  In the same way that micro-mechanical resonators resemble guitar strings and
drums, Surface Acoustic Waves (SAW) resemble the sound these instruments
produce, but moving over a solid surface rather than through air. In contrast
with oscillations in suspended resonators, such propagating mechanical waves
have not before been studied near the quantum mechanical limits. Here, we
demonstrate local probing of SAW with a displacement sensitivity of
30amRMS/sqrt(Hz) and detection sensitivity on the single-phonon level after
averaging, at a frequency of 932MHz. Our probe is a piezoelectrically coupled
Single Electron Transistor, which is sufficiently fast, non-destructive and
localized to let us track pulses echoing back and forth in a long acoustic
cavity, self-interfering and ringing the cavity up and down. We project that
strong coupling to quantum circuits will allow new experiments, and hybrids
utilizing the unique features of SAW. Prospects include quantum investigations
of phonon-phonon interactions, and acoustic coupling to superconducting qubits,
for which we present favourable estimates.
