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Superradiant to subradiant phase transition in the open system Dicke model: dark state cascades

Gegg, Michael; Carmele, Alexander; Knorr, Andreas; Richter, Marten

AG Nichtlineare Optik und Quantenelektronik

Collectivity in ensembles of atoms gives rise to effects like super- and subradiance. While superradiance is well studied and experimentally accessible, subradiance remains elusive since it is difficult to track experimentally as well as theoretically. Here we present a new type of phase transition in the resonantly driven, open Dicke model that leads to a deterministic generation of subradiant states. At the transition the system switches from a predominantly superradiant to a predominantly subradiant state. Counterintuitively, the cavity decay is the crucial parameter for subradiant state generation and not the individualizing process of spontaneous decay. The observed effect is thus a cavity assisted generation of subradiant quantum coherences. Clear experimental signatures for the effect are presented and entanglement properties are discussed. Letting the system relax into the ground state generates a cascade of dark Dicke states, with dark state populations up to unity. Furthermore we introduce a collectivity measure that allows to quantify collective behaviour.