Real-time magnetic field noise correction using trapped-ion monitor qubits

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 $^{40}\mathrm{Ca}^{+}$ 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 ($1/{f}^{2}$), the protocol maintains coherence and, when compared with interleaved calibration, it extends usable data-qubit probe times by up to a factor of approximately $\sqrt{2}$ and doubles the experimental duty cycle. These results establish monitor qubits as a scalable tool for real-time recalibration in quantum information processors.