  A promising way to store quantum information is by encoding it in the bosonic
excitations of microwave resonators. This provides for long coherence times,
low dephasing rates, as well as a hardware-efficient approach to quantum error
correction. There are two main methods used to make superconducting microwave
resonators: traditionally machined out of bulk material, and lithographically
fabricated on-chip in thin film. 3D resonators have few loss channels and
larger mode volumes, and therefore smaller participations in the lossy parts,
but it can be challenging to reach high material qualities. On-chip resonators
can use low-loss thin films, but confine the field more tightly, resulting in
higher participations and additional loss channels from the dielectric
substrate. In this work, we present a design in which a dielectric scaffold
supports a thin-film conductor within a 3D package, thus combining the low
surface participations of bulk-machined cavities with the high quality and
control over materials of thin-film circuits. By incorporating a separate chip
containing a transmon qubit, we realize a quantum memory and measure
single-photon lifetimes in excess of a millisecond. This hybrid 3D architecture
has several advantages for scaling, as it relaxes the importance of the package
and permits modular construction with separately-replaceable qubit and
resonator devices.
