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

TL;DR

This paper presents a real-time magnetic noise correction protocol using a dedicated monitor qubit in trapped-ion systems, significantly improving coherence times and operational duty cycles.

Contribution

The authors introduce a novel monitor qubit scheme that tracks magnetic-field drifts in real time without disrupting data qubit operations, enhancing quantum processor stability.

Findings

Maintains coherence under 1/f^2 magnetic noise spectrum.

Extends data-qubit probe times by up to a factor of √2.

Doubles the experimental duty cycle.

Abstract

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 ${\sim}\sqrt{2}$ and doubles the experimental duty cycle. These results establish monitor qubits as a scalable tool for real-time recalibration in quantum information processors.