Counting individual photons in the visible is routine. Silicon avalanche photodiodes detect single visible photons with efficiency exceeding 90%. Each absorbed photon triggers a cascade of charge carriers, producing a measurable electrical pulse — a click.
In the mid-infrared — wavelengths from 3 to 25 micrometers, where warm objects glow and molecules absorb — single-photon detection is far harder. The photon energy drops with wavelength: a 25-micrometer photon carries 40 times less energy than a visible photon. Thermal background radiation at room temperature peaks in this range, drowning individual photons in noise. Semiconductor detectors designed for the mid-infrared suffer from dark counts — false clicks from thermal excitation of charge carriers in the detector material itself.
Ras-Vinke, Kouwenhoven, Baselmans, and collaborators (arXiv 2602.22970, February 2026) demonstrate single-photon counting at 3.8, 8.5, 18.5, and 25 micrometers using kinetic inductance detectors — superconducting resonators whose resonant frequency shifts when a photon is absorbed. The photon breaks Cooper pairs in the superconducting film, increasing the kinetic inductance, which shifts the resonance. Each photon produces a measurable frequency shift proportional to its energy.
The key innovation is the membrane geometry. The superconducting film is deposited on a thin silicon nitride membrane — only 110 nanometers thick — suspended over a cavity. The membrane serves two purposes: it minimizes the volume of superconductor that the photon must perturb (smaller volume means larger frequency shift per photon), and the cavity beneath enhances absorption by creating an optical resonance. The membrane-based design outperforms conventional solid-substrate configurations at shorter wavelengths, where the smaller photon energy produces smaller signals that require greater sensitivity to detect.
At 3.8 micrometers, the energy resolution is sufficient to distinguish individual photon energies. At 25 micrometers — the longest wavelength demonstrated — the resolution is lower, but individual photon arrivals are still clearly resolved above the noise floor. Background count rates are minimal.
The application is exoplanet spectroscopy. The atmospheres of temperate exoplanets — planets with surface temperatures comparable to Earth — emit thermal radiation peaking in the mid-infrared. Molecular absorption features of water, carbon dioxide, methane, and ozone lie in this wavelength range. Detecting these features in the spectra of Earth-like planets around other stars requires mid-infrared detectors sensitive enough to count individual photons arriving at rates of one per second or less from objects billions of times fainter than their host stars. Each photon counted is a datapoint. Each datapoint constrains the atmospheric composition of a world you cannot visit.