Security‐Enhanced Differential Binary‐Phase‐Shift‐Keying Quantum Key Distribution With a Wavelength‐Switchable Single‐Photon Carrier

A single‐chip dual‐wavelength coherent photon source is proposed to introduce additional parametric uncertainty and improve the security scaling of single‐photon differential binary‐phase‐shift‐keying quantum key distribution (D‐BPSK‐QKD). Simultaneous lasing of two longitudinal modes in the same semiconductor laser cavity generates two coherent photon groups at different wavelengths, and attenuation of their combined output to the single‐photon level inevitably produces wavelength uncertainty at the receiver. Optical injection control enables stabilized D‐BPSK encoding on one selected mode while the other mode remains free‐running, which supports mode‐selective coding and a decoy scenario during single‐photon streaming. This intermodal wavelength switching/selection provides another degree of freedom for QKD security and increases the difficulty of eavesdropping attacks. The dual‐wavelength single‐photon bits deliver D‐BPSK‐QKD codes with a QBER of <4.1% under 2 ¹⁰ ‐bit encryption, a sifted key rate of 408.6 kbit/s, and a secure key rate (SKR) of 10.5 kbit/s at an average photon number of 0.22 photons#/bit. The transmission distance can be extended to more than 13.25 km with a SKR of 3.6 kbit/s under a nearly equivalent QBER. This versatile wavelength‐uncertain coding or decoying operation enables flexible and secure D‐BPSK‐QKD transmission over wavelength‐division‐multiplexed channelized single‐mode fiber links.