Identifying Noise Sources via Multi-level Quantum Noise Spectroscopy*

Identifying dominant sources of noise and mitigating or eliminating them is crucial for engineering robust quantum devices. Although existing quantum noise spectroscopy (QNS) protocols provide insight into noise sources by measuring the noise power spectral density, they do not directly identify the physical origin of the noise process. Here, we develop and experimentally validate a multi-level QNS protocol using a superconducting transmon qubit as a spectrometer. This technique has two notable implications: First, it expands the applicability and spectral range of a qubit spectrometer beyond the two-level approximation. Second, it can be applied to higher excited levels, which enables us to extract and identify the noise contributions from multiple noise sources.