Infrared photons are invisible to photon counters based on semiconductors because they have less energy than the energy gap: photons must supply at least this much energy to change the electrical characteristics of the semiconductor. Sobolewski and colleagues realised that superconductors could be more sensitive to lower-energy photons because their energy gaps are thousands of times smaller.

Conventional superconductors lose their electrical resistance when electrons pair up and flow through the superconductor crystal, aided by lattice vibrations known as phonons. The energy gap in such materials is the energy needed to split the electron pairs.

Sobolewski's team deposited a strip of niobium nitrite just a few atoms thick onto a sapphire substrate. The strip was 0.2 micrometres thick and 1 micrometre long, and became a superconductor when it was cooled to 4.2 kelvin. When a photon with more energy than the superconducting energy gap is absorbed by this strip, it breaks apart the electron pairs and creates a local pool of energetic electrons - or a 'hotspot' - in which superconductivity breaks down. Since the hotspot is as wide as the strip, it blocks superconductivity and a voltage is registered.

After about 30 picoseconds - that is, 30 x 10-12 seconds - the energetic electrons in the hotspot spread out and lose their energy through collisions with phonons. This restores superconductivity and the device is ready to detect the next photon.

This gigahertz repetition rate enables the device to detect extremely brief bursts of photons, such as the fleeting pulse of infrared light emitted by a transistor when it switches. This light reveals whether the transistor is switching at the correct time. According to Sobolewski and colleagues, their device could identify a single misbehaving component among the billions in a modern computer.

Operating at cryogenic temperatures, the new device can detect single photons because it is less prone to thermal 'noise'. This could make the detector useful in communications between Earth and Mars, and has attracted the interest of NASA. "When you're dealing with such incredible distances, you may only be able to catch a few photons from a transmitter on Mars", explains Sobolewski.