Optimizing Anharmonicity in Nanoscale Weak Link Josephson Junction Oscillators

TL;DR

This paper explores how to engineer nonlinearity in nanoscale weak link Josephson junctions, enabling their use as sensitive quantum circuits and bifurcation amplifiers with potential advantages over traditional tunnel junctions.

Contribution

It provides a detailed calculation of the current phase relation in nanoscale weak links and demonstrates their suitability for nonlinear oscillators and qubits.

Findings

Nanoscale aluminum weak links can be used to create nonlinear oscillators.

These oscillators exhibit sufficient anharmonicity for qubit applications.

Weak link junctions offer a practical alternative to tunnel junctions in quantum circuits.

Abstract

Josephson tunnel junctions are widely used as nonlinear elements in superconducting circuits such as low noise amplifiers and quantum bits. However, microscopic defects in the oxide tunnel barrier can produce low and high frequency noise which can potentially limit the coherence times and quality factors of resonant circuits. Weak link Josephson junctions are an attractive alternative provided that sufficient nonlinearity can be engineered. We compute the current phase relation for superconducting weak links, with dimensions comparable to the zero temperature coherence length, connected to two and three dimensional superconducting electrodes. Our results indicate that 50-100 nm long aluminum nanobridges connected with three dimensional banks can be used to construct nonlinear oscillators for bifurcation amplification. We also show that under static current bias, these oscillators have a sufficiently anharmonic energy level structure to form a qubit. Such weak link junctions thus present a practical new route for realizing sensitive quantum circuits.