Fully passive quantum bits encoding scheme based on dual-laser interference

This Letter presents a transmitter-side fully passive QKD encoding scheme based on dual-laser interference, eliminating delay-line structures and active modulation components. Two independent laser diodes are precisely controlled in time and frequency domains to achieve consistent pulse timing, intensity, and spectra. Experimental interference results agree well with theoretical intensity distributions. Numerical simulations using the FP-QKD protocol based on a dual-laser passive encoding source show secure key extraction over distances exceeding 100 km with a 10¹² block size, and an SKR above 10⁵ bits/s for short distances. The scheme offers improved stability and significantly higher encoding rates compared to prior implementations.