  We measure the maximal distance at which two absorbed photons can jointly
trigger a detection event in NbN nanowire superconducting single photon
detector (SSPD) microbridges by comparing the one-photon and two-photon
efficiency of bridges of different overall lengths, from 0 to 400 nm. We find a
length of $23 \pm 2$ nm. This value is in good agreement with to size of the
quasiparticle cloud at the time of the detection event.
