Measurement of superconducting single-crystal Al resonators on Si and sapphire substrates

Al made via molecular beam epitaxy offers improvements to existing superconducting qubit architectures due to decreased loss at the interface between the Al and the substrate [1]. We have studied this loss by measuring the quality factors of a variety of superconducting quarter-wave resonators fabricated under different conditions from single-crystal aluminum on both silicon and sapphire substrates. The resonators, which have resonant frequencies between 4.5 and 6 GHz, were measured at a temperature of 25 mK and from an average stored photon number n$\sim$1 up to 10$^{6}$. At low photon numbers, we consistently observe Q\textgreater 200k on both substrates. We will discuss potential limitations on the measured loss and steps taken to mitigate them. \\[4pt] [1] A. Megrant, et al., App. Phys. Lett., {\bf 100}, 113510 (2012).