Collective resonances of two atoms revealed by zero-quantum two-dimensional coherent spectroscopy
Phys. Rev. A 114, 022825 – Published 25 August, 2026
DOI: https://doi.org/10.1103/mzkr-4jnd
Abstract
Optical two-dimensional coherent spectroscopy (2DCS) provides sensitive detection of collective resonances of two or more atoms, usually by using double- or multiquantum excitations. Here, we report the observation of two-atom collective resonances by using zero-quantum 2DCS. We perform one-quantum (1Q) and zero-quantum (0Q) 2DCS on rubidium atoms in a three-level ladder configuration involving the , and states. The 1Q 2D spectra exhibit the expected diagonal and cross peaks corresponding to the ladder transitions and their couplings. In contrast, the 0Q 2D spectra obtained at elevated excitation power reveal additional peaks at nonzero mixing frequencies that cannot be explained by an isolated single-atom ladder scheme. Density-matrix simulations confirm that the single-atom picture produces only zero-frequency features in the 0Q 2D spectrum. We show that the observed nonzero-frequency peaks originate from collective resonances of two atoms, enabled by higher-order nonlinear excitation pathways that create coherences between two doubly excited collective states. The power dependence of each peak confirms the order of the corresponding nonlinear processes. A theoretical model incorporating two-atom collective states reproduces the experimental 0Q 2D spectrum. These results show that zero-quantum 2DCS can provide a complementary higher-order pathway for observing collective resonances and many-body correlations in atomic and other systems with ladder-type energy-level structures.
