Frequency-Division Multiplexed CV-QKD System

Abstract

We propose a frequency-division multiplexed (FDM) continuous-variable quantum key distribution (CV-QKD) system with enhanced spectral efficiency through dense multiplexing of low-symbol-rate signals. A four-channel 10-Mbaud FDM-CV-QKD system was experimentally demonstrated using Gaussian modulation, a transmitted local oscillator, and homodyne detection. Under a finite-size scenario (N = 10^7), the system achieved a 3.7-fold back-to-back secret key rate gain and outperformed the single-channel system for distances up to 41.1 km.

Figures (5)

• Figure 1: (a) Experimental setup for the FDM-CV-QKD system (BB: baseband signal, Clk: clock). (b) Schematic illustration of the Gaussian-modulated CV-QKD signal distribution in phase space. (c) Electrical spectra of an IF signal ($f_{\text{IF}}$ = 100 MHz) generated from a 10-Mbaud baseband signal, with and without a 10-MHz LPF applied.
• Figure 2: (a) Estimated excess noise as a function of the main carrier power of the CV-QKD signal. (b) Measured spectra of detector noise, shot noise, and total noise including frequency noise at main carrier powers of $-56$ and $-48$ dBm.
• Figure 3: (a) Estimated excess noise as a function of $f_{\text{IF}}$ for 10-, 15-, and 20-Mbaud single-channel signals. (b) Ratio of detector noise to shot noise at various $f_{\text{IF}}$, measured with a 10-MHz noise bandwidth.
• Figure 4: (a) Estimated excess noise of each channel versus channel spacing in a two-channel FDM system (10 Mbaud per channel), with the first channel at $f_{\text{IF}}=64$ MHz. (b) Estimated excess noise for all channels as a function of the number of channels, with the first channel at $f_{\text{IF}}=64$ MHz and a 40-MHz channel spacing. Markers represent individual channel excess noise, while lines indicate the average.
• Figure 5: (a) Total SKR as a function of transmission distance for FDM-CV-QKD with 10-Mbaud signals, comparing finite- ($N=10^7, m=1.25 \times 10^6$) and infinite-size key scenarios. Markers represent experimental verification via emulated channel loss using VOAs. (b) SKR gain as a function of transmission distance for multi-channel 10-Mbaud FDM signals and single-channel high-baud-rate signals.