Significant efforts have been made to interface cold atoms with micro- and nano-photonic systems in recent years. Originally, it was envisioned that the migration to these systems from free-space atomic ensemble or macroscopic cavity QED experiments could dramatically improve figures of merit and facilitate scalability. However, a more interesting question is whether nanophotonic systems can yield new paradigms for controlling quantum light-matter interactions, which are intrinsically different than in their macroscopic counterparts.
Here, we describe one paradigm for novel physics, based upon the coupling of atoms to photonic crystals. In particular, we show that atoms can become dressed by localized photonic "clouds" of tunable size. This cloud behaves much like an external cavity, but one which follows the position of the atom. This dynamically induced cavity can then mediate long-range spin interactions or forces between atoms. We discuss some interesting phenomena that can emerge, such as highly spatially non-local interactions between photons propagating through the system, and phases of ultracold atoms with strong entanglement between spin and motional degrees of freedom.