**DARRICK CHANG**Theoretical Quantum-Nano Photonics

ICFO - The Institute of Photonic Sciences

A central goal within quantum optics is to realize efficient, controlled interactions between photons and atomic media. A fundamental limit in nearly all applications based on such systems arises from spontaneous emission, in which photons are absorbed by atoms and then re-scattered into undesired channels. In typical theoretical treatments of atomic en- sembles, it is assumed that this re-scattering occurs independently, and at a rate given by a single isolated atom, which in turn gives rise to standard limits of fidelity in applications such as quantum memories for light or photonic quantum gates. However, this assumption can in fact be dramatically violated. In particular, it has long been known that spontaneous emission of a collective atomic excitation can be significantly suppressed through strong interference in emission between atoms. While this concept of ``subradiance” is not new, thus far the techniques to exploit the effect have not been well-understood.

Here, we provide a comprehensive treatment of this problem. First, we show that in ordered atomic arrays in free space, subradiant states acquire an elegant interpretation in terms of optical modes that are guided by the array, which only emit due to scattering from the ends of the finite system. We shown then that these states can be naturally coupled to, by inter- facing the atomic chain with a nanophotonic system. Exploiting subradiant states in such a system allows for a photon storage error that scales exponentially better with number of at- oms than previously known bounds, suggesting that subradiance can completely re-define the rules for quantum optics and atomic physics.

**Summer Lecture, August 21, 2018, 12:00. Blue Lecture Room**