Unimon: an island-free superconducting qubit (Part 2/2)*

In the design of superconducting qubits, one strives for a circuit that is insensitive to common sources of decoherence while retaining a high anharmonicity and a high controllability. We propose a novel superconducting qubit that is insensitive to low-frequency charge noise thanks to the island-free structure of the circuit. The proposed qubit, nicknamed as the unimon, is constructed of a grounded coplanar waveguide resonator incorporating a single embedded Josephson junction in its center conductor. In addition to being insensitive to charge noise, the unimon qubit has a flux-insensitive sweet spot and a pair of gradiometric loops that provide protection against magnetic flux noise. At the sweet spot, the unimon qubit has an anharmonicity that is positive and a few times higher than in the conventionally used transmon qubits. We present a theoretical treatment of the circuit and first experimental results demonstrating that it is possible to implement fast and high-fidelity single-qubit gates using the unimon qubit.