Investigating microwave losses in β-tantalum-based superconducting circuits

State-of-the-art tantalum-based superconducting circuits almost exclusively consist of thin films grown at elevated temperatures in the alpha phase (α-Ta). While tantalum readily nucleates in the beta phase (β-Ta) at room temperature, α-Ta is preferred due to its higher superconducting transition temperature and lower normal state resistivity. However, a systematic comparison of microwave losses for the two different phases has not been reported. Here, we compare the materials-related microwave losses in resonators made from α-Ta and β-Ta at few-photon powers and millikelvin temperatures. Finally, we report coherence times of transmon qubits with capacitor pads made from β-Ta using a room temperature fabrication process.