Coherent polarization self-rotation

Dense alkali-metal vapors in the SERF regime combine long-lived collective spin with extreme optical depth. A typical dense and warm apparatus with high-pressure buffer gas exhibits pressure broadening, unresolved hyperfine structure, and rapid spin exchange. These effects often wash out coherence in light-matter interfaces that depend on narrow optical resonances or sensitive dark-state conditions.

We report coherent polarization self-rotation (CPSR), a two-photon interaction between spin-polarized atoms and a probe, detuned from the optical transition, whose polarization contains a small oscillatory component. In this regime, the oscillatory polarization component exchanges coherence with the transverse collective spin. This produces narrow spectroscopic features near the electronic Larmor frequency even in pressure-broadened, optically thick cells. We observe near-unity-contrast lines in rubidium and a two-photon linewidth as small as 10 Hz in potassium. Measurements agree with a quantitative physical model.

CPSR establishes a scalable light-spin interface in dense vapors, relevant to audio-band quantum-enhanced metrology and to coherent linking of light with ultra-long-lived noble-gas spins via alkali mediation.