Direction-dependent collective speed-up of spontaneous emission in a nanofiber-coupled cloud of atoms

We experimentally investigate collective enhancement of light-matter coupling by interfacing a cloud of laser-cooled cesium (Cs) atoms with photons in the evanescent field of an optical nanofiber. We explore the system dynamics by exciting the atoms with short pulses of nanofiber-guided light, whose rise and fall times are much shorter than the atomic lifetime. We analyze the temporal response of the system by recording the power of the light transmitted and reflected by the atomic ensemble. We observe superradiant decay in the forward direction, for which the decay rate of the ensemble increases linearly with optical depth. Notably, in the backward direction, the decay rate of the emitted light is independent of the optical depth and approximately equal to the inverse of the intrinsic lifetime of the Cs 6P3/2 excited state. These results suggest that our system is a promising platform for unveiling novel aspects of collective effects with cold atoms. In particular, in the next step, we plan to integrate the nanofiber in a fiber ring-resonator with a variable in- and out-coupling rate. This will allow us to investigate the physics of collective enhancement while continuously transitioning from the regime of waveguide quantum electrodynamics to cavity quantum electrodynamics.