Detecting half-quantum superconducting vortices by spin-qubit relaxometry

TL;DR

This paper proposes a new method using spin-qubit relaxometry to directly detect half-quantum vortices in spin-triplet superconductors, which could host Majorana zero modes relevant for topological quantum computing.

Contribution

The authors introduce a novel experimental approach to measure magnetic fluxes of half-quantum vortices via spin-qubit relaxometry, enabling direct detection of these vortices in candidate materials.

Findings

Relaxation rate peaks when vortex crossing matches qubit transition frequency.

Magnetic flux of a vortex can be calculated from voltage and transition frequency.

Feasibility discussed for materials like UTe$_2$, UPt$_3$, URhGe.

Abstract

Half-quantum vortices in spin-triplet superconductors are predicted to harbor Majorana zero modes and may provide a viable avenue to topological quantum computation. Here, we introduce a novel approach for directly measuring the half-integer-quantized magnetic fluxes, $\Phi = h / (4e)$, carried by such half-quantum vortices via spin-qubit relaxometry. We consider a superconducting strip with a narrow pinch point at which vortices cross quasi-periodically below a spin qubit as a result of a bias current. We demonstrate that the relaxation rate of the spin qubit exhibits a pronounced peak if the vortex-crossing frequency matches the transition frequency of the spin qubit and conclude that the magnetic flux $\Phi$ of a single vortex can be obtained by dividing the corresponding voltage along the strip with the transition frequency. We discuss experimental constraints on implementing our proposed setup in spin-triplet candidate materials like UTe$_2$, UPt$_3$, and URhGe.