Abstract
We consider a superconducting half-wavelength resonator that is grounded at its both ends and contains a single Josephson junction. Previously this circuit was considered as a unimon qubit in the single-mode approximation where dc-phase-biasing the junction to π leads to increased anharmonicity and 99.9% experimentally observed single-qubit gate fidelity. Inspired by the promising first experimental results, we develop here a theoretical and numerical model for the detailed understanding of the multimode physics of the unimon circuit. To this end, first, we consider the high-frequency modes of the unimon circuit and find that even though these modes are at their ground state, they imply a significant renormalization to the Josephson energy. We introduce an efficient method to fully account for the relevant modes and show that unexcited high-lying modes lead to corrections in the qubit energy and anharmonicity. Interestingly, provided that the junction is offset from the middle of the circuit, we find strong cross-Kerr coupling strengths between a few low-lying modes. This observation paves the way for the utilization of the multimode structure, for example, as several qubits embedded into a single unimon circuit.
| Original language | English |
|---|---|
| Article number | 033001 |
| Journal | Physical review research |
| Volume | 6 |
| Issue number | 3 |
| DOIs | |
| Publication status | Published - Jun 2024 |
| MoE publication type | A1 Journal article-refereed |
Funding
The authors acknowledge funding from the Academy of Finland Project no. 316619 and the Academy of Finland Centre of Excellence program (project Nos. 352925 and 336810), the European Research Council under Advanced Grant no. 101053801 (ConceptQ), FICORE project Speedup of Quantum Computations by Many-Qubit Logic under MEC Global program pilot initiatives, and the Vilho, Yrj\u00F6 and Kalle V\u00E4is\u00E4l\u00E4 Foundation of the Finnish Academy of Science and Letters.