  Mechanical resonators are a promising way for interfacing qubits in order to
realize hybrid quantum systems that offer great possibilities for applications.
Mechanical systems can have very long energy lifetimes, and they can be further
interfaced to other systems. Moreover, integration of mechanical oscillator
with qubits creates a potential platform for exploration of quantum physics in
macroscopic mechanical degrees of freedom. Utilization of high overtone bulk
acoustic resonators coupled to superconducting qubits is an intriguing platform
towards these goals. These resonators exhibit a combination of high frequency
and high quality factors. They can reach their quantum ground state at dilution
refrigeration temperatures and they can be strongly coupled to superconducting
qubits via their piezoelectric effect. In this report, we demonstrate our
system where bulk acoustic phonons of a high overtone resonator are coupled to
a transmon qubit in a planar circuit architecture. We show that the bulk
acoustic phonons are interacting with the qubit in the simple design
architecture at the quantum level, representing further progress towards
quantum control of mechanical motion.
