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

Abstract: The non-reciprocal (unidirectional) atom-photon couplings are crucial for modern photonics ranging from chiral quantum networks to cold-atom many-body physics. In the presented experiment, we demonstrated unidirectional Raman emission of Stokes photons from 87Rb atoms in a ring cavity. A bias magnetic field B0 is applied along z-direction on the atoms to define the quantum axis, which breaks the time inverse symmetry. By transversely applying write laser pulses to drive a \pi-transition of the atoms, we generate spontaneous Raman emissions of Stokes photons from a chiral (\sigma+) transition. The emissions are coupled into the clock-wise (z-direction) and counter-clock-wise (-z-direction) modes of a running-wave cavity, respectively. According to the mirror (parity) symmetry of the atom-light coupling, we demonstrated that spins (polarizations) of the Stokes fields are correlated with their propagation directions along z and -z -axis. The Stokes emissions constrained to the spin-momentum correlation are found to be violation of Kirchhoff's law of thermal radiation. Based on the correlation, we demonstrated that the Stokes emissions propagate along the clock-wise or counter-clock-wise via polarization dissipation. The directional factor is up to 1500:1.