Effects of electron-electron interaction and spin-orbit coupling on Andreev pair qubits in quantum dot Josephson junctions

TL;DR

This paper studies how electron interactions and spin-orbit coupling influence Andreev bound states in quantum dot Josephson junctions, revealing effects on spin polarization, decoherence, and control of Andreev pair qubits.

Contribution

It analyzes the even-parity sector of the superconducting Anderson impurity model with advanced numerical methods, highlighting the impact of interactions on qubit properties.

Findings

Electron-electron interactions enhance spin transitions via YSR components.

ABS states become sensitive to magnetic fluctuations, affecting decoherence.

Strong interactions near the ABS-YSR crossover enable spin control and quantum transduction.

Abstract

We investigate the superconducting Anderson impurity model for interacting quantum dot Josephson junctions with spin-orbit coupling and a term accounting for tunnelling through higher-energy orbitals. These elements establish the conditions required for spin polarization in the absence of external magnetic field at finite superconducting phase bias. This Hamiltonian has been previously used to model the Andreev spin qubit, where quantum information is encoded in spinful odd-parity subgap states. Here we instead analyse the even-parity sector, i.e., the Andreev pair qubit based on Andreev bound states (ABS). The model is solved using the zero-bandwidth approximation and the numerical renormalization group, with further insight from variational calculations. Electron-electron interaction admixes single-occupancy Yu-Shiba-Rusinov (YSR) components into the ABS states, thereby strongly enhancing spin transitions in the presence of spin-orbit coupling. The ABS states can thus become sensitive to local magnetic field fluctuations, which has implications for decoherence in Andreev pair qubits. For strong interaction $U$, especially in the cross-over region between the ABS and YSR regimes for $U \sim 2\Delta$, charge, spin, and inductive transitions can all become strong, offering avenues for spin control and quantum transduction.