Extending the low-frequency limit of qubit noise spectroscopy beyond the inverse dephasing time

Noise-aware quantum control strategies have shown great promise to reach gate fidelities necessary for operating fault-tolerant quantum computers. These protocols rely on detailed knowledge of the noise, which requires an accurate, full bandwidth characterisation. Conventional noise spectroscopy protocols [1] fail to provide information about the noise spectrum below the inverse of the T₂ coherence time. This limitation is particularly severe in most solid-state quantum systems, where 1/f noise dominates. Here, we demonstrate a novel spectroscopy protocol that circumvents this limitation by employing control sequences which allow moving the sampling regime to lower frequency regions. We apply this method to ³¹P donor qubits in silicon [2] and estimate the basic properties of the low-frequency noise by applying a Bayesian reconstruction algorithm. We verify our approach by detecting noise that we intentionally introduce to the system. With the information gained from this method we expect to design noise-aware quantum gates that further increase gate fidelities beyond fault-tolerant thresholds.



[1] G.A. Álvarez and D. Suter D., Phys. Rev. Lett. 107, 230501 (2011)

[2] M.T. Madzik, S. Asaad et al. Nature 601, 348 (2022)