The challenge of maintaining photon indistinguishability over long distances has been a significant hurdle for quantum communication protocols. These protocols—including quantum teleportation, entanglement swapping, and Measurement-Device-Independent Quantum Key Distribution (MDI-QKD)—require single photons that remain indistinguishable even when separated by considerable distances.
The core problem is time jitter during synchronization between remote nodes, which reduces temporal overlap and compromises photon indistinguishability. Previous solutions attempted to address this by extending photon coherence time through narrowband filtering, but this dramatically decreased photon rates, making practical applications unfeasible.
This research presents a novel approach using common laser pulses to generate intrinsically synchronized single photons, eliminating the need for extended coherence time. The team achieved a four-fold coincidence count rate more than two orders of magnitude higher than previous methods over 50 kilometers of fiber transmission—exceeding 16 counts per second compared to previous rates. This breakthrough enabled the implementation of MDI-QKD with passive decoy states.
While Spontaneous Parametric Down-Conversion (SPDC) sources are promising candidates for generating high-quality single photons due to their industrial maturity and room temperature compatibility, maintaining their indistinguishability over distance has been challenging. The picosecond-range time jitter between remotely synchronized clocks typically exceeds the duration of single photon pulses, resulting in poor temporal indistinguishability.
Previous solutions included attempts to reduce time jitter (requiring delicate optical designs), applying narrowband filters (sacrificing efficiency), and using CW-pumped sources with time-resolved detection (resulting in low coincidence rates). A promising approach by D’Auria involved placing a common seed pulsed laser at a central location, but this only demonstrated local Hong-Ou-Mandel interference and still used inefficient narrowband filters.
The current research extends this approach by carefully designing the seed pulse profile and filtering approach to generate spectrally and temporally indistinguishable photons with high brightness and efficiency. The researchers achieved a raw visibility of 0.822 at an average photon number of 0.028, demonstrating the feasibility of employing SPDC sources for practical quantum communication protocols.
Reference: Zhan, XH., Zhong, ZQ., Ma, JY. et al. Experimental demonstration of long distance quantum communication with independent heralded single photon sources. npj Quantum Inf 11, 73 (2025). doi:10.1038/s41534-025-01025-w