High coherence hybrid superconducting qubit

Abstract

We measure the coherence of a new superconducting qubit, the {\em low-impedance flux qubit}, finding $T_2^{\ast }\sim T_1\sim 1.5\mu$s. It is a three-junction flux qubit, but the ratio of junction critical currents is chosen to make the qubit's potential have a single well form. The low impedance of its large shunting capacitance protects it from decoherence. This qubit has a moderate anharmonicity, whose sign is reversed compared with all other popular qubit designs. The qubit is capacitively coupled to a high-Q resonator in a $\lambda /2$ configuration, which permits the qubit's state to be read out dispersively.

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