Zero-quantum dual-comb spectroscopy of collective resonances in rubidium vapor

We report dual-comb spectroscopy of the collective zero-quantum response of rubidium atoms. The observed signals include two-atom collective resonances at 2(ω _{D 2} -ω _{D 1} ), ω_5P3/2→5D-ω _{D 2} , ω_5P3/2→5D-ω _{D 1} , and (ω _{D 2} -ω _{D 1} )+(ω_5P3/2→5D-ω _{D 1} ), as well as higher-order three-atom resonances. Unlike double-quantum spectroscopy, which probes sum-frequency resonances of multiple atoms, the zero-quantum pathway probes difference-frequency correlations between optical transitions. In a dual-comb implementation, these signals are independent of the carrier-envelope offset frequencies of both combs, enabling rapid measurements without active carrier-envelope offset stabilization. These results show that zero-quantum dual-comb spectroscopy is a powerful approach for probing collective coherences and many-body correlations in atomic systems.