Reduced Active Area in Transition-Edge Sensors for Visible-NIR Photon Detection: A Comparison of Experimental Data and Two-Fluid Model

Transition-edge sensors (TESs) are very versatile superconducting devices used to detect radiation from gamma-rays to visible and submillimeter. The intrinsic capability to measure the energy of the absorbed photons with very high energy resolution and the related possibility to resolve the number of incident photons distinguish photon-number resolving (PNR) devices from any other photon detectors. PNR detectors are fundamental for the measurement of the photon-number distribution of singlephoton emitters and for the progress of quantum information technology and quantum metrology. By reducing the active area of TESs for visible-NIR light from typical values of 10^-10 m² toward 10^-12 m² and by increasing the TES operating temperature, we should be able to combine the high energy resolution permitted by very low heat capacity with fast response time. To support the future design and development of this new type of detector, in this work we compare experimental data with circuit simulation results based on the two-fluid theory.