NbN

NbN in numbers

Transition temperature

16 K

Made as

film

Nitrogen line

Below 77 K

Why engineers choose NbN

A superconducting nanowire single-photon detector works by sitting just under its critical current, so that one photon carries enough energy to push a segment normal and produce a voltage pulse. What matters is not how cold the wire has to be but how quickly it recovers and re-arms. In NbN the excited electrons hand their energy to the lattice quickly, so the detector resets in nanoseconds and can count at high rates. Its transition temperature also sits comfortably above what a closed-cycle cryocooler reaches, which the designer spends as margin rather than as performance.

Where NbN stops

The detector is a wire a few nanometers thick and a fraction of a micron wide, and it performs like the worst constriction anywhere along its length. Making one is a film problem. Making a thousand on a chip that all work is a uniformity problem, and that, not sensitivity, is where the array count stops.