Frequency domain side channel on lithium niobate phase modulator in high-rate quantum key distribution system

Constructing high-rate quantum key distribution (QKD) systems is an effective way to increase secure key rates. However, in such systems, modulation strengthens the correlation between signaling and non-signaling states, allowing an eavesdropper to extract information from the latter. While frequency-domain side-channel leakage from light sources has been recognized, leakage arising from modulation in high-rate QKD systems has not yet been reported. In this paper, we first constructed BB84 phase and polarization coding units and observed frequency-domain variations in increasing rates of lithium niobate (LN) modulation. Using an information-theoretic model, we evaluated the correlation between frequency and coded bits, finding side-channel mutual information of 2.44 × 10^-2 and 8.81 × 10^-3 bits/pulse at 594 MHz and 668 MHz for phase and polarization units, respectively. Information leakage increased with higher modulation rate and voltage, and potential defense schemes are briefly discussed.