Scientist use superconducting detectors (MKIDs) to capture single photons coming from exoplanets. MKIDs constantly monitor their own kinetic inductance, which changes proportionally to the energy of an incoming photon. Researchers from SRON Netherlands Institute for Space Research have now more than doubled their spectral resolution by re-trapping most of the leaked energy. The research was published in Physical Review Applied.

In a superconductor at low temperature, most electrons live in pairs. An oscillating current accelerates and decelerates these pairs, giving rise to an effect called kinetic inductance. When a photon strikes a superconductor, its energy cascades through the material, breaking up thousands of electron pairs. A lower density of pairs means a higher kinetic inductance.

Scientists use this property to detect single visible and near-infrared photons, for example from exoplanets, by building superconducting single-photon detectors in the shape of microwave resonators, called Microwave Kinetic Inductance Detectors (MKIDs). These detectors constantly measure the kinetic inductance of their material and deduce if a photon has hit. And if so, with what wavelength, so that each pixel can also measure a spectrum. Pieter de Visser at SRON Netherlands Institute for Space Research and colleagues have now modified the design of MKIDs to achieve a 2.5-fold increase in the precision with which the device can measure a photon’s wavelength.

Currently, conventional single photon detectors are superconducting circuits, deposited on a thick (>300 μm) silicon or sapphire substrate. The spectral resolution of these detectors is limited, because part of the initial energy from the detected photon can leak away into the substrate through acoustical waves—phonons— before it is registered. This energy loss increases the statistical variance of the kinetic-inductance signal used to detect a photon, which broadens the measured spectrum.