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.
@article{arxiv.2003.02782,
title = {Multi-level Quantum Noise Spectroscopy},
author = {Youngkyu Sung and Antti Vepsäläinen and Jochen Braumüller and Fei Yan and Joel I-Jan Wang and Morten Kjaergaard and Roni Winik and Philip Krantz and Andreas Bengtsson and Alexander J. Melville and Bethany M. Niedzielski and Mollie E. Schwartz and David K. Kim and Jonilyn L. Yoder and Terry P. Orlando and Simon Gustavsson and William D. Oliver},
journal= {arXiv preprint arXiv:2003.02782},
year = {2021}
}