A superconducting strip thinner than a human hair lies on a silicon substrate. Next to it is a tiny waveguide through which a beam travels. When a photon hits the strip, it heats up by a fraction of a degree and immediately changes its resistance. This is how light turns into an electrical signal.
Four such strips are assembled on a single chip, and light is fed to each one through a separate waveguide that fits into a special groove. The groove is carved with micron precision so that the beam hits the waveguide's end face exactly. The device responds to a signal that changes three billion times per second — a million times faster than a household light bulb blinks.
Such a receiver is needed where it's important to catch a single photon without confusing it with noise — for example, in quantum computers or radio telescopes listening to distant galaxies.
The superconducting sensor is so sensitive that a single photon can heat it up. For comparison: to heat a glass of water by one degree, you'd need about a hundred billion billion photons.
In the series "The Expanse," ships catch signals from other worlds with hypersensitive receivers — similar technologies are needed for communicating with distant probes.