Improving Coherence in Tantalum Transmon Qubits

Tantalum (Ta) transmon qubits exhibit record high relaxation and coherence times; however, a significant increase in coherence times is necessary to develop scalable quantum processors. Coherence times are limited by microwave dielectric loss in the constituent device materials; therefore, understanding and mitigating loss in these materials is important for achieving higher coherence devices. Recent work has quantified materials-based losses in Ta resonators and demonstrated that loss is dominated by saturable two-level systems (TLSs) in the amorphous tantalum surface oxide and bulk sapphire substrate. By studying the dependence of loss on temperature, microwave photon number, and device geometry, we investigate alternative materials and fabrication methods to further reduce surface and bulk losses in Ta-based resonators. We then apply these insights to the fabrication of transmon qubits to achieve higher relaxation and coherence times.