Dual-frequency Doppler-free crossover resonance with suppressed magnetic-field sensitivity for compact optical clocks

We report the observation and characterization of a high-contrast dual-frequency Doppler-free ground-state crossover resonance in the D1 line of 87Rb. The crossover appears at the two-photon detuning δ exceeding the natural linewidth of the excited state and is formed by the hyperfine optical pumping. We show that the previously proposed resonance at zero two-photon detuning is sensitive to magnetic-field fluctuations due to residual ellipticity of the optical fields that produces the profile asymmetry and frequency shifts, while the crossover resonance is largely immune to this effect. Our theoretical analysis attributes the observed sensitivity to the dispersive contribution of ground-state coherences to absorption. Stability measurements under magnetic-field fluctuations demonstrate that using the crossover resonance at large δ provides more than an order-of-magnitude improvement, making it a promising reference for frequency stabilization in compact, field-deployable optical standards.