Observation of individual vortex penetration in a coplanar superconducting resonator

TL;DR

Problem: detecting and controlling single Abrikosov vortices in superconducting microwave resonators is challenging due to small impedance changes and background nonlinearities. Approach: engineer a narrow neck at the grounded end of a λ/4 CPW Nb resonator to concentrate current and enable single-vortex entry, with verification via NV-center QDM imaging. Findings: discrete steps in $f_0$ (≈0.2–0.5 MHz) accompany each vortex entry, with concurrent $Q_i$ drops; step amplitudes scale as $Z_v(\\omega) \\propto W^{-2}$ and depend on neck width; NV imaging shows vortices nucleating near the neck and obeying geometrical barriers. Significance: enables on-chip single-vortex metrology, fast multiplexed readout, and a platform to study vortex pinning/depinning and mitigate vortex-induced loss in superconducting microwave circuits.

Abstract

We demonstrate the detection and control of individual Abrikosov vortices 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 NV center magnetometry revealing discrete vortex entry events on increasing field. Our results establish a method to investigate and manipulate the states of Abrikosov vortices with microwaves.

Figures (4)

• Figure 1: (a) Optical image of the chip showing the common feedline and four multiplexed $\lambda/4$ resonators. (b) Zoom in on the grounded end of one resonator, showing a constriction in the center conductor. (c) Further magnification of the 1 $~µ m$-wide neck used to pin individual vortices. The narrow neck at the grounded end is designed to enhance current density and increase sensitivity to vortex entry.
• Figure 2: (Left) Resonator frequency shift as a function of applied magnetic field current, showing discrete steps corresponding to vortex entry events. (Right) Transmission spectra taken at three values of magnetic field (vertical dashed lines), indicating a progressive decrease in the internal quality factor $Q_i$.
• Figure 3: (Top) QDM Magnetic field map over a wide field of view in the neck region of the device with a $2~µm$-wide neck after field cooling in $113~µT$. (Bottom left) Zoomed-in view of the red dashed rectangular region in the top panel. (Bottom right) Zoomed-in view of the black dashed rectangular region in the top panel.
• Figure 4: (a) Magnetic-field imaging. (ZF) Magnetic-field map after cooling in a near-zero field ($\lesssim$1µT). (540 µ T) After increasing the external field to $\sim$540µT. (540 µ T+MW) After sweeping the microwave (MW) drive across resonance. (630 µ T+MW) After increasing the field to $\sim$630µT and repeating the MW sweep. (b) Fine view of the optical image obtained during QDM magnetometry measurement in (a, 540 µ T). The field of view corresponds to the red dashed rectangular in (a). The narrowing geometry is visible. The blue dashed line corresponds to the stripline shape for eyeguide. (c) Fine views of the magnetic images for (540 µ T) and (540 µ T+MW) (Red dashed region). The slow background gradient observable in (a) is subtracted by another magnetic image taken under preceding measurement under 450 µ T, with multiplying the average difference SI. Vortices are located near the edge.