Optimizing Designs and Materials for Transmon Qubits

Superconducting qubits based on Tantalum (Ta) have achieved some of the longest reported lifetimes to date. However, recent measurements of Ta lumped element (LE) resonators suggest dielectric loss at the capacitor surface may not be the only limitation for their transmon counterparts. This implies losses from other sources, such as Josephson junction interfaces and substrates, have to be mitigated if further improvements in lifetimes are to be achieved. In this talk, we discuss some progress in addressing these other loss channels, focusing especially on losses from junction interfaces. We also discuss the status of a broader materials search involving transmons and resonators, and give an outlook for further progress in optimizing transmon designs, materials, and fabrication.