Mutual inductance sensing SQUID: Cryogenic microcalorimeter based on mutual inductance readout of superconducting temperature sensors

Abstract

Superconducting microcalorimeters, such as superconducting transition-edge sensors and magnetic microcalorimeters, have emerged as state-of-the-art detectors for x-ray emission spectroscopy by combining near-unity quantum efficiency with excellent energy resolution. Despite these achievements, their resolving power has not yet reached the level required to rival modern wavelength-dispersive grating or crystal spectrometers. Here, we introduce a next-generation superconducting quantum interference device-based microcalorimeter concept that exploits the strong temperature dependence of the magnetic penetration depth of a superconductor operated close to its critical temperature. The resulting mutual inductance-based readout enables in situ tunable signal amplification, while inherently avoiding hysteretic effects that commonly limit superconducting sensors. Experiments with prototype devices demonstrate robust and reproducible operation over a wide temperature range. Based on our measurements and modeling, we project that, using an optimized absorber–sensor combination, an energy resolution below 100 meV (full width at half maximum) should be achievable for soft x-ray photons with energies below 800 eV. This approach therefore represents a promising pathway toward next-generation cryogenic detectors for high-precision x-ray emission spectroscopy.