  The strong anharmonicity and high coherence times inherent to fluxonium
superconducting circuits are beneficial for quantum information processing. In
addition to requiring high-quality physical qubits, a quantum processor needs
to be assembled in a manner that minimizes crosstalk and decoherence. In this
paper, we report work on fluxonium qubits packaged in a flip-chip architecture,
where a classical control and readout chip is bump-bonded to the quantum chip,
forming a multi-chip module (MCM). The modular approach allows for improved
connectivity between the qubits and control/readout elements, and separate
fabrication processes. We characterize the coherence properties of the
individual fluxonium qubits, demonstrate high fidelity single-qubit gates with
6 ns microwave pulses (without DRAG), and identify the main decoherence
mechanisms to improve on the reported results.
