[ED7] Quantum Information Techonology 2

2:30 PM - 2:45 PM JST(5:30 AM - 5:45 AM UTC)

[ED7-04] Observation of Individual Vortex Penetration in a Superconducting Resonator

*Kirill Shulga¹ (1. The University of Tokyo (Japan))

Keywords:

Abrikosov vortex,superconducting resonator,microwave spectroscopy,single-vortex detection

We present the observation of discrete, quantized frequency shifts in superconducting Nb thin-film λ/4 resonators caused by the entry of individual Abrikosov vortices. Using microwave transmission spectroscopy at 20 mK, we detect sharp, reproducible steps in the resonance frequency during perpendicular magnetic field sweeps. These steps, typically 0.2–0.5 MHz, are attributed to single-vortex penetration and pinning in a purpose-designed constriction at the grounded end of the resonator. The device geometry, originally developed for dispersive readout of flux-type qubits, was modified to include constrictions of varying widths (1–10 µm) in the center conductor. This design enhances local current density, maximizing the reactive response to vortex trapping. Comparative measurements reveal that narrower constrictions produce larger frequency shifts per vortex, confirming the role of current crowding in coupling vortex dynamics to the resonator’s electromagnetic mode. In addition to the frequency steps, each vortex entry event is accompanied by a measurable drop in the loaded quality factor, reflecting increased dissipation from vortex motion and core losses. This simultaneous access to reactive and dissipative changes enables full characterization of the electromagnetic impact of a single vortex. The observed phenomena are modeled within the Coffey–Clem framework for AC vortex response, incorporating realistic values for vortex viscosity, pinning strength, and local current enhancement. The model reproduces the step magnitudes and explains the hysteresis between vortex entry and exit, which arises from strong pinning in the constriction. These pinned states persist down to zero applied field, leaving a remanent frequency shift. Our results demonstrate that superconducting resonators with engineered constrictions are sensitive probes of single-vortex dynamics. This capability opens pathways for magnetic single-particle detection, real-time monitoring of vortex-bound excitations, and improved understanding of vortex behavior in superconducting thin films under extreme conditions.