Long-distance cascaded fluorescence of cold cesium atoms coupled to an optical nanofiber

We demonstrate an experimental realization of cascaded resonance fluorescence over a 64-meter propagationdelay time between two spatially and temporally independent ensembles of laser-cooled cesium atoms coupledto an optical nanofiber. Spontaneously emitted photons from a strongly driven first ensemble are guided througha standard fiber, reflected by a fiber Bragg grating mirror, and interact with a second ensemble, producing a unidirectional two-node cascaded system. The cascaded fluorescence spectrum is broadened and blueshifted relative to the original fluorescence spectrum. Our simple model reproduces the power broadening and the cascaded fluorescence spectrum, as well as the ratio of cascaded to original photon flux, giving insight into non-Markovian dynamics. Our results establish a long-distance one-way cascaded atom-photon interface, providing a stepping stone toward a fiber-based platform for quantum networking.