Single Quantum’s 50-km Interferometer Achieved 4.42×10⁻⁶ Rad Phase Sensitivity

Single Quantum’s technology has enabled the successful operation of a 50-kilometer single-photon fiber interferometer, achieving a phase sensitivity of 4.42×10⁻⁶ rad RMS. This level of precision is sufficient to detect gravitational time dilation in a table-top experiment, representing a crucial advance in the pursuit of more ambitious tests of quantum gravity. A key to this achievement was overcoming a saturation bottleneck with a multi-pixel SNSPD system, allowing for heralding count rates approaching 40 MHz despite substantial losses within the 50-kilometer fiber network. The researchers reported that this detector system proved highly reliable during extended measurement campaigns and was straightforward to operate, facilitating the large-scale quantum optics experiment and enabling future investigations with entangled states of light.

Achieving this sensitivity required overcoming substantial signal loss inherent in a 50-kilometer fiber optic system; the team addressed this by maximizing photon-pair generation rates at the source. Counterintuitively, the primary limitation wasn’t loss within the interferometer’s measurement arm, but saturation of conventional single-pixel detectors in the heralding system, which would have restricted the achievable photon flux. Single Quantum’s multi-pixel superconducting nanowire single-photon detector (SNSPD) system proved essential, enabling heralding count rates approaching 40 MHz, a significant improvement over detectors limited to 1 or 2 MHz. This higher rate allowed the source brightness to be increased without detector saturation, directly translating into shorter data acquisition times and lessening demands on long-term interferometer stability. The experiment, detailed in Physical Review Letters, represents a step toward utilizing multi-photon entangled states for even more ambitious tests of fundamental physics, leveraging a platform that balanced performance with practical usability. Maintaining high count rates throughout the extended measurement period was critical to the success of the large-scale quantum optics experiment.

The pursuit of increasingly sensitive quantum experiments has long been constrained by detector limitations; conventional single-photon detectors struggle to maintain performance as photon fluxes increase, hindering progress in long-baseline interferometry. This capability translated directly into reduced acquisition times and lessened demands on long-term interferometer stability, ultimately supporting the demonstration of a phase sensitivity of 4.42×10⁻⁶ rad RMS.