Superconducting Nanowire Single Photon Detectors (SNSPDs) are a leading technology for quantum optics and information, offering fast recovery, low timing jitter, high detection efficiency, and intrinsic photon number resolution, making them ideal for future quantum applications [1]. We introduce a Monte-Carlo-Markov-Chain (MCMC) approach to Detector Tomography for nanobridge NbTiN devices, separating internal and external quantum efficiencies. This is key to understanding SNSPD detection mechanisms and performance trade-offs.

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.