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arXiv:2512.04681

Fast and efficient formation of stable tetraatomic molecules from ultracold atoms via generalized stimulated Raman exact passage

Jia-Hui Zhang, Wen-Yuan Wang, Fu-Quan Dou

cond-mat.quant-gasquant-phSubmitted Dec 4, 2025• Announced Dec 5, 2025• 0 citations
v1 · Dec 4, 2025v2 · Dec 5, 2025v3 · Dec 9, 2025v4 · Jan 20, 2026
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Abstract

The study of the conversion of ultracold atoms into molecules has long remained a hot topic in atomic, molecular, and optical physics. However, most prior research has focused on diatomic molecules, with relatively scarce exploration of polyatomic molecules. Here we propose a two-step strategy for the formation of stable ultracold tetraatomic molecules. We first suggest a generalized nonlinear stimulated Raman exact passage (STIREP) technique for the coherent conversion of ultracold atoms to tetraatomic molecules, which is subsequently followed by a chainwise-STIREP technique to transfer the resulting molecules into a sufficiently stable ground state. Through systematic numerical analysis, we demonstrate that the proposed two-step strategy holds great potential for enabling the robust, fast, and efficient formation of stable ultracold tetraatomic molecules.

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The study of the conversion of ultracold atoms into molecules has long remained a hot topic in atomic, molecular, and optical physics. However, most prior research has focused on diatomic molecules, with relatively scarce exploration of polyatomic molecules. Here we propose a two-step strategy for the formation of stable ultracold tetraatomic molecules. We first suggest a generalized nonlinear stimulated Raman exact passage (STIREP) technique for the coherent conversion of ultracold atoms to tetraatomic molecules, which is subsequently followed by a chainwise-STIREP technique to transfer the resulting molecules into a sufficiently stable ground state. Through systematic numerical analysis, we demonstrate that the proposed two-step strategy holds great potential for enabling the robust, fast, and efficient formation of stable ultracold tetraatomic molecules.
We suggest and analyze a technique for efficient and robust creation of dense ultracold molecular ensembles in their ground rovibrational state. In our approach a molecule is brought to the ground state through a series of intermediate vibrational states via a multistate chainwise stimulated Raman adiabatic passage technique. We study the influence of the intermediate states decay on the transfer process and suggest an approach that minimizes the population of these states, resulting in a maximal transfer efficiency. As an example, we analyze the formation of 87Rb2^{87}\mathrm{Rb}_{2}87Rb2​ starting from an initial Feshbach molecular state and taking into account major decay mechanisms due to inelastic atom-molecule and molecule-molecule collisions. Numerical analysis suggests a transfer efficiency >90>90%>90, even in the presence of strong collisional relaxation as are present in a high density atomic gas.
Robust stimulated Raman exact passages are requisite for controlling nonlinear quantum systems, with wide applications, ranging from ultracold molecules to nonlinear optics to superchemistry. Inspired by shortcuts to adiabaticity, we propose the fast-forward scaling of stimulated Raman adiabatic processes with nonlinearity involved, describing the transfer from an atomic Bose-Einstein condensate to a molecular one by controllable external fields. The fidelity and robustness of atom-molecule conversion are shown to surpass those of conventional adiabatic passages, assisted by the fast-forward driving field. Finally, our results are extended to fractional stimulated Raman adiabatic processes for the coherent superposition of atomic and molecular states.
We have studied the formation of ultracold RbSr molecules with laser pulses. After discussing the advantages of the Mott insulator phase for the control with pulses, we present two classes of strategies. The first class involves two electronic states. Two extensions of stimulated Raman adiabatic passage (STIRAP) for multilevel transitions are used: alternating STIRAP (A-STIRAP) and straddle STIRAP (S-STIRAP). Both transfer dynamics are modeled and compared. The second class of strategies involves only the electronic ground state and uses infrared (IR) and terahertz (THz) pulses. The molecular bond is first created by the application of a THz chirped pulse or $\ensuremath{\pi}$-pulse. Subsequently, the molecules are transferred to their rovibrational ground level using IR pulses. For this last step, different optimized pulse sequences that were obtained through optimal control techniques have been studied. The relative merits of these strategies in terms of efficiency and robustness are discussed with respect to the experimental feasibility based on present laser technologies.
We develop a theoretical description of photoassociative stimulated Raman adiabatic passage (STIRAP) near a Feshbach resonance in a thermal atomic gas. We show that it is possible to use low-intensity laser pulses to directly excite pairs of atoms in the continuum near a Feshbach resonance and to transfer most of the atomic cloud to the lowest rovibrational level of the molecular ground state. For a broad resonance, commonly found in several mixtures of alkali atoms, our model predicts a transfer efficiency up to 97% for a given atom pair, and up to 70% when averaged over an atomic ensemble. The laser intensities and pulse durations needed for such an optimal transfer, 10 2 –10 3 W cm −2 and several microseconds, are easily achievable experimentally. A single pair of STIRAP pulses converts an estimated fraction f ∼10 −6 –10 −4 of atoms in an atomic ensemble, leading to the production of 10–1000 molecules in a large sample of 10 7 atoms. A total of ∼10 4 –10 6 pulse pairs are thus required to transfer most atoms into molecules. Such an efficiency compares with or surpasses currently available techniques for creating stable diatomic molecules, and the versatility of this approach simplifies its potential use for many molecular species.
Chainwise stimulated Raman adiabatic passage (C-STIRAP) in a M-type molecular system is a good alternative in creating ultracold deeply bound molecules when the typical STIRAP in a $\mathrm{\ensuremath{\Lambda}}$-type system does not work due to weak Frank-Condon factors between states. However, its creation efficiency under smooth evolution is generally low. During the process, the population in the intermediate states may decay out quickly and the strong laser pulses may induce multiphoton processes. In this paper, we find that the shortcut to adiabaticity (STA) fits very well in improving the performance of the C-STIRAP. Currently, related discussions on the so-called chainwise stimulated Raman shortcut-to-adiabatic passage (C-STIRSAP) are rare. Here, we investigate this topic under adiabatic elimination. Given a relation among the four incident pulses, it is quite interesting that the M-type system can be generalized into an effective $\mathrm{\ensuremath{\Lambda}}$-type structure with the simplest resonant coupling. Consequently, all possible methods of STA for the three-state system can be borrowed. We take the counterdiabatic driving and ``chosen path'' methods as instances to demonstrate our treatment of the molecular system. Although the chosen path method does not work well in a real three-state system if there is strong decay in the excited state, our C-STIRSAP protocol under both methods can create ultracold deeply bound molecules with high efficiency in the M-type system. The evolution time is shortened without strong laser pulses and the robustness of STA is well preserved. Finally, the detection of ultracold deeply bound molecules is discussed.
We propose a coherent optical population transfer of weakly bound field-linked (FL) tetratomic molecules (tetramers) to deeper FL bound states using stimulated Raman adiabatic passage. We consider static-electric-field shielded polar alkali-metal diatomic molecules and corresponding FL tetramers in their X^{1}Σ^{+}+X^{1}Σ^{+} ground electronic state. We show that the excited metastable X^{1}Σ^{+}+b^{3}Π electronic manifold supports FL tetramers in a broader range of electric fields with collisional shielding extended to zero field. We calculate the Franck-Condon factors between the ground and excited FL tetramers and show that they are highly tunable with the electric field. We also predict photoassociation of ground-state shielded molecules to the excited FL states in free-bound optical transitions. We propose proof-of-principle experiments to implement stimulated Raman adiabatic passage and photoassociation using FL tetramers, paving the way for the formation of deeply bound ultracold polyatomic molecules.