Two-Color Optical Nonlinearity in an Ultracold Rydberg Atom Gas Mixture

Recent progresses on Rydberg atoms have significantly advanced their applications to quantum computation, quantum simulation, and quantum optics. Most systems studied so far are based on single atomic species. In this work, we report the experimental observation of strong two-color optical nonlinearity in an ultracold gas of ⁸⁵Rb-⁸⁷Rb atom mixture. By coupling light fields of two different frequencies respectively to Rydberg states of ⁸⁵Rb and ⁸⁷Rb atom via two distinct EIT pathways, we map the Rydberg interaction between different atomic species onto photonic interaction between two nondegenerate optical modes. Such a cross-absorption modulation can be described by a two-component nonlinear wave equations we develope. We further demonstrate that the two-color optical nonlinearity can be tuned by varying the density ratio of different atomic isotopes, which highlights its potential for exploring strongly interacting multi-component fluids of light. Our work opens up an exciting avenue towards generating strong cross-Kerr nonlinearity between two-color optical modes with ultracold atom gas mixture, and the potential of generating quantum correlation between two photons using strong Rydberg atom-atom interaction as a bridge to connect light fields of different colors.