Coherent polarization self-rotation

TL;DR

This paper introduces coherent polarization self-rotation (CPSR), a novel two-photon interaction in dense alkali vapors that enables narrowband spectroscopy and robust light-spin coupling, with potential applications in quantum metrology and transduction.

Contribution

It presents CPSR as a new mechanism requiring initial spin polarization, demonstrating high-contrast and ultra-narrow linewidths in alkali vapors, expanding quantum optics capabilities.

Findings

Achieved near-unity contrast in rubidium CPSR.

Demonstrated 10 Hz linewidth in potassium.

Enabled robust spin-light coupling in optically thick vapors.

Abstract

We introduce and study coherent polarization self-rotation (CPSR), a two-photon light-matter interaction in dense alkali-metal vapors that enables both narrowband optical spectroscopy of magnetic transitions and coherent coupling between light and collective atomic spins. Unlike conventional polarization self-rotation, CPSR requires initial spin polarization and a predominantly linearly polarized probe. It operates efficiently even in optically thick vapors with high buffer-gas pressure, rapid spin-exchange collisions, and optically-unresolved hyperfine structure. We demonstrate CPSR with near-unity contrast in rubidium and achieve an exceptionally narrow two-photon linewidth of 10 Hz in potassium. CPSR realizes a coherent interface between one optical quadrature and the long-lived collective electronic spin, offering a robust and scalable spin-light coupling in optically thick platforms. This opens new opportunities for quantum optics, including quantum-enhanced metrology in the audio-frequency band and coherent transduction between light and ultra-long-lived noble-gas spins via alkali spins.