SPAM-Robust Multi-axis Quantum Noise Spectroscopy in Temporally
  Correlated Environments
Characterizing temporally correlated (``non-Markovian'') noise is a key
prerequisite for achieving noise-tailored error mitigation and optimal device
performance. Quantum noise spectroscopy can afford quantitative estimation of
the noise spectral features; however, in its current form it is highly
vulnerable to implementation non-idealities, notably, state-preparation and
measurement (SPAM) errors. Further to that, existing protocols have been mostly
developed for dephasing-dominated noise processes, with competing dephasing and
relaxation effects being largely unaccounted for. We introduce quantum noise
spectroscopy protocols inspired by spin-locking techniques that enable the
characterization of arbitrary temporally correlated multi-axis noise on a qubit
with fixed energy splitting, while remaining resilient to realistic static SPAM
errors. By validating our protocol's performance in both numerical simulation
and cloud-based IBM quantum processors, we demonstrate the successful
separation and estimation of native noise spectrum components as well as SPAM
error rates. We find that SPAM errors can significantly alter or mask important
noise features, with spectra overestimated by up to 26.4% in a classical noise
regime. Clear signatures of non-classical noise are manifest in the
reconstructed IBM-qubit dephasing spectra, once SPAM-error effects are
compensated for. Our work provides a timely tool for benchmarking realistic
sources of noise in qubit devices.
Muhammad Qasim Khan Wenzheng Dong Leigh M. Norris Lorenza Viola