# Zeeman-tunable Modulation Transfer Spectroscopy

## TL;DR

This paper introduces a magnetically tunable modulation transfer spectroscopy method that allows for Zeeman-shifting error signals over a broad frequency range, enhancing laser stabilization flexibility in atomic physics experiments.

## Contribution

The authors demonstrate for the first time a magnetically tunable MTS error signal, enabling arbitrary frequency locking across the rubidium spectrum using a simple two-magnet setup.

## Key findings

- Zeeman-shifted MTS error signals over >15 GHz range.
- Successful locking to multiple rubidium transitions.
- Versatility for locking Raman and lattice lasers.

## Abstract

Active frequency stabilization of a laser to an atomic or molecular resonance underpins many modern-day AMO physics experiments. With a flat background and high signal-to-noise ratio, modulation transfer spectroscopy (MTS) offers an accurate and stable method for laser locking. Despite its benefits, however, the four-wave mixing process that is inherent to the MTS technique entails that the strongest modulation transfer signals are only observed for closed transitions, excluding MTS from numerous applications. Here, we report for the first time the observation of a magnetically tunable MTS error signal. Using a simple two-magnet arrangement, we show that the error signal for the $^{87}$Rb $F=2 \rightarrow F'=3$ cooling transition can be Zeeman-shifted over a range of $>$15 GHz to any arbitrary point on the rubidium $\text{D}_2$ spectrum. Modulation transfer signals for locking to the $^{87}$Rb $F=1 \rightarrow F'=2$ repumping transition as well as 1 GHz red-detuned to the cooling transition are presented to demonstrate the versatility of this technique, which can readily be extended to the locking of Raman and lattice lasers.