The ultra-fast dynamics of superconducting vortices harbors rich physics generic to nonequilibrium collective systems. The phenomenon of flux-flow instability (FFI), however, prevents its exploration and sets practical limits for the use of vortices in various applications. To suppress the FFI, a superconductor should exhibit a rarely achieved combination of properties: weak volume pinning, close-to-depairing critical current, and fast heat removal from heated electrons. Here, we demonstrate experimentally ultra-fast vortex motion at velocities of 10–15 km s−1 in a directly written Nb-C superconductor with a close-to-perfect edge barrier. The spatial evolution of the FFI is described using the edge-controlled FFI model, implying a chain of FFI nucleation points along the sample edge and their development into self-organized Josephson-like junctions (vortex rivers). In addition, our results offer insights into the applicability of widely used FFI models and suggest Nb-C to be a good candidate material for fast single-photon detectors.

We fabricated and characterised nanowire superconducting single-photon detectors (SSPDs) made of 4 nm thick amorphous Mo x Si 1−x films. At 1.7 K the best devices exhibit a detection efficiency up to 18% at 1.2 µm wavelength of unpolarised light, a characteristic response time of about 6 ns and timing jitter of 120 ps. The detection efficiency was studied in wavelength range from 650 nm to 2500 nm. At wavelengths below 1200 nm these detectors reach their maximum detection efficiency limited by photon absorption in the thin MoSi film.

The photon count rate of superconducting single photon detectors made of MoSi films shaped as a 2 µm-wide strip and a 115 nm-wide meander stripline is studied experimentally as a function of the dc biasing current at different values of the perpendicular magnetic field. For the wide strip a crossover current Icross is observed, below which the photon count rate increases with increasing magnetic field and above which it decreases. This behavior contrasts with the narrow MoSi meander for which no crossover current is observed, thus suggesting different photon detection mechanisms in the wide and narrow strips. Namely, we argue that in the wide strip the absorbed photon destroys superconductivity locally via the vortex-antivortex mechanism for the emergence of resistance while in the narrow meander superconductivity is destroyed across the whole stripline, forming a hot belt. Accordingly, the different photon detection mechanisms associated with vortices and the hot belt stipulate the qualitative difference in the dependence of the photon count rate on the magnetic field.