Observation of Individual Vortex Penetration in a Coplanar Superconducting Resonator

We demonstrate the microwave signatures of individual Abrikosov-vortex penetration events in superconducting microwave resonators. λ/4 resonators with a narrowed region near the grounded end acting as a vortex trap were fabricated and studied using microwave transmission spectroscopy at millikelvin temperatures. Sharp stepwise drops in resonance frequency are detected as a function of increasing external magnetic field, attributed to the entry of individual Abrikosov vortices in the narrow region. This interpretation is confirmed by nitrogen-vacancy center magnetometry, revealing discrete vortex entry events on increasing field and the absence of such drops in a device with a wider narrowed region. Our results establish a method to investigate and manipulate the states of Abrikosov vortices with microwaves access and readout vortex configurations using magnetic-field sweeps and microwave drive.