Vacuum-induced collective dynamics in three-level V-type atomic systems*

The phenomenon of spontaneous radiation refers to the decay of an excited atom via interaction with the vacuum electromagnetic (EM) field. When there are many atoms interacting coherently with a common vacuum EM field, the collective atom-field interaction can lead to the atomic ensemble decaying with an enhanced or suppressed rate [1]. We extend the theory to an ensemble of multiple three-level V-type atoms that allows for quantum beats. We analytically solve the collective atomic dynamics following the sudden turn-off of a weak drive field resonant with one excited state, assuming all atoms to be localized within a sub-wavelength region. Interference between two different transitions, as well as among multiple atoms, manifests as collective quantum beat dynamics in subsequent emission. Surprisingly, collective quantum beats appear even when the atomic ensemble starts from only one excited level as the vacuum field provides required coherence through second-order coupling to the other excited level. Our calculated field intensity with no initial superposition matches well with experiment [2], where we detect absorption in the forward direction where the phases add constructively, providing superradiance and enhanced quantum beat amplitudes.  Our work is the first demonstration of quantum beats without initial superposition due to vacuum-induced coupling of excited levels and shows fascinating interplay between multi-level and multi-atom features.

[1] R. H. Dicke, Phys. Rev. 93, 99 (1954).

[2] H. S. Han, A. Lee, K. Sinha, F. K. Fatemi, and S. L. Rolston, arXiv:2102.11982 [quant-ph] (2021).