Characterization of multi-level dynamics and decoherence in a high-anharmonicity capacitively shunted flux circuit

We present the design and characterization of a three-Josephson-junction superconducting loop circuit with three large shunt capacitors. The circuit used as a qubit shows long energy relaxation times, of the order of 40 μs, and a spin-echo dephasing time of 9.4 μs. The circuit has high anharmonicity, of 2π×3.69 GHz. We extract the multi-level 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 99.92% average gate fidelity, characterized by randomized benchmarking. These results demonstrate interesting potential use for fast nanosecond time scale two-qubit gates and multi-level quantum logic