Quantum tomography of superconducting single photon detectors

Nanowire superconducting single photon detectors (SSPDs) consist of a thin (~ 4 to 6 nm) and narrow (~ 50 to 200 nm) superconducting wire biased close to the critical current. Operating conditions are such that the absorption of one or more photons locally transfers the detector to its normal state, which then results in a detectable electrical pulse. SSPDs are versatile detectors that operate in a wide wavelength range (from UV to mid-infrared) with almost negligible dark counts. They are typically shaped in the form of a meander to cover a sizeable fraction of a sufficiently large 2D surface, but other shapes have also been produced and are often easier to analyse. Until recently, the working principle of SSPDs was badly understood and several competing theory were formulated.