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

Abstract

We present the design and characterization of a three-Josephson-junction\nsuperconducting loop circuit with three large shunt capacitors. Used as a\nqubit, the circuit shows long energy-relaxation times, of the order of 40 ${\n\\unicode[Times]{x3BC}}$s and a spin-echo dephasing time of 9.4 ${\n\\unicode[Times]{x3BC}}$s. The circuit has high anharmonicity, of\n2$\\pi$$\\times$3.69 GHz. We extract the multilevel relaxation and dephasing\nrates of the circuit used as a qutrit and discuss the possible sources for the\ndecoherence. The high anharmonicity allows for fast qubit control with\nnanosecond-range gate durations and a measured average gate fidelity of 99.92%,\ncharacterized by randomized benchmarking. These results demonstrate interesting\npotential use for fast nanosecond-time-scale two-qubit gates and multilevel\nquantum logic.\n