This paper reports how the superconducting micro-resonators work as high-sensitivity photon detectors, from millimeter-wave signals to high-energy particles. The resonators are called microwave kinetic inductance detectors and are suitable for use in a large format array. The resonators are easily fabricated applying a few photolithographic processes, and the resonator yield is more than 90%. Hundreds of superconducting resonators can be biased with a single readout line, enabling them to be read out together with frequency multiplexing. We introduce the recent progress of scientific projects relying on this technique, and summarize our results for millimeter-wave astronomy and a high-energy particle detection system. The device was made using epitaxial aluminum on a silicon wafer and its noise was as low as the background limit, approximately 6 × 10−18 W/Hz1/2 in the millimeter-wave range. For high-energy particle detection, we investigated two materials: niobium and a hightemperature superconductor, YBa2Cu3O7-δ. The niobium-based device detected the alpha line (5.4 MeV) events. The pulse decay time was approximately 5 μs and energy resolution was approximately 1 MeV.
