Characterization of Multilevel 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. Used as a qubit, the circuit shows long energy-relaxation times, of the order of 40 mu s and a spin-echo dephasing time of 9.4 mu s. The circuit has high anharmonicity, of 2 pi x 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-timescale two-qubit gates and multilevel quantum logic.

Automated summary

The three-Josephson-junction superconducting loop circuit with three large shunt capacitors demonstrates long energy-relaxation times, high anharmonicity, and fast qubit control capability, which show potential applications for fast nanosecond-timescale two-qubit gates and multilevel quantum logic.