  Recent significant advances in coupling superconducting qubits to acoustic
wave resonators has led to demonstrations of quantum control of surface and
bulk acoustic resonant modes as well Wigner tomography of quantum states in
these modes. These advances were achieved through the efficient coupling
afforded by piezoelectric materials combined with GHz-frequency acoustic
Fabry-Perot cavities. Quantum control of itinerant surface acoustic waves
appears in reach, but is challenging due to the limitations of conventional
transducers in the appropriate GHz-frequency band. In particular, GHz-frequency
unidirectional transducers would provide an important addition to the desired
quantum toolbox, promising unit efficiency with directional control over the
surface acoustic wave emission pattern. Here we report the design, fabrication
and experimental characterization of unidirectional distributed acoustic
reflection transducers (DARTs) demonstrating a high transduction frequency of
4.8 GHz with a peak directivity larger than 25 dB and a directivity greater
than 15 dB over a bandwidth of 17 MHz. A numerical model reproduces the main
features of the transducer response quite well, with ten adjustable parameters
(most of which are constrained by geometric and physical considerations). This
represents a significant step towards quantum control of itinerant quantum
acoustic waves.
