  A hybrid system with tunable coupling between phonons and qubits shows great
potential for advancing quantum information processing. In this work, we
demonstrate strong and tunable coupling between a surface acoustic wave (SAW)
resonator and a transmon qubit based on galvanic-contact flip-chip technique.
The coupling strength varies from $2\pi\times$7.0 MHz to -$2\pi\times$20.6 MHz,
which is extracted from different vacuum Rabi oscillation frequencies. The
phonon-induced ac Stark shift of the qubit at different coupling strengths is
also shown. Our approach offers a good experimental platform for exploring
quantum acoustics and hybrid systems.
