We demonstrate the use of a flexible and highly accurate Markov chain Monte Carlo quantum detector tomography method as a minimization algorithm to best describe the response of an efficient 120 nm wide NbTiN superconducting nanobridge single photon detector. Separation of the internal quantum efficiency and external quantum efficiency is possible due to the difference in saturation behavior of an ideal one-photon threshold detector as compared to a detector with non-unity one-photon internal quantum efficiency. From a statistical analysis of our measurements (at T = ( 4.23 ± 0.01 ) K , I = ( 29.4 ± 0.1 ) μ A , I / I switch = ( 0.90 ± 0.01 ) ), we find an external quantum efficiency of η = ( 1.60 ± 0.05 ) × 10 − 6 , a one-photon internal quantum efficiency of p 1 = 0.568 ± 0.008 , and a unity multi-photon (two or more) internal quantum efficiency.

We present the first observations of laser synchronised relaxation oscillations in superconducting nanowire single photon detectors. Understanding the thermal feedback behind these oscillations aids the development of photon number resolving and higher count rate detectors.