Direct observation of photon-induced vortices in superconducting films
Nucleation of vortex-antivortex pairs (VAPs) is believed to play a central role in the photon detection mechanism of superconducting detectors; however, their direct dynamic observation has remained challenging. Here, we report the direct observation of photon-induced VAP dynamics in a current-carrying superconductor as quantized voltage signals following photon absorption. The observed signals are interpreted as discrete phase-slip events, where each vortex traversal induces a 2-pi phase change of the superconducting order parameter, resulting in a quantized voltage pulse whose time integral is given by the magnetic flux quantum. We analyze the resulting quantized signals as a function of bias current, base temperature, and input photon-number states, and find that the number of VAPs generated per absorbed photon becomes effectively stabilized under specific conditions. Under these conditions, we demonstrate photon-number-resolving capability by directly counting phase-slip-induced voltage quanta. Our results reveal a detection mechanism governed by phase dynamics rather than conventional resistive transitions. We further show that photon-number resolution emerges when the fluctuation of photon-induced vortex-antivortex pair generation becomes statistically suppressed. These findings establish a new route toward photon-number-resolving detection based on phase-slip counting and open opportunities for high-speed superconducting detectors for quantum optics and photonic quantum technologies.