Andreev bound states in a superconducting qubit at odd parity

TL;DR

This paper investigates the unique Andreev bound states in a superconducting qubit with odd parity, revealing novel low-energy structures that differ from even parity states, across various regimes, with potential experimental verification.

Contribution

It introduces a new theoretical analysis of Andreev bound states in a single Josephson junction qubit with odd parity, highlighting distinct spectral features.

Findings

Distinct low-lying states in odd parity sector

Differences between Coulomb and Josephson regimes

Predictions testable in superconductor/semiconductor junctions

Abstract

The quantum mechanics of the Josephson effect is the core ingredient for quantum technologies with superconducting circuits. A new avenue was recently opened in this field by predicting that the Josephson quantum mechanics in the odd parity sector, when a quasiparticle in trapped in an Andreev bound state, is fundamentally different from the conventional one in the even sector. The focus was then on a Josephson junction surrounded by an electromagnetic environment formed of a collection of bosonic modes, including the case of an ohmic environment. Here we consider the distinct case of a superconducting qubit made of a single Josephson junction whose environment reduces to a capacitance. We find a novel structure for the low-lying discrete states in the odd sector, which is altogether different from the one that appears in the even sector. Our study of the bound-state spectrum ranges from the Coulomb-dominated (Cooper pair box) to the Josephson-dominated (transmon) regime. Our prediction could be tested in forthcoming experiments with superconductor/semiconductor/superconductor junctions, which have been studied intensively in recent years, both using nanowires as well as two-dimensional electron gases.