Reconfigurable directional Raman amplifier controlled by the internal state of cold atoms chirally coupled to a nanophotonic waveguide

We experimentally demonstrate a scheme for nonreciprocal amplification of light, which uses atoms that are chirally coupled to a nanophotonic waveguide. Rather than a magnetic field, in this scheme, it is the atomic spin that breaks Lorentz reciprocity [1]. Specifically, we show directional amplification of guided light using cold cesium atoms coupled to the evanescent field surrounding an optical nanofiber. By preparing the atoms in different Zeeman states of the ground state manifold, we can control the direction in which amplification occurs. We observe an exponential increase of the optical output power with the number of atoms and obtain up to 50 % single-pass gain for about 2000 atoms. Our results contribute to establishing a new class of spin-controlled, nonreciprocal integrated optical devices.

[1] Nature 541, 473–480 (2017)