Spectroscopy of high-frequency TLS defects in superconducting qubits using a spin-locking pulse sequence*

Improving coherence times of superconducting qubits requires the identification of relevant noise sources. Noise spectroscopy using a spin-locking pulse sequence has gained increasing attention as a tool for the characterization of low-frequency noise mechanisms. Here, we demonstrate that, in addition to the low-frequency noise, the spin-locking sequence can be used to identify high-frequency two-level-system (TLS) defects, both below and above the qubit frequency. Measurements were performed using a capacitively-shunted flux qubit embedded in a 3D cavity [1]. The amplitude of the spin-locking pulse was varied in the range 0-90 MHz in the units of the corresponding Rabi frequency. Spectral features were observed when the Rabi frequency was equal to the frequency detuning between the qubit and a high-frequency TLS defect [2]. Thus, spin-locking noise spectroscopy can be used for the detection of off-resonant TLS defects which can be particularly useful for the case of fixed-frequency superconducting qubits.

[1] Abdurakhimov et al., Appl. Phys. Lett. 115, 262601 (2019)
[2] Abdurakhimov et al., Phys. Rev. B 102, 100502(R) (2020)