(a) Schematic of the device. Two different species (yellow and blue) populate the waveguide. Two counterpropagating quantum light fields () and two classical control beams () create spin and charge separation, represented by the red and green wavefronts above the waveguide. (b) Energy level diagrams for the two species of atom. Four atomic levels are exploited for optimal tunablility. The strongly interacting polariton gas is created by shifting in the fourth level after a pulse of photons enters the fiber.