We present the design and characterization of a three-Josephson-junction superconducting loop circuit with three large shunt capacitors. Used as a qubit, the circuit shows long energy-relaxation times, of the order of 40 \unicode[Times]x3BCs and a spin-echo dephasing time of 9.4 \unicode[Times]x3BCs. The circuit has high anharmonicity, of 2π×3.69 GHz. We extract the multilevel relaxation and dephasing rates of the circuit used as a qutrit and discuss the possible sources for the decoherence. The high anharmonicity allows for fast qubit control with nanosecond-range gate durations and a measured average gate fidelity of 99.92%, characterized by randomized benchmarking. These results demonstrate interesting potential use for fast nanosecond-time-scale two-qubit gates and multilevel quantum logic.
@article{arxiv.2008.00593,
title = {Characterization of multi-level dynamics and decoherence in a high-anharmonicity capacitively shunted flux circuit},
author = {M. A. Yurtalan and J. Shi and G. J. K. Flatt and A. Lupascu},
journal= {arXiv preprint arXiv:2008.00593},
year = {2022}
}