Origin of anomalously low SPCM detection efficiency

Determine the dominant mechanisms causing the unexpectedly low single-photon detection efficiency of the Excelitas SPCM-AQ4C near 737 nm within the described microscope optical path, beyond the identified fiber-coupling loss, and quantify their contributions relative to the expected approximately 60% efficiency for this wavelength range.

Background

The authors measured the detection efficiency of their single-photon counting module (SPCM) as part of a system efficiency audit and found it to be substantially lower than expected for the 737 nm wavelength region. While some loss is attributed to fiber coupling, the majority of the shortfall remains unexplained.

This unresolved discrepancy affects the overall collection efficiency from the photonic device and could confound quantitative comparisons, such as cooperativity estimates or count-rate expectations. Identifying and mitigating the source(s) of this loss is therefore important for reliable performance assessment and scalability.

References

We note that this is anomalously lower than the expected SPCM detection efficiency of roughly 60% in this wavelength range. The reduced efficiency, of roughly 35%, is due in part to the fiber coupling loss but is largely of unknown origin.

— A scalable gallium-phosphide-on-diamond spin-photon interface  (2601.04733 - Yama et al., 8 Jan 2026) in Section SI.3, Subsection "System efficiency"