Localization of light in ensembles of cold atoms
Final Report Abstract
Cooperative effects first predicted by Dicke in 1954 have been topic of intense research in the past years. Especially super- and subradiance in the linear optics regime or single excitation limit had been at the center of interest. Subradiance which leads to a collective reduction of the spontaneous emitted light and therefor to a significant extended lifetime of the excitation was recently observed for N identical cold atoms. The manipulation and enhancement of these collective long living modes are of interest for the transition to localized long living states (Anderson localization). Approaches used for cooperative effects were applied in the context of localization. Beside subradiance and localization also radiation trapping leads to long lifetimes of scattered photons in an ensemble of scatterers (Rubidium atoms), even though the origins are different. Therefore, it is crucial to distinguish these effects. We studied experimentally as well as numerically the interplay between subradiance and radiation trapping (multiple scattering). A simultaneous observation of both decays was accomplished and enabled to find different scaling laws for both. Numerical calculation of the coupled-dipole-model as well as random walk simulations are in good agreement with the measured data and support the interpretation of the different origins for these effects. Furthermore, the influence of finite temperatures (~100µK), which change the scaling of radiation trapping, was determined. Since subradiance is a collective effect, widely interpreted as interference effect, involving very long timescales, it is suspected to be fragile to thermal motion. Therefore, we studied the impact of thermal decoherence on the subradiant lifetimes. The temperature of the atom cloud was varied from the µK to the mK range and the temporal decay dynamics was recorded. Intuitively the subradiant lifetime should not exceed the inverse of the Doppler width. This was refuted by our measurements, which show only a slight decrease of the lifetimes with increasing temperature. The robustness of subradiance against thermal decoherence was confirmed by the coupled-dipole model. Within the model subradiance lifetimes were calculated and a scaling law for high temperatures was observed, which give for the first time rise to room temperature subradiance. Finally the temporal switch-on dynamics was studied, which contains damped Rabi oscillations. The oscillations show an irregularity in the oscillation frequency due to frequency beating. The frequencies can be interpreted as collective Rabi splitting, similar to the ones observed in optical cavities. Analogues to this the splitting can be explained by a diffusion theory, while the splitting scales with the on resonant optical depth. The damping rate of the Rabi oscillations turned out the be the superradiant decay rate, previously observed at the switch-off. Both measurements of the superradiant decay rate, at the switch-on as well as the switch-off are in good agreement and can be described with the coupled dipole model. Also measurements at a high saturation parameter were done. These are no longer in agreement with the coupled-dipole model, as expected, but can be described by a mean field model. The investigation of a high saturation parameter on the subradiant decay is topic of recent measurements. In the high excitation regime one can expect effects which are beyond the mean field model, like entanglement, which can be examined in future projects.
Publications
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Subradiance and radiation trapping in cold atoms. New J. Phys., 20:063024 (2018)
P. Weiss, M. O. Araújo, R. Kaiser and W. Guerin
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Collective Multimode Vacuum Rabi Splitting. Phys. Rev. Lett., 123:243401 (2019)
W. Guerin, T. S. do Espirito Santo, P. Weiss, A. Cipris, J. Schachenmayer, R. Kaiser and R. Bachelard
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Robustness of Dicke subradiance against thermal decoherence. Phys. Rev. A, 100:033833 (2019)
P. Weiss, A. Cipris, M. O. Araújo, R. Kaiser and W. Guerin
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Collective excitation dynamics of a cold atom cloud. Phys. Rev. A, 101:013617 (2020)
T.S. do Espirito Santo, P. Weiss, A. Cipris, R. Kaiser, W. Guerin, R. Bachelard and J. Schachenmayer
