To study distant planets, SRON researchers are developing detectors that register the colour of each incoming photon. They are searching for a suitable material with a high degree of ‘disorder’ and therefore high electrical resistance in order to absorb as much light as possible. They have now discovered that too much disorder is also not ideal because it results in signal loss. Publication in Nature Communications.
Planets orbiting other stars are so far away from us that a large telescope has to wait a few seconds before it can capture even a single photon. In comparison: at one metre distance from a light bulb, your eye captures billions of photons per second. In order to obtain useful information about a so-called exoplanet, it is essential that the detector inside the telescope does not lose any of the scarce light.
Superconducting detectors
SRON researchers are developing Kinetic Inductance Detectors (KIDs) that register every incoming visible photon, including their colour. They cool the detectors to near absolute zero to make them superconducting. Electrons then form (Cooper) pairs that flow through the material without resistance. When a photon enters, it breaks up thousands of pairs, resulting in an avalanche of so-called quasiparticles.
Quasiparticles
Quasiparticles behave like loose electrons that quickly form pairs again when they encounter each other. They reveal their quantity by generating a voltage in the material. And that is exactly what the SRON researchers need to determine the colour of each photon. The “bluer” an incident photon, the more quasiparticles. The “redder”, the fewer.
High resistance
A prerequisite for this method is that the detector actually absorbs photons in the first place. Superconductivity does not play a role in absorption because photons break up Cooper pairs, so the intrinsic resistance of the material becomes relevant again. The more resistance the electrons encounter, the easier it is for them to absorb the energy of light. Pieter de Visser (SRON) and his group are therefore studying the usability in KIDs of materials with high resistance—so-called disordered superconductors.
Marbles in a well
They have now discovered that within a disordered superconductor, each quasiparticle becomes trapped like a marble in a well. ‘You would think that this would prevent them from rearranging into pairs,’ says first author Steven de Rooij (SRON). ‘But when they break free from their well, we see that they immediately end up in another well, where another quasiparticle is already present, so that they actually encounter each other more quickly. They therefore disappear more quickly, which results in a weaker signal when a photon strikes. It turns out that we should not use a superconductor with too much disorder. On the other hand, too little disorder results in too little absorption of incoming light, so we now know that we have to choose the middle ground.’
Publication
Steven A. H. de Rooij, Remko Fermin, Kevin Kouwenhoven, Tonny Coppens, Vignesh Murugesan, David J. Thoen, Jan Aarts, Jochem J. A. Baselmans & Pieter J. de Visser, ‘Recombination of localized quasiparticles in disordered superconductors’, Nature Communications volume 16, Article number: 8465 (2025)