Fig. 1. Optical spectra of different detection schemes with quadrature components representation. (a) Two-quadrature encoding with phase-diversity homodyne detection (2Q2D); (b) two-quadrature encoding with LO phase switching (2Q1D), showing a π/2 phase difference between the solid LO and the dashed LO; (c) single-quadrature encoding with LO phase-locked to the signal (1Q1D); (d) two-quadrature encoding with heterodyne detection at IF stage (sub-2Q2D); (e) phase-conjugated subcarrier encoding with phase-sensitive heterodyne detection (PCS).

Fig. 2. Loss-only (nex=0) AWGN capacities for different CVQKD schemes as functions of received signal photon numbers (ns). The PCS scheme with phase-sensitive detection offers the highest classic capacity closest to the Holevo bound.

Fig. 3. Simulated secure key rate of the PCS scheme, with and without the loose assumption, and other typical GG02 schemes as functions of received signal photon numbers (ns). We consider transmittance τ=0.3, nex=0.001, β=1, and perfect detectors. We use VA=ns and Vex=nex for the loose assumption and VA=2ns and Vex=2nex otherwise.

Fig. 4. Experimental setups. (a) Block diagram of the CVQKD system with phase-conjugated subcarrier modulation and the phase-sensitive heterodyne detector; (b) OIL setup with an electrical phase-locked loop (PLL). AM, amplitude modulator; BPF, bandpass filter; BS, beam splitter; EDFA, erbium-doped fiber amplifier; LNA, low-noise amplifier; LPF, lowpass filter; PD, photodetector; PBC, polarization beam combiner; PBS, polarization beam splitter; PC, polarization controller; PID, proportional-integral-differential; PZFS, piezoelectric fiber stretcher.

Fig. 5. (a) Experimental results of AWGN capacity of the proposed scheme and the sub-2Q2D scheme, and the theoretical curve plotted with zero excess noise; (b) the corresponding excess noise photons of the experimental points.

Fig. 6. (a) Experimental results of secure key rate of the proposed scheme and the sub-2Q2D scheme; (b) corresponding excess noise of photons of the experimental points.

Fig. 7. (a) Estimated excess noise; (b) SKR in the asymptotic regime. The standard deviation of the pilot phase is compared for (c) the sub-2Q2D and (d) the PCS scheme with phase-sensitive detection.

Fig. 8. Normalized power at the pilot frequency in the LO phase-scanned and phase-locked cases.