Cooperative ultrastrong atom-light coupling from an effective mapping picture of single-photon superradiance.

As a manifestion of cooperative atom-light interaction from a cluster of atoms, single-photon superradiance has attracted considerable attention for its salient feature of a fast collective spontaneous emission rate and an enhanced coupling strength. It is found that when a cluster of atoms in a dense atom cloud or spatially distributed QED system obtains a largest collective spontaneous emission rate, which is the superadiant emission rate, their optical response is always indistinguishable with a single effective atom. From such effective mapping picture, one can obtain the superradiance conditions for a QED system. Specifically, we have analytically calculated the superradiance conditions for atoms trapped along a waveguide and for atoms coupled to whispering gallery mode resonators and computationally validated them. Based on strong coupling regime, an ultrastrong coupling strength proportional to atom number is accessible by exploiting the superradiance condition, and meanwhile the coherence between atoms remains without distorting the profile of optical response from a single atom. The ultrastrong coupling strength would benefit in the design of ultrafast optical devices, atomic mirror, and controllable optic add/drop filter.