Mutual Inductance Sensing SQUID: Cryogenic microcalorimeter based on mutual inductance readout of superconducting temperature sensors
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 SQUID-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 100meV (FWHM) should be achievable for soft X-ray photons with energies below 800eV. This approach therefore represents a promising pathway towards next-generation cryogenic detectors for high-precision X-ray emission spectroscopy.