Towards a tunable, wide-band acoustic transducer operating in the quantum regime.

Abstract: In the past decade we have seen fast development of new quantum technologies that promise to revolutionise communications and computing. Many different routes are explored to physically implement such quantum technologies. Among others, we can mention superconducting circuits, spin-based devices and photonic devices.  An active area of research concerns hybrid quantum systems, which aim at combining the best properties of these different implementations. Recently quantum acoustics has been gaining interest as a potential intermediate in such hybrid systems. Indeed, phonons can couple to many different degrees of freedom and could therefore form an interface between different quantum systems.One method to convert microwaves to an acoustic signal relies on the exploitation of the piezoelectric effect. However, for transducer devices based on the piezoelectric effect, the operating bandwidth has remained limited. By using tunable matching circuits, we propose a device capable of performing piezoelectric transduction over large bandwidths. The tunability is achieved by using SQUID transmission lines. This work reports some preliminary studies towards the creation of such a platform. We conduct measurements at cryogenic temperatures on suspended lithium niobate delay lines. These allow us to characterise the dependence of the IDT response on various parameters such as the number of fingers and the angle relative to the underlying crystal. We show that certain angles favour transduction into a single acoustic mode. In parallel, we characterise transmission lines consisting of one-dimensional arrays of SQUIDs. By applying magnetic flux to these lines, we show that it is possible to tune their impedance. These experiments improved our understanding of both IDTs and SQUID transmission lines and pave the way for the implementation of our experimental platform.