Many-body interactions and correlations in atomic ensembles are fundamental in understanding many-body effects such as collective and emergent phenomena and also play an important role in various atom-based applications. Optical two-dimensional coherent spectroscopy (2DCS) provides a powerful tool to measure many-body interactions and correlations. Here, we present the study of many-body dipole-dipole interactions and correlations in potassium and rubidium atomic vapors by using double-quantum and multi-quantum 2DCS. The results show that double-quantum 2DCS provides sensitive and background-free detection of weak dipole-dipole interaction between atoms with a mean separation up to about 16 μm, and multi-quantum 2DCS can excite and detect multi-atom states (Dicke states) with up to eight correlated atoms. The technique of optical 2DCS can provide a new approach to study many-body physics in atomic ensembles and can be potentially implemented to measure many-body effects in cold atoms and other atomic/molecular systems.
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.
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.
Quantum coherence of collective states plays a pivotal role in understanding many-body cooperative behaviors. Collective coherences between the ground state and excited states in potassium atoms were previously revealed; however, the coherence between collectively excited states has hitherto been overlooked, and its observation has remained challenging. Here we present a joint experimental and theoretical investigation of coherence effects in K atomic vapor using two-dimensional coherent spectroscopy. By collecting zero-quantum, double-quantum, and double-quantum--zero-quantum two-dimensional spectra, distinct coherence signatures are spectroscopically accessible. The experimental spectra are well reproduced by numerically solving the time dependence of the Lindblad equation, and further analyses unveil nonradiative coherence as the origin of the observations. Our work provides comprehensive insight into the collective coherence effect in multiatom systems.
Collective resonance of interacting particles has important implications in many-body quantum systems and their applications. Strong interactions can lead to a blockade that prohibits the excitation of a collective resonance of two or more nearby atoms. However, a collective resonance can be excited with the presence of weak interaction and has been observed for atoms in the first excited state (P state). Here, we report the observation of collective resonance of rubidium atoms in a higher excited state (D state) in addition to the first excited state. The collective resonance is excited by a double-quantum four-pulse excitation sequence. The resulting double-quantum two-dimensional (2D) spectrum displays well-isolated peaks that can be attributed to collective resonances of atoms in P and D states. The experimental one-quantum and double-quantum 2D spectra can be reproduced by a simulation based on the perturbative solutions to the optical Bloch equations, confirming collective resonances as the origin of the measured spectra. The experimental technique provides a new approach for preparing and probing collective resonances of atoms in highly excited states.
Frequency-comb based multidimensional coherent spectroscopy is a novel optical method that enables high-resolution measurement in a short acquisition time. The method's resolution makes multidimensional coherent spectroscopy relevant for atomic systems that have narrow resonances. We use double-quantum multidimensional coherent spectroscopy to reveal collective hyperfine resonances in rubidium vapor at 100 °C induced by dipole-dipole interactions. We observe tilted and elongated line shapes in the double-quantum 2D spectra, which have never been reported for Doppler-broadened systems. The elongated line shapes suggest that the signal is predominately from the interacting atoms that have a near zero relative velocity.