  System noise identification is crucial to the engineering of robust quantum
systems. Although existing quantum noise spectroscopy (QNS) protocols measure
an aggregate amount of noise affecting a quantum system, they generally cannot
distinguish between the underlying processes that contribute to it. Here, we
propose and experimentally validate a spin-locking-based QNS protocol that
exploits the multi-level energy structure of a superconducting qubit to achieve
two notable advances. First, our protocol extends the spectral range of weakly
anharmonic qubit spectrometers beyond the present limitations set by their lack
of strong anharmonicity. Second, the additional information gained from probing
the higher-excited levels enables us to identify and distinguish contributions
from different underlying noise mechanisms.
