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

TL;DR

This paper reports on a superconducting circuit with high anharmonicity exhibiting long coherence times, fast control, and high-fidelity operations, advancing the development of multilevel quantum computing components.

Contribution

The study introduces a novel three-Josephson-junction circuit with high anharmonicity and detailed multilevel decoherence characterization, enabling fast and high-fidelity quantum control.

Findings

Energy relaxation time of ~40 μs

Dephasing time of 9.4 μs

Gate fidelity of 99.92%

Abstract

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 ${ \unicode[Times]{x3BC}}$s and a spin-echo dephasing time of 9.4 ${ \unicode[Times]{x3BC}}$s. The circuit has high anharmonicity, of 2$\pi$$\times$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.