  We propose a universal, on-chip quantum transducer based on surface acoustic
waves in piezo-active materials. Because of the intrinsic piezoelectric (and/or
magnetostrictive) properties of the material, our approach provides a universal
platform capable of coherently linking a broad array of qubits, including
quantum dots, trapped ions, nitrogen-vacancy centers or superconducting qubits.
The quantized modes of surface acoustic waves lie in the gigahertz range, can
be strongly confined close to the surface in phononic cavities and guided in
acoustic waveguides. We show that this type of surface acoustic excitations can
be utilized efficiently as a quantum bus, serving as an on-chip, mechanical
cavity-QED equivalent of microwave photons and enabling long-range coupling of
a wide range of qubits.
