Optical two-dimensional coherent spectroscopy (2DCS) has become a powerful tool for studying energy-level structure, dynamics, and coupling in many systems including atomic ensembles. Various types of two-dimensional (2D) spectra, including the so-called single-quantum, zero-quantum, and double-quantum 2D spectra, of both
D lines (
D1 and
D2 transitions) of potassium (K) atoms have been reported previously. For rubidium (Rb), a major difference is that the
D lines are about 15 nm apart as opposed to only about 3 nm for K. Simultaneously exciting both
D lines of Rb atoms requires a broader laser bandwidth for the experiment. Here, we report a broadband optical 2DCS experiment in a Rb atomic vapor. A complete set of single-quantum, zero-quantum, and double-quantum 2D spectra including both
D lines of Rb atoms were obtained. The experimental spectra were reproduced by simulated 2D spectra based on the perturbation solutions to the optical Bloch equations. This work in Rb atoms complements previous 2DCS studies of K and Rb with a narrower bandwidth that covers two
D lines of K or only a single
D line of Rb. The broadband excitation enables the capability to perform double-quantum and multiquantum 2DCS of both
D lines of Rb to study many-body interactions and correlations in comparison with K atoms.
is the Bohr radius. The experimental results confirm the long range nature of the dipole-dipole interaction, which is critical for understanding many-body physics in atoms/molecules. The long range interaction also has implications in atom-based applications involving many-body interactions. Additionally, we demonstrated that double-quantum 2DCS is sufficiently sensitive to probe dipole-dipole interaction at densities that can be achieved with cold atom in a magneto-optical trap, paving the way for double-quantum 2DCS studies of cold atoms and molecules. The method can also open a new avenue to study long-range interactions in solid state systems such as quantum dots and color centers in diamonds.
We report a novel, to the best of our knowledge, double-quantum-zero-quantum two-dimensional coherent spectroscopy (2DCS) that allows direct detection of the quantum coherence between multiparticle collective states. Through correlating the double-quantum coherence and the zero-quantum coherence, signatures for coherence between collective states can be well isolated as side peaks and readily identified in the 2D spectrum. The experiment is implemented in a vapor of rubidium atoms in a collinear 2DCS setup. Good agreement with a theoretical simulation using density matrix confirms the essential role of the interatomic correlation effect in generating the side peak signals. This 2D spectrum technique paves a new avenue for studying the coherent coupling of highly excited states and many-body properties.
Abstract Two-dimensional coherent spectroscopy (2DCS) combined with dual-comb technology offers unprecedented resolution for probing many-body interactions and correlations in atomic vapors, yet its application to high-order multi-quantum transitions remains challenging due to weaker nonlinear signals and phase instability. Here, we demonstrate a triple-quantum dual-comb 2DCS technique that achieves λ/110 phase stability through digital correction, enabling the observation of velocity-synchronized Dicke states in a thermal rubidium vapor. By resolving collective hyperfine resonances of ⁸⁵Rb and ⁸⁷Rb isotopes with 100 MHz spectral resolution, we reveal that triple-quantum dipole-dipole correlations (0.95) surpass double-quantum counterparts (0.90), indicating tighter velocity matching in higher-order Dicke states. This work establishes a pathway for manipulating multi-atom correlations in Doppler-broadened systems and extends 2DCS to research the many-body interaction in semiconductor exciton and 2D materials.