Low-Loss Dielectric Materials and the Merged Element Transmon*

Josephson junctions, a crucial component in quantum bits (qubits), are commonly composed of a pair of superconducting aluminum films separated by a thin layer of amorphous aluminum oxide. In order to avoid the high density of two-level states (TLS) in amorphous oxides, Josephson junctions are designed to be small, thus reducing the participation of the lossy material. However, the persistence of lossy materials in qubits leads to diminishing returns with this strategy. An alternative approach to TLS loss minimization in a transmon qubit junction is to combine the qubit’s nonlinear inductance and capacitance into a single trilayer junction with extremely low dielectric loss, i.e., a merged-element transmon. In this work, we characterize dielectric thin films within lumped-element resonators to determine microwave losses in the single-photon regime and to identify dielectric barrier materials for a merged element transmon. In addition to amorphous solid barriers, we measure microwave-frequency loss of single-crystal epitaxial superconductor-insulator-superconductor trilayers.