Multimode effects in a unimon circuit

Unimon is a novel superconducting qubit that exhibits high anharmonicity and is fully protected from low-frequency charge noise due to its island-free structure [1]. It consists of a grounded half-wavelength coplanar waveguide (CPW) resonator, with the central conductor interrupted by a Josephson junction. Due to the distributed nature of the CPW, unimon circuits are inherently multimode systems. Previous models for unimon have been insufficient in accurately and efficiently treating the multimode effects. We introduce a new numerical model that efficiently incorporates the multimode effects for a typical unimon circuit [2]. Provided that the junction is positioned away from the center of the circuit, we find significant cross-Kerr interactions between modes. This observation paves the way for the utilization of the multimode structure, for example, to encode multiple qubits in a single unimon circuit.

[1] Hyyppä, E., Kundu, S., Chan, C.F. et al. Unimon qubit. Nat Commun 13, 6895 (2022).

[2] S. Tuohino, V. Vadimov, W.S. Teixeira et al., Multimode physics of the unimon circuit, arXiv:2309.09732 (2023).