BEGIN:VCALENDAR VERSION:2.0 PRODID:Icfo X-PUBLISHED-TTL:P1W BEGIN:VEVENT UID:69d8d24833acb DTSTART:20221028T080000Z SEQUENCE:0 TRANSP:OPAQUE LOCATION:ICFO Auditorium and Online (Teams) SUMMARY:ICFO | STEFANO GRAVA CLASS:PUBLIC DESCRIPTION:The interface between light and cold atomic ensembles is a fund amental platform to unravel the quantum world and develop quantum technolo gical applications. Its success relies on the simple idea that the efficie ncy of such an interface can be collectively enhanced by the use of many a toms. While the interaction between its building blocks\, a single photon\ , and a single atom\, is theoretically and experimentally understood\, ins tead\, the interaction between light and a macroscopic ensemble of motionl ess atoms is generically a complex system featuring multiple scattering an d many-body dipole interactions. To avoid the complexity\, typical theorie s of atom-light interactions treat the atomic medium as smooth. However\, it is well-known that microscopic optical effects driven by atomic granula rity can lead to important effects\, especially in dense media. These phen omena and their consequences on the performance of applications are not co mpletely understood. To take them into account exactly\, Chapter 1 introdu ces a ``spin model'' for light-matter interaction. The rest of the thesis is then divided into three chapters\, which push forward our understanding of the interaction of light with dense atomic media.\nIn Chapter 2 it is argued that because of the overwhelming collective macroscopic response an ensemble can exhibit (well captured by the standard theory)\, many micros copically-driven effects that have been predicted\, have also been challen ging to observe so far. An essential step is thus to suppress the macrosco pic light propagation\, so as to allow the microscopic correlations to bui ld up and to be analyzed in a background-free fashion. To solve this issue \, a technique to suppress the macroscopic optical dynamics in free space\ , which allows to precisely investigate many-body aspects of light-matter interaction\, will be presented and demonstrated. In particular\, we unrav el and precisely characterize a microscopic\, density-dependent dipolar de phasing effect that generally limits the lifetime of the optical spin-wave order in ensemble-based atom-light interfaces.\nIn Chapter 3 we will go b eyond the short-time and dilute limits considered previously\, to develop a comprehensive theory of dephasing dynamics for arbitrary times and atomi c densities. In particular\, our non-perturbative approach is based on the strong-disorder renormalization group (RG)\, in order to quantitatively p redict the dominant role that near-field optical interactions between near by neighbors have in driving the dephasing process. This theory also enabl es one to capture the key features of the many-atom dephasing dynamics in terms of an effective single-atom model. These results should shed light o n the limits imposed by near-field interactions on quantum optical phenome na in dense atomic media\, and illustrate the promise of strong disorder R G as a method of dealing with complex microscopic optical phenomena in suc h systems.\nChapter 4 tries to answer the question of why ordinary materia ls exhibit a refractive index of order unity and if the answer can come fr om an electro-dynamical argument. While textbook theories predict nonphysi cal values when extrapolated to densities of solids\, here\, we will evalu ate the exact linear optical response of a three-dimensional lattice of tw o-level atoms\, first from the band structure and then from a direct numer ical simulation. Interestingly\, when multiple scattering of light is exac tly taken into account\, as a result of perfect interference\, it is found that an ideal unity-filled array of atoms can have a refractive index tha t grows with the density and is furthermore real. This implies that a satu ration mechanism for the index should come from the quantum chemistry inte ractions that arise in real materials. Whether saturation could be circumv ented\, could lead to novel optical materials with transformative technolo gical potential.\n \;\nThesis Director: Prof Dr. Darrick Chang DTSTAMP:20260410T103448Z END:VEVENT END:VCALENDAR