Detection of TLS defects coupled to a c-shunt flux qubit via critical-current fluctuations*

Two-level-system (TLS) defects in superconducting qubits are one of the major noise mechanisms limiting qubit coherences. TLS defects are various atomic-scale defects of different microscopic nature which are mainly formed in amorphous layers such as the aluminum-oxide tunnel barrier of an Al/AlOx/Al Josephson junction. Conventionally, it is assumed that charge TLS defects are coupled to a superconducting qubit via charge fluctuations. However, TLS defects can also interact with a qubit through critical-current fluctuations, although such type of interaction has not been detected in experiments consistently.

In this work, we report the detection of critical-current-coupled TLS defects in a c-shunt flux qubit using a strong resonant qubit drive. The qubit is dynamically coupled to a critical-current-fluctuation TLS defect when the relation Ω=|ω_TLS-2ω_q| is met, where ω_q is the qubit frequency, ω_TLS is the defect frequency, and Ω is the Rabi frequency. For a charge-fluctuation TLS defect, the condition is different and given by Ω=|ω_TLS-ω_q|. By measuring the qubit excited-state population as a function of an applied magnetic flux and qubit drive amplitude, we can distinguish between TLS defects with different types of qubit-defect interaction.