Unidirectional Raman emissions of Stokes photons via chiral atom-photon coupling in a ring cavity

In the presented experiment, we demonstrated unidirectional Raman emission from ⁸⁷ Rb atoms in a ring cavity. A bias magnetic field B ₀ is applied along z-direction to define the quantum axis. By transversely applying write laser pulses to drive a π-transition of the atoms, we generate spontaneous Raman emissions of Stokes photons from a chiral (σ ⁺ ) transition. The emissions are coupled into the clock-wise (z-direction) and counter-clock-wise modes (-z-direction) of a running-wave cavity, respectively. We reveal spin-momentum correlation in the Stokes emissions. The Stokes emissions constrained to the spin-momentum correlation are found to be a violation of the Kirchhoff’s law of thermal radiation. Thus, we demonstrated that the Stokes emissions can propagate along the clock-wise or counter-clock-wise mode via polarization dissipation based on the spin-momentum correlation. The directional factor is up to 1500:1.