Anharmonicity and Charge-Noise Sensitivity of Fraunhofer Qubit

TL;DR

This paper introduces the Fraunhofer qubit, a flux-tunable superconducting qubit whose anharmonicity can be significantly enhanced near one flux quantum, enabling better flux control and charge-noise protection.

Contribution

It develops a theoretical model of the Fraunhofer qubit based on Josephson interference, demonstrating anharmonicity enhancement through flux averaging effects.

Findings

Anharmonicity increases near one flux quantum due to flux averaging.

Microscopic simulations confirm anharmonicity enhancement with disorder.

Framework for flux control balancing anharmonicity and charge noise.

Abstract

We present a theory of a flux-tunable superconducting qubit, the "Fraunhofer qubit," based on the Fraunhofer interference in a wide ballistic Josephson junction. As magnetic flux threads the junction, the Josephson potential is effectively averaged over a phase window proportional to flux. For perfectly transmitting junctions, as flux approaches one flux quantum h/2e, the flux averaging transforms the potential near its minimum from a quadratic to a triangular shape, resulting in significantly enhanced anharmonicity. This enhancement persists for junctions with lower transparency conducting channels. Microscopic tight-binding simulations that include inhomogeneous electrostatic potential and disorder confirm the enhancement of anharmonicity. These results establish a framework for flux control in hybrid superconducting circuits, providing an operating point where anharmonicity and charge-noise protection can be optimally balanced.