Origin of bulk two-level system loss in superconducting circuits on silicon: Part I Experiment*

Two-level systems (TLS) in amorphous materials are the dominant loss mechanism for superconducting quantum circuits. While the microscopic origin of amorphous TLSs is still unknown, recent efforts enabled reduction in interface TLS losses via reduced surface participation and improved surface treatment. With these improvements, experiments are now approaching a regime where bulk TLS losses may have a significant contribution to qubit loss. In this work, we present the first microscopic identification of a TLS bath for superconducting circuits based on defects in bulk silicon. We use low surface-participation microwave resonators to study the impact of common acceptors and donors on microwave losses. We show that spin-orbit acceptor systems lead to a large microwave loss tangent at the single-photon level. In this talk, we describe experiments where we study two-level system loss as a function of dopant concentration, temperature, and magnetic field, and demonstrate the spin-orbit nature of the bulk TLS bath in silicon.