The Andreev spin qubit

Short answer: The Andreev spin qubit uses the spin of an electronic quasiparticle trapped in the supercurrent-carrying Andreev levels of a Josephson semiconductor nanowire. A 2021 Science paper, with an arXiv preprint version, reports coherent spin manipulation with a spin-flip time of 17 microseconds and a spin coherence time of 52 nanoseconds. The coherence time is short, and this is an early laboratory demonstration.

The idea

The abstracts start from two architectures for solid-state quantum information processing: electron spins confined in semiconductor quantum dots, and the collective electromagnetic modes of superconducting circuits. According to the authors, superconducting qubits involve macroscopic numbers of electrons and are more easily coupled, but are relatively large, while electrostatically confined spins are compact but more difficult to link. The preprint gives rough scales of millimetres for superconducting qubits and micrometres for confined spins.

The Andreev spin qubit

The authors combine features of both platforms in what they call the Andreev spin qubit. Its qubit is the spin degree of freedom of an electronic quasiparticle trapped in the Andreev levels of a Josephson semiconductor nanowire, levels that carry a supercurrent. Because the spin sits in a superconducting circuit, it can in principle be read through circuit quantum electrodynamics, the readout technique used for superconducting qubits.

What was measured

The authors report coherent spin manipulation that combines single-shot circuit quantum electrodynamics readout with spin-flipping Raman transitions. They find a spin-flip time of 17 microseconds and a spin coherence time of 52 nanoseconds. They say these results herald a regime of supercurrent-mediated coherent spin-photon coupling at the single-quantum level. The preprint adds that the work furthers understanding and control of Andreev levels, which it describes as the parent states of Majorana zero modes in semiconductor-superconductor heterostructures. That is context from the authors, and the abstracts report no Majorana result.

Why the readout matters

Single-shot readout means the spin state is determined in one measurement, without averaging over many repeats. The authors pair this with Raman transitions that flip the spin, which is how they demonstrate control and measure the spin-flip time. Having both readout and control in a circuit is what lets the lifetimes above be measured directly instead of inferred.

Limits

The two figures differ by a large factor. The spin lives for tens of microseconds before flipping, but stays coherent for only tens of nanoseconds, so the coherence is the limiting number. The abstracts describe one device and one set of measurements, and they do not report two-qubit operations or error rates. Claims about future spin-photon coupling are the authors' outlook. The Yale preprint copy listed as a third source is the same work and adds no independent confirmation.

FAQ

What is an Andreev spin qubit?

A qubit stored in the spin of a quasiparticle trapped in the Andreev levels of a Josephson semiconductor nanowire.

How is it read out?

By single-shot circuit quantum electrodynamics readout, as in superconducting circuits.

What lifetimes were reported?

A spin-flip time of 17 microseconds and a spin coherence time of 52 nanoseconds.

Why combine the two platforms?

The authors say superconducting qubits couple easily but are large, while confined spins are compact but harder to link.

Is it ready for computing?

No. The abstracts describe a first demonstration of a single device.