Phononic Losses in Superconducting Coplanar Waveguide Resonators on Piezoelectric Substrates

In recent years there has been an increasing number of experiments involving superconducting qubits on piezoelectric substrates to investigate quantum acoustics. In this context Coplanar Waveguide (CPW) resonators provide a well established way for qubit manipulation and readout. However CPW resonators on piezoelectric substrates perform poorly, with two orders of magnitude lower internal quality factor (Q) compared with similar devices on low loss dielectrics.
In this work we present an investigation of the phononic loss channel for CPW resonators fabricated on a piezoelectric substrate. With the help of finite element method (FEM) simulations, we calculate the energy loss due to electromechanical conversion, and derive an upper limit for the internal Q. We fabricate quarter wavelength CPW resonators on GaAs and measure their internal Q as function of frequency, power and temperature. We obtain a linear increase of the internal Q with respect to frequency as expected for a frequency independent electromechanical coupling. Moreover we find a weak power dependence and a negligible temperature dependence around 10mK, excluding two level systems and non-equilibrium quasiparticles as the main source of losses.