Quantum noise spectroscopy for multiaxis noise models

Characterizing decoherence that arises from coupling to noisy environments is essential for optimized control and error correction strategies in realistic quantum information processors. Motivated by this challenge, quantum noise spectroscopy (QNS) seeks to estimate the spectral properties of noise affecting quantum systems. To date, QNS protocols have largely focused on platforms dominated by dephasing (T₂) processes, rendering them inapplicable to systems in which longitudinal relaxation (T₁) processes occur on a comparable timescale. To move beyond dephasing-dominated platforms, we extend frequency-comb based QNS to a multi-axis qubit noise model that takes into account both T₁ and T₂ processes from either classical or quantum environments. Targeted control of the qubit permits a complete spectral reconstruction, including arbitrary cross-axis correlations. Using a novel spherical representation for the noise operators, we show that three noise spectra characterize the reduced dynamics in a regime where the qubit energy splitting is large. This spherical representation enables a straightforward multi-axis extension to QNS protocols based on continuous driving, such as spin-locking.