Vortices in Granular Superconducting Films Exhibit Quantum Two-Level Behaviour

Vortices trapped in granular superconducting films can behave as two-level systems with microsecond-range quantum coherence and energy relaxation times that reach fractions of a millisecond, according to a paper published in Nature. A defining characteristic of superconductors is their tendency to expel magnetic fields.

Above a critical threshold, magnetic flux penetrates superconductors in discrete quanta carried by Abrikosov vortices.

The superconducting gap is completely suppressed at the vortex core, rendering them dissipative, semi-classical entities. Abrikosov vortices impact applications from high-current-density wires to quantum devices. Material disorder can drive a crossover to vortices that preserve an energy gap at the core, owing to intrinsic or emergent granularity on the scale of the coherence length.

Quantum vortex behaviour could emerge in the effective tunnel-junction regime. Coherent manipulation of vortex states has remained elusive prior to the reported work. Using the tools of circuit quantum electrodynamics, researchers performed coherent manipulation and quantum non-demolition readout of vortex states in granular aluminium microwave resonators.

The current paper is titled 'Quantum coherent manipulation and readout of superconducting vortex states'. @Nature reported that the experiments demonstrate vortices trapped in granular superconducting films can behave as two-level systems exhibiting the measured coherence and relaxation properties.

The work heralds future directions for quantum information processing, materials characterization and sensing.

A related paper titled 'Magnetic memory and spontaneous vortices in a van der Waals superconductor' was published on 27 July 2022. The superconducting gap is completely suppressed at the vortex core in conventional Abrikosov vortices. In contrast, the granular aluminium system preserves an energy gap at the core, allowing the observed quantum coherent behaviour.

Prior signatures of quantum vortex behaviour had been observed in diverse systems. Yet the ability to perform coherent manipulation had remained out of reach until the circuit quantum electrodynamics approach succeeded in the granular aluminium resonators.

The energy relaxation times measured in the vortex states reach fractions of a millisecond while coherence persists in the microsecond range. These timescales position the vortex-based two-level systems as potential building blocks for new forms of quantum devices.

Applications that stand to benefit range from high-current-density wires, where vortex motion creates dissipation, to quantum information processors that could exploit the long-lived states.

The quantum non-demolition readout technique preserves the vortex state during measurement, a key requirement for quantum information protocols.