************************
Appendix Principle of operation
************************

Superconducting-nanowire single-photon detectors transition from the superconducting state to the normal state upon the absorption of a photon. This transistion gives a fast electrical pulse that is amplified by the SNSPD driver.
The operation principle of an SNSPD is outlined in the figure below. 

The superconducting nanowire is biased with a direct current (Ibias) close to the critical current (Ic), meaning that the current density flowing through the wire is near the critical current density, above which the superconductivity of the nanowire breaks down. When an incident photon of sufficient energy is absorbed by the wire, the superconductivity in that region is disrupted and a part of the detector becomes resistive (Rn >1 kΩ). This is modelled as a switch which is closed when the SSPD is superconducting and opened after a photon is absorbed. When the detector is resistive, the bias current is reflected towards the amplification electronics, in the SNSPD driver, and creates a voltage pulse. Quickly after the detection event takes place the superconductivity is recovered, the biasing current returns through the nanowire and the device is ready to detect the next incoming photon.
