Unidirectional Raman emission of Stokes photons from a circularly polarized transition of atoms in a chiral ring cavity

In this study, we demonstrated unidirectional Raman emission from cold ⁸⁷Rb atoms with a ring cavity. A bias magnetic field <i>B₀</i> is applied along the z direction to define the quantization axis. By transversely applying pumping laser pulses on a π-transition of the atoms, we generate spontaneous Raman emissions of Stokes photons from an individual circularly polarized (<i>σ</i>⁺) transition. The emissions are coupled into the running-wave cavity where two quarter-wave plates are inserted. We observed that the Stokes field is right-hand-circularly (left-hand-circularly) polarized when propagating along the z (-z) direction. The measured polarization extinction of the Stokes field escaped from the ring cavity is beyond 1400:1. A polarization interferometer is inserted in the cavity and the two orthogonal polarization modes are spatially split when propagating in the interferometer. By blocking one arm of the interferometer, we demonstrated a chiral cavity system with the atoms in the presence of the magnetic field. On this basis, we demonstrated unidirectional Raman emissions of Stokes photons in the ring cavity. The unidirectional factor reaches ∼99.8%.