Title: Waveguide-coupled atoms: how a simple quantum optical system can amaze you

Abstract: 
The collective absorption and emission of light by an ensemble of atoms is at the heart of many fundamental quantum optical effects and the basis for numerous applications. Here, I report on experiments, in which we study these collective phenomena by analyzing the transmission of light past an ensemble of atoms that are optically interfaced using the evanescent field surrounding an optical nanofiber. Upon weak pulsed excitation, we unveil the microscopic dynamics of the system, showing that collective interactions between the atoms and guided light at the single photon level gradually build up along the atomic ensemble. Furthermore, we observe coherent oscillations between super- and fully subradiant states of the ensemble, leading to sudden, temporary switch-offs of the emitted optical power. We also explore the regime beyond weak excitation, where we excite about 80% of the atoms, and study their subsequent radiative decay into the waveguide modes. Finally, and surprisingly, we demonstrate that even when the system has reached its steady state transmission for long excitation pulses, the collectively enhanced nonlinear atom-light interaction induces correlations between simultaneously arriving photons. As a consequence, depending on the number of atoms, we experimentally observe strong photon bunching or antibunching of the transmitted light. Our results contribute to the fundamental understanding of the collective interaction of light and matter and are relevant for applications ranging from quantum memories to sources of nonclassical light to optical frequency standards.