Probing the Hotspot Interaction Length in NbN Nanowire Superconducting   Single-Photon Detectors
  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. 
J.J. Renema, R. Gaudio, Q. Wang, M.P. van Exter, A. Fiore, M.J.A. de   Dood
cond-mat.supr-con physics.optics quant-ph