  Solid-state quantum acoustodynamic (QAD) systems provide a compact platform
for quantum information storage and processing by coupling acoustic phonon
sources with superconducting or spin qubits. The multi-mode composite
high-overtone bulk acoustic wave resonator (HBAR) is a popular phonon source
well suited for QAD. However, scattering from defects, grain boundaries, and
interfacial/surface roughness in the composite transducer severely limits the
phonon relaxation time in sputter-deposited devices. Here, we grow an
epitaxial-HBAR, consisting of a metallic NbN bottom electrode and a
piezoelectric GaN film on a SiC substrate. The acoustic impedance-matched
epi-HBAR has a power injection efficiency > 99% from transducer to phonon
cavity. The smooth interfaces and low defect density reduce phonon losses,
yielding fxQ products and phonon lifetimes up to 1.36 x 10^17 Hz and 500
microseconds respectively. The GaN/NbN/SiC epi-HBAR is an electrically
actuated, multi-mode phonon source that can be directly interfaced with
NbN-based superconducting qubits or SiC-based spin qubits.
