Collective states of multi-level emitters: The role of multi-level interferences

Research Question and Approach

This paper investigates how the multi-level internal structure of quantum emitters—such as atoms or molecules—affects the collective states and dynamical properties of emitter arrays. While many theoretical studies simplify emitters to two-level systems, this approximation often fails when transitions are closely spaced. The authors derive a master equation that accounts for interferences between these near-resonant transitions without relying on common simplifications like mean-field treatments. They then perform numerical simulations on a three-emitter array to compare the dynamics of interfering versus non-interfering models across varying spatial separations.

The researchers find that for emitters separated by distances significantly smaller than the excitation wavelength, multi-level interferences fundamentally change the system's collective behavior. Specifically, these interferences open additional decay channels, which slows the overall relaxation to the ground state compared to non-interfering models. Most notably, the study identifies a lower bound on inter-emitter separation for the emergence of superradiance; at ultra-small distances, the multi-level nature of the emitters can suppress the superradiant burst entirely. These effects are most pronounced when the energy separation between transitions is very small (below 10⁻⁵ of the excitation energy) and the spatial separation is between 0.01 and 0.05 times the excitation wavelength.

As experimental platforms like optical lattices reach the regime of ultra-small emitter separations, the standard two-level approximation becomes increasingly unreliable. This work provides a necessary theoretical framework for predicting spectroscopic signals in dense arrays of complex emitters. By identifying the specific conditions under which multi-level interference becomes significant, the study offers guidance for designing experiments in quantum sensing, metrology, and many-body physics where precise control over collective light-matter interactions is required.