Atom–atom interactions around the band edge of a photonic crystal waveguide

Significance In recent years, there has been considerable effort to bring ultracold atoms into the realm of nanophotonics. Nanoscopic dielectric devices offer unprecedented opportunities to engineer novel capabilities for the control of atom–photon interactions. In particular, photonic crystals are periodic dielectric structures that display a photonic bandgap where light cannot propagate and provide a new setting for coherent photon-mediated interactions between atoms with tunable range. Here, we report the initial observation of cooperative atom–atom interactions around the band edge of a photonic crystal waveguide. Our experiment opens the door to fascinating scenarios, such as exploring many-body physics with large spin exchange energies and low dissipation. Tailoring the interactions between quantum emitters and single photons constitutes one of the cornerstones of quantum optics. Coupling a quantum emitter to the band edge of a photonic crystal waveguide (PCW) provides a unique platform for tuning these interactions. In particular, the cross-over from propagating fields E(x)∝e±ikxx outside the bandgap to localized fields E(x)∝e−κx|x| within the bandgap should be accompanied by a transition from largely dissipative atom–atom interactions to a regime where dispersive atom–atom interactions are dominant. Here, we experimentally observe this transition by shifting the band edge frequency of the PCW relative to the D1 line of atomic cesium for N¯=3.0±0.5 atoms trapped along the PCW. Our results are the initial demonstration of this paradigm for coherent atom–atom interactions with low dissipation into the guided mode.