Andreev spin qubit

Description

Andreev spin qubit: the spin degree of freedom of an electronic quasiparticle trapped in the supercurrent-carrying Andreev levels of a Josephson semiconductor nanowire.

Figure

Hamiltonian

A minimal Andreev-spin-qubit model uses spin-split Andreev bound states in a phase-biased Josephson weak link:

$H=E_A(\phi ){\tau }_z+\frac{1}{2}g{\mu }_B\mathbf{B}\cdot \mathbf{\sigma }+H_{\text{SO}}+H_{\text{drive}}$

with Andreev level dispersion (short-junction limit):

$E_A(\phi )=\Delta \sqrt{1-\tau {\sin }^2(\phi /2)}$

where $\tau$ is channel transparency, $\phi$ the superconducting phase difference, and spin-orbit coupling $H_{\text{SO}}$ enables electrically driven spin control and spin-dependent supercurrent readout.

Motivation

The most promising solid-state approaches for developing quantum information-processing systems have been based on the circulating supercurrents of superconducting circuits and manipulating the spin properties of electrons in semiconductor quantum dots. The Andreev spin qubit is an attempt to combine the desirable aspects of both approaches, the scalability of the superconducting circuits and the compact footprint of the quantum dots, to design and fabricate a superconducting spin qubit.

References

• https://science.sciencemag.org/content/373/6553/430
• https://science.sciencemag.org/content/349/6253/1199.abstract
• https://www.nature.com/articles/nphys4150

Linked Papers

• hays-2021-andreev-spin-qubit

Related Entries

• transmon
• spin-qubit
• gatemon
• gatemonium

Seed Metadata

• date_published: 2021-07-23

Physics

Qubit encoded in the spin degree of freedom of an Andreev bound state in a semiconductor-superconductor nanowire Josephson junction. Spin-orbit coupling in the semiconductor (InAs) creates spin-split Andreev levels $|\uparrow ⟩$, $|\downarrow ⟩$ below the superconducting gap. The different spin states carry different supercurrents, enabling dispersive readout via a coupled microwave resonator — bridging spin and superconducting qubit paradigms.

Key Metrics

Metric	Value	Notes	Fidelity reference
Qubit coherence $T_1$	1–10 μs	Limited by quasiparticle poisoning	Hays et al. 2021
Qubit coherence $T_2$	0.1–1 μs	Early devices (2021)	Hays et al. 2021
Gate fidelity (1Q)	~95%	Microwave-driven spin transitions	Hays et al. 2021
Readout fidelity	~90%	Dispersive via resonator	Hays et al. 2021
Qubit footprint	~1 μm junction	Nanowire device	—
Operating temperature	10–30 mK	Dilution refrigerator	—

Related Qubits

• gatemon — same material platform, charge degree of freedom
• gatemonium — semiconductor-superconductor hybrid
• majorana-topological-qubit — related InAs-Al platform
• transmon — shares dispersive readout mechanism