We demonstrate a trapped-ion protocol in which a nearby, dedicated "monitor" qubit tracks magnetic-field drifts in real time without interrupting data-qubit operations. Using two 40Ca+ ions and the optical--metastable--ground architecture, we encode the data qubit in the ground-state manifold and the monitor qubit in a metastable-state manifold to achieve spectral separation. The monitor qubit senses common magnetic fluctuations during data-qubit experiments, enabling feedforward corrections to the qubit-control drives. Under applied magnetic noise with a realistic spectrum (1/f2), the protocol maintains coherence and, when compared with interleaved calibration, it extends usable data-qubit probe times by up to a factor of ∼2 and doubles the experimental duty cycle. These results establish monitor qubits as a scalable tool for real-time recalibration in quantum information processors.
@article{arxiv.2511.04767,
title = {Real-time magnetic field noise correction using trapped-ion monitor qubits},
author = {Kyle DeBry and Agustin Valdes-Martinez and David Reens and Colin D. Bruzewicz and John Chiaverini},
journal= {arXiv preprint arXiv:2511.04767},
year = {2025}
}