Spin-Qubit Relaxometry Detects Half-Vortex Magnetic Fluxes of ½ in Superconductors

Why It Matters

Identifying half‑quantum vortices confirms spin‑triplet pairing and opens a pathway to harness Majorana modes for fault‑tolerant quantum bits, accelerating topological quantum computer development.

Key Takeaways

• Spin‑qubit relaxometry measures Φ0/2 magnetic flux.
• Half‑vortices may host Majorana zero modes.
• Technique distinguishes half‑quantum from full‑quantum vortices.
• Experiments resolve relaxation times around one millisecond.
• Applicable to candidate materials UTe₂, UPt₃, URhGe.

Pulse Analysis

The quest for topological quantum computation has long hinged on the experimental verification of half‑quantum vortices in unconventional superconductors. These vortices, predicted to carry half of the magnetic flux quantum (Φ0/2), are expected to bind Majorana zero modes—non‑abelian excitations that can encode quantum information immune to local noise. Yet direct observation has remained elusive because conventional magnetometry lacks the sensitivity to resolve the minute flux difference. The recent Oak Ridge study bridges this gap by leveraging quantum sensing techniques that operate at the single‑spin level.

Spin‑qubit relaxometry exploits the magnetic‑field fluctuations generated as a vortex traverses a nanometer‑scale constriction beneath a proximal spin qubit. The qubit’s longitudinal relaxation time (T1) shortens in proportion to the vortex‑induced field, allowing researchers to infer both the vortex velocity and its flux content from the measured relaxation spectrum. By tuning the bias current and geometry, the team achieved a resolvable T1 of under one millisecond, sufficient to discriminate Φ0/2 from Φ0 with a clear spectral peak at the fundamental crossing frequency. This approach offers a non‑invasive, high‑bandwidth probe compatible with existing superconducting device architectures.

The demonstrated sensitivity opens immediate avenues for testing candidate spin‑triplet superconductors such as UTe₂, UPt₃ and URhGe, where confirming half‑quantum vortices would settle long‑standing debates about pairing symmetry. Beyond fundamental physics, the ability to reliably locate Majorana‑hosting vortices paves the way for engineered networks of topologically protected qubits, a cornerstone of fault‑tolerant quantum processors. Future work will likely focus on integrating spin‑qubit sensors with scalable nanofabrication, optimizing qubit‑vortex spacing, and extending the technique to explore vortex dynamics under varying temperature and magnetic‑field conditions.