Vacuum-induced interference in light scattering by multilevel atomic chains

This study investigates how vacuum-induced interference—specifically cross-damping and cross-shifts—affects the collective optical response of ordered chains of multilevel atoms. While standard models often treat atoms as two-level systems, real atoms possess multiple transitions that can couple to the same vacuum modes. The authors aim to determine how this quantum interference, when combined with Bragg scattering in periodic arrays, alters the excitation spectrum and collective decay rates.

Using a model of coherent dipoles (MCD), the authors analyze a one-dimensional chain of multilevel emitters driven by an external laser. They derive a mean-field solution to account for the interplay between the two quasi-resonant transitions. By comparing numerical simulations with perturbative analytical results, they quantify the impact of cross-interference terms on the photon-count signal. The study specifically tests these effects using parameters corresponding to the D2 lines of sodium (23Na) and lithium (7Li), which have different energy gaps between their transitions.

The researchers demonstrate that cross-interference significantly modifies the spectroscopic properties of the emitted light. In periodic chains, these effects manifest as distinct deviations from the predictions of standard two-level models, particularly in the collective Lamb shift (CLS) and the cooperative decay rate (CDR). The impact is highly dependent on the ratio of the transition linewidth to the energy gap between transitions; consequently, lithium (with a smaller relative gap) exhibits much stronger interference effects than sodium. These effects are most pronounced when the interatomic spacing is an integer multiple of the transition wavelength or in the subwavelength regime.

This work highlights that high-precision spectroscopy in dense atomic arrays cannot always be simplified to two-level dynamics. As experimental platforms like optical tweezers and lattices reach higher densities and better spatial control, the subtle quantum interference between multiple atomic transitions becomes a critical factor in understanding collective light-matter interactions. These findings provide a framework for predicting and potentially engineering the optical response of complex atomic systems.