  Piezoelectric optomechanical platforms represent one of the most promising routes towards achieving quantum transduction of photons between the microwave and optical frequency domains. However, there are significant challenges to achieving near-unity transduction efficiency. We discuss such factors in the context of the two main approaches being pursued for high efficiency transduction. The first approach uses one-dimensional nanobeam optomechanical crystals excited by interdigitated transducers, and is characterized by large single-photon optomechanical coupling strength, limited intracavity pump photon population to avoid absorption-induced heating, and low phonon injection efficiency from the transducer to the optomechanical cavity. The second approach uses (quasi) bulk acoustic wave resonators integrated into photonic Fabry-Perot cavity geometries, and is characterized by low single-photon optomechanical coupling strength, high intracavity pump photon population without significant heating, and high phonon injection efficiency. After reviewing the current status of both approaches, we discuss the need for co-designing the electromechanical and optomechanical sub-systems in order to achieve high transduction efficiencies, taking the GaAs piezo-optomechanical platform as an example. 
Piezoelectric optomechanical approaches for efficient quantum   microwave-to-optical signal transduction: the need for co-design