  Superfluid helium's low-loss dielectric properties, excellent thermal
conductivity, and unique collective excitations make it an attractive candidate
to incorporate into superconducting qubit systems. We controllably immerse a
three-dimensional superconducting transmon qubit in superfluid helium-4 and
measure the spectroscopic and coherence properties of the system. We find that
the cavity, the qubit, and their coupling are all modified by the superfluid,
which we analyze within the framework of circuit quantum electrodynamics
(cQED). At at temperatures relevant to quantum computing experiments, the
energy relaxation time of the qubit is not significantly changed by the
presence of the superfluid, while the pure dephasing time modestly increases,
which we attribute to improved thermalization of the microwave environment via
the superfluid.
