Fiber-based double-pass single-crystal photon-pair source for quantum key distribution in a network
Maximilian Tippmann, Erik Fitzke, Oleg Nikiforov, Philipp Kleinpaß, Till Dolejsky, Maximilian Mengler, Thomas Walther · Physical Review Applied · 2025
Entanglement-based quantum key distribution (QKD) protocols require robust and stable photon-pair sources in terms of high heralding efficiencies or photon-pair generation rates even under harsh environmental conditions, e.g., when operated in the field. Here we report on a type-0 spontaneous parametric down-conversion photon source in periodically poled crystals based on simultaneous operation of second harmonic generation and spontaneous parametric down-conversion (SPDC) in a double-pass configuration within the same nonlinear crystal, further simplifying the hardware requirements of SPDC photon sources. Our source is flexible, tunable, alignment-free, relatively broadband, and completely fiber coupled. Its rack-compatible and modular setup allows a straightforward plug-and-play integration of coding modules, e.g., interferometers to enable various QKD protocols such as phase or phase-time coding. We compare the double-pass configuration with our single-pass type-II and type-0 SPDC source configurations. To evaluate the conversion efficiencies of our modules, we use data postprocessing to remove artifacts from detector after-pulsing and dead times of the detectors.