Towards measuring quantum sound

Licentiate thesis, 2016

One of the most fundamental phenomena occurring in nature is the interaction between single atoms and electromagnetic fields. This is studied in quantum optics to probe the quantum nature of the system. In appended Paper I, we demonstrated that an artificial atom can couple to propagating sound, which is the acoustic analogue of quantum optics. This thesis covers experiments where surface acoustic waves (SAWs) are used in superconducting circuits at gigahertz frequencies with the aim for quantum applications. Since SAWs are mechanical vibrations that propagate on the surface of solids and dissipate little power, they can propagate freely over long distances before and after they interact with the artificial atom. Their five order of magnitude slower speed and smaller wavelength than light at gigahertz frequencies, open up for exploring new regimes of quantum physics. In addition, the coupling between the artificial atom and the SAWs is promising for reaching ultrastrong coupling limits, which is hard in other types of systems. Many of the future applications are discussed in appended Paper II, together with a more detailed description of the theory and fabrication of the devices. Although the possibilities are many, a variety of potential experiments would benefit from higher conversion efficiency between electric signals and SAWs. Therefore, improvements of this conversion are studied in appended Paper III, making use of the many advances in classical SAW devices. More specifically, the conversion of unidirectional transducers (UDTs) on lithium niobate is studied and compared to symmetric interdigital transducers (IDTs). The results show that 99.4 % of the acoustic power can be focused in the desired direction and that the conversion between electric signals and SAWs is greatly improved by using UDTs, eliminating the largest contribution to loss of the IDTs. However, there is a trade-off between conversion efficiency and bandwidth. This knowledge allows us to better tailor potential quantum experiments based on SAWs, possibly towards measuring quantum sound.