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.
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.
Abstract Ultracold polyatomic molecules offer opportunities 1 in cold chemistry 2,3 , precision measurements 4 and quantum information processing 5,6 , because of their rich internal structure. However, their increased complexity compared with diatomic molecules presents a challenge in using conventional cooling techniques. Here we demonstrate an approach to create weakly bound ultracold polyatomic molecules by electroassociation 7 (F.D. et al., manuscript in preparation) in a degenerate Fermi gas of microwave-dressed polar molecules through a field-linked resonance 8–11 . Starting from ground-state NaK molecules, we create around 1.1 × 10 3 weakly bound tetratomic (NaK) 2 molecules, with a phase space density of 0.040(3) at a temperature of 134(3) nK, more than 3,000 times colder than previously realized tetratomic molecules 12 . We observe a maximum tetramer lifetime of 8(2) ms in free space without a notable change in the presence of an optical dipole trap, indicating that these tetramers are collisionally stable. Moreover, we directly image the dissociated tetramers through microwave-field modulation to probe the anisotropy of their wavefunction in momentum space. Our result demonstrates a universal tool for assembling weakly bound ultracold polyatomic molecules from smaller polar molecules, which is a crucial step towards Bose–Einstein condensation of polyatomic molecules and towards a new crossover from a dipolar Bardeen–Cooper–Schrieffer superfluid 13–15 to a Bose–Einstein condensation of tetramers. Moreover, the long-lived field-linked state provides an ideal starting point for deterministic optical transfer to deeply bound tetramer states 16–18 .
Abstract Heteronuclear alkali-metal dimers represent the class of molecules of choice for creating samples of ultracold molecules exhibiting an intrinsic large permanent electric dipole moment. Among them, the KCs molecule, with a permanent dipole moment of 1.92 Debye still remains to be observed in ultracold conditions. Based on spectroscopic studies available in the literature completed by accurate quantum chemistry calculations, we propose several optical coherent schemes to create ultracold bosonic and fermionic KCs molecules in their absolute rovibrational ground level, starting from a weakly bound level of their electronic ground state manifold. The processes rely on the existence of convenient electronically excited states allowing an efficient stimulated Raman adiabatic transfer of the level population.
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 tetratomic molecules. We first suggest a generalized nonlinear stimulated Raman exact passage (STIREP) technique for the coherent conversion of ultracold atoms to tetratomic 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 the robust, fast, and efficient formation of stable ultracold tetratomic molecules.
We report the creation of ultracold ground state ^{6}Li^{40}K polar molecules with high efficiency. Starting from weakly bound molecules, stimulated Raman adiabatic passage is adopted to coherently transfer the molecules to their singlet rovibrational ground state |X^{1}Σ^{+},v=0,J=0⟩. By employing a singlet stimulated Raman adiabatic passage pathway and low-phase-noise narrow-linewidth lasers, we observed a one-way transfer efficiency of 96(4)%. Held in an optical dipole trap, the lifetime of the ground state molecules is measured to be 5.0(3) ms. The large permanent dipole moment of LiK is confirmed by applying a dc electric field on the molecules and performing Stark shift spectroscopy of the ground state. With recent advances in the quantum control of collisions, our work paves the way for exploring quantum many-body physics with strongly interacting ^{6}Li^{40}K molecules.
Ultracold paramagnetic and polar diatomic molecules are among the promising systems for quantum simulation of lattice-spin models. Unfortunately, their experimental observation is still challenging. Based on our recent ab initio calculations, we analyze the feasibility of all-optical schemes for the formation of ultracold 87Rb84Sr bosonic molecules. A first possibility is photoassociation followed by spontaneous emission. The photoassociation rate coefficients toward electronic states converging to the 87Rb(5s2S1/2)+84Sr(5s5p3P0,1,2) asymptotes are particularly small for vibrational levels close to the asymptote. The creation of molecules would be more interesting by using deeply bound levels which preferentially relax to the ${v}^{\ensuremath{'}\ensuremath{'}}=0$ level of the ground state. On the other hand, the photoassociation rate coefficients toward electronic states correlated to the Rb(5p2P1/2,3/2)+Sr(5s21S0) are significant for levels close to the asymptote. The spontaneous emission thus creates weakly bound molecules in a single vibrational level. A second option relies on stimulated Raman adiabatic passage implemented in a tight optical trap. It efficiently creates weakly bound ground-state molecules in a well-defined level, thus providing a promising alternative to magnetic Feshbach resonances for further population transfer toward the absolute ground state of the RbSr molecule.
The assembly and coherent control of ultracold molecules have opened new horizons in quantum science with applications in ultracold chemistry, quantum information, quantum simulation, precision metrology and fundamental physics. While considerable progress has been achieved in controlling diatomic molecules at ultracold temperatures, extending to atom-diatom collisions and triatomic systems introduces additional internal structure, anisotropy, and interaction complexity, enabling access to many-body phenomena. This dissertation presents a detailed theoretical study on the production and stabilization of ultracold alkali-metal triatomic molecules, with a particular focus on the NaK2 molecular species. First, long-range photoassociation in ultracold NaK(X^1Σ+) + K(4s) mixtures is investigated near the molecular transition X^1Σ+ → b^3Π. Photoassociation rates into weakly bound trimers in the electronically excited 3^2A' state are evaluated under experimentally relevant conditions. The calculated photoassociation rates indicate experimentally feasible and spectrally selective routes for detecting ultracold atom–diatom photoassociation resonances, while suppressing competing diatomic loss channels. Second, a microwave-assisted association mechanism is introduced for coherently binding ultracold atoms and polar molecules into weakly bound triatomic molecules on the ground-state 1^2A' potential energy surface. This process relies on the interaction between the electric component of the microwave field and the permanent electric dipole moment of the NaK molecule. In the presence of the anisotropic atom–diatom potential, which couples the molecular rotation to the relative angular motion, the microwave field can drive transitions from scattering states to weakly bound trimer states entirely in the electronic ground-state of the triatomic molecule, without involving spontaneous emission. This process is therefore intrinsically coherent and particularly suited to closed-shell polar molecules for which magnetically tunable Feshbach resonances are weak or absent. Finally, candidate pathways toward coherent population transfer to the absolute rovibronic ground state of NaK2 are explored using bridge states belonging to the excited 3^2A' electronic state, such as those identified in [Ref.1]. The relevant potential energy surfaces, transition dipole moments, and rovibrational structures are analyzed, revealing favorable conditions for future stimulated Raman adiabatic passage scheme. The theoretical modeling combines high-level electronic-structure calculations with nuclear-motion methods across multiple energy and length scales. Three-dimensional potential energy surfaces and electronic transition dipole moment surfaces were obtained from ab initio multireference configuration interaction calculations employing effective-core pseudopotentials and core-polarization potentials. Nuclear motion was treated using coupled-channel methods for describing weakly bound states and discrete variable representation methods for deeply bound rovibrational states. The resulting wavefunctions were subsequently used to evaluate Franck–Condon factors, photo- and microwave association rates, and transition strengths relevant to experimental implementation. Overall, this dissertation provides an extensive theoretical framework for the formation, detection and ground-state stabilization of ultracold NaK2 trimers. Reference: [1] J. Cao, B.-Y. Wang, H. Yang, Z.-J. Fan, Z. Su, J. Rui, B. Zhao, and J.-W. Pan, “Observation of photoassociation resonances in ultracold atom-molecule collisions,” Phys. Rev. Lett., vol. 132, p. 093403, 2024.