In “50-km Fiber Interferometer for Testing Gravitational Signatures in Quantum Interference” (published in Physical Review Letters, 135, 093604 (2025)), researchers demonstrated a 50-km single-photon fiber interferometer designed to explore the interplay between quantum interference and gravity. The experiment achieved a shot-noise limited phase sensitivity of (4.42\times10^{-6}) rad RMS, sufficient to resolve the expected gravitational time dilation-induced phase shift for a table-top experiment and representing an important step toward future experiments with multi-photon entangled states of light.

A key enabling technology for the experiment was Single Quantum’s multi-pixel SNSPD system, which was used as the heralding detector for the photon-pair source. The substantial losses accumulated in the 50-km interferometer required operating the source at high photon-pair generation rates in order to maintain useful coincidence rates at the interferometer output. However, because the heralding arm experienced only minimal losses, increasing the source brightness would quickly saturate conventional single-pixel detectors, effectively limiting the achievable photon flux and, consequently, the coincidence rate.

The multi-pixel SNSPD system overcame this bottleneck by supporting heralding count rates approaching 40 MHz. This allowed the source brightness to be increased without saturating the heralding detector, enabling significantly higher coincidence rates despite the large interferometer losses. The increased count rate translated directly into shorter acquisition times and reduced requirements on long-term interferometer stability. In contrast, detector systems limited to standard count rates of only 1–2 MHz would have required substantially longer integration times, making the experiment considerably more challenging.

Beyond its performance, the detector system proved highly reliable during extended measurement campaigns and was straightforward to operate. The combination of high count-rate capability, robustness, and ease of use made the Single Quantum platform an important component in realizing this large-scale quantum optics experiment.