Towards non-suspended electromechanical crystals for quantum transduction

Acoustic systems are on the rise for storing and transferring quantum information, for example as mediators between microwave and optical photons. While a host of quantum acoustic devices have been demonstrated, such as bulk acoustic wave (BAW)-, surface acoustic wave (SAW)- and phononic crystal resonators, current devices are either bulky or require suspension, which hinders the development of complex phononic circuitry or severely limits thermal anchoring. We present a release-free, i.e. non-suspended, lithium niobate electromechanical crystal at frequencies around 5 GHz. Microwave reflection measurements show an external coupling rate of 47 kHz to a microwave feedline -- equivalent to a coupling rate of 24 MHz to a 70 fF transmon and comparable with those of traditional suspended phononic crystals. This paves the way for connecting release-free electro- and optomechanical devices-- thus realizing a fully non-suspended piezo-optomechanical transducer between microwave and optical photons.