Epitaxial GaAs Loss Measurements and the Merged Element Transmon*

Transmon qubits are traditionally composed of two components: a Josephson junction, which is commonly made up of a pair of superconducting aluminum films separated by a thin layer of amorphous aluminum oxide, and a large paddle capacitor. The presence of lossy materials in this design, as well as the significant size of the capacitor paddles, limits the transmon’s performance and scalability. In contrast, the merged element transmon design combines the qubit's nonlinear inductance and capacitance into a single trilayer junction with extremely low dielectric loss. GaAs is a good candidate test material for these trilayer junctions as its epitaxial growth and interface with Al is well-characterized, and single crystal growth and lattice matching is possible. In this work, we use dielectric loss extraction methods to accurately measure the TLS loss for Al/GaAs/Al trilayers.