Josephson harmonics-like features in tunnel-junction SQUIDs

Josephson tunnel junctions are an essential element for superconducting quantum computing. However, higher Josephson harmonics of a tunnel junction, one of its fundamental physical properties, have traditionally been neglected. Recently, signatures of higher harmonics in tunnel junctions were observed in the spectra of single-junction transmons [1]. Here, we introduce a direct method to probe higher harmonics in tunnel junctions using symmetric SQUIDs. Across multiple devices with different circuit parameters, we observe a significant change in the qubit transition spectrum near half-flux that cannot be explained by the standard cosine potential. We discuss two possible models that agree with these experimental results: a Josephson harmonics model and a parasitic inductance model.

[1] D. Willsch et al., Nat. Phys. 20, 815-821 (2024)