Study of Two-Level-System losses in Tantalum Superconducting Microwave Coplanar Waveguide Resonators*

Superconducting qubits based on tantalum have achieved record lifetimes and coherence times for planar transmons [1]. The microscopic mechanisms for loss in these devices are poorly understood. In this work, we use systematic measurements of losses in Ta coplanar waveguides to disentangle loss channels. Specifically, we use temperature and power dependent measurements to investigate the contribution of two-level systems (TLSs), and we vary the surface participation ratio, geometry, frequency, and environment of the device to investigate the contribution of surface losses, bulk losses, and packaging. We quantitatively parametrize different sources of loss, and we conclude that TLSs are a significant loss channel, particularly for smaller devices. Complementarily, we perform direct materials characterization of the Ta films using several methods including variable photon X-ray photoelectron spectroscopy, X-ray diffraction, resistivity, and electron microscopy. By correlating direct materials characterization with systematic resonator and qubit measurements, we identify candidates for microscopic sources of loss and decoherence.

[1] A.P.M. Place et al New material platform for superconducting transmon qubits with coherence times exceeding 0.3 milliseconds Nat. Commun. 12, 1779, 2021.