Observation of coherent coupling between super- and subradiant states of an ensemble of cold atoms collectively coupled to a single propagating optical mode

Abstract

We discuss the evolution of the quantum state of an ensemble of atoms that are coupled via a single propagating optical mode. We theoretically show that the quantum state of N atoms, which are initially prepared in the timed Dicke state, evolves through all the N − 1 states that are subradiant with respect to the propagating mode. We predict this process to occur for any atom number and any atom-light coupling strength. These findings are supported by measurements performed with cold cesium atoms coupled to the evanescent field of an optical nanofiber. We experimentally observe the evolution of the state of the ensemble passing through the first two subradiant states, leading to sudden, temporary switch-offs of the optical power emitted into the nanofiber. Our results contribute to the fundamental understanding of collective atom-light interaction and apply to all physical systems, whose description involves timed Dicke states.

Collective excitation and decay of waveguide-coupled atoms: from timed Dicke states to inverted ensembles

“…There, the theoretical description becomes increasingly complex due to the exponential scaling of the system's Hilbert space with the number of emitters [19][20][21][22][23][24][25]. Recently, nanofiber-based atom-light interfaces have opened a new experimental avenue for studying collective radiative dynamics with waveguide-coupled atoms [26][27][28][29][30][31][32][33]. There, all emitters couple efficiently to the guided optical mode, and propagation-direction-dependent coupling can be implemented, providing access to the field of chiral quantum optics [34].…”

Observation of superradiant bursts in waveguide QED

“…For larger atom numbers, η f increases as N 1.2 (1) , i.e., forward scattering is indeed collectively enhanced compared to independent decay. We note that this mechanism is fundamentally different from the case of coherent forward scattering in the weak excitation regime, where a similar phase pattern is imprinted on the ensemble by the exciting laser field [24,36].…”

“…There, the waveguide mediates long-range dipole-dipole interactions between the emitters, which can be engineered to be almost unidirectional [19]. These unique interactions make the study of collective radiative effects in wQED an especially intriguing research direction [20][21][22][23][24][25][26][27].…”

Large-N limit of spontaneous superradiance

“…This is also motivated by technological applications of superradiance (and subradiance) as a way to improve light-matter interfaces, which play a central role in many quantum technology platforms [34][35][36]. Several experiments have probed radiance in the linear regime [37][38][39][40][41][42][43]. Another possibility is raised by the emergence of waveguide QED [44][45][46][47], where quantum emitters are coupled to one-dimensional optical fields, with diverse implementations ranging from cold atoms near waveguides [48][49][50][51][52], quantum dots [53,54], or nitrogen-vacancy centres [55][56][57].…”