the quantum signal. There are 10,000 samples in the quantum signal constellation, while the four branches of the classical signal add up to 10,000 samples. This explains why the GMCS constellation looks denser than the QPSK one.

Fig. 4. Power spectrum diagram of the original signal received by Bob. The peak at 100 MHz is the classical signal and the peak at 500 MHz is the quantum signal.

Fig. 5. (a) The excess noise and secret key rate of the quantum signal. (b) The bit error rate and bit rate of the classical signal.