Optimizing optical TESs for high-efficiency Multimode Fiber coupling

Transition-Edge Sensors (TESs) are superconducting photon detectors offering high quantum efficiency, energy resolution, and photon-number discrimination, making them attractive for quantum sensing applications. We report on the design, fabrication, and characterization of a Ti/Au/Ti TES optimized for multimode fiber coupling in NV-center experiments. The device integrates a bi-layer anti-reflective coating optimized for 700 nm, the emission wavelength range of NV centers in which we are interested. Optical characterization in a cryogenic setup demonstrated an energy resolution below 2 eV and photon-number resolution capability. A dedicated calibration setup was used to evaluate the system detection efficiency (SDE), yielding 0.85 ± 0.02. These results confirm the suitability of large-area TESs for efficient multimode fiber coupling and highlight their potential for improving the performance of NV-center-based quantum sensing experiments.