Ultracold Li Cr : A New Pathway to Quantum Gases of Paramagnetic Polar Molecules
Stefano Finelli, A. Ciamei, Beatrice Restivo, Max Schemmer, A. Cosco, M. Inguscio, A. Trenkwalder, Klaudia Zaremba-Kopczyk, Marcin Gronowski, Michał Tomza, Matteo Zaccanti
Quantum gases of doubly polar molecules represent appealing frameworks for a variety of cross-disciplinary applications, encompassing quantum simulation and computation, controlled quantum chemistry, and precision measurements. Through a joint experimental and theoretical study, here we explore a novel class of ultracold paramagnetic polar molecules combining lithium alkali and chromium transition metal elements. Focusing on the specific bosonic isotopologue 6 Li 53 Cr , leveraging on the Fermi statistics of the parent atomic mixture and on suitable Feshbach resonances recently discovered, we produce up to 50 × 10 3 ultracold Li Cr molecules at peak phase-space densities exceeding 0.1, prepared within the least bound rotationless level of the Li Cr electronic ground state X 6 Σ + . By also developing new probing methods, we thoroughly characterize the molecular gas, demonstrating the paramagnetic nature of Li Cr dimers and the precise control of their quantum state. We investigate their stability against inelastic processes and identify a parameter region where pure Li Cr samples exhibit lifetimes exceeding 0.2 s. Parallel to this, we employ state-of-the-art quantum chemical calculations to accurately predict the properties of Li Cr ground and excited electronic states. This model, able to reproduce the experimental Li - Cr high-spin, scattering length, allows us to identify both efficient paths to coherently transfer weakly bound Li Cr dimers to their absolute ground state, and suitable transitions for their subsequent optical manipulation. Our studies establish Li - Cr as a prime candidate to realize ultracold gases of doubly polar molecules with significant electric (3.3 D) and magnetic ( 5 μ B ) dipole moments. Published by the American Physical Society 2024
Quantum gases of doubly polar molecules represent appealing frameworks for a variety of cross-disciplinary applications, encompassing quantum simulation and computation, controlled quantum chemistry, and precision measurements. Through a joint experimental and theoretical study, here we explore a novel class of ultracold paramagnetic polar molecules combining lithium alkali and chromium transition metal elements. Focusing on the specific bosonic isotopologue 6 Li 53 Cr , leveraging on the Fermi statistics of the parent atomic mixture and on suitable Feshbach resonances recently discovered, we produce up to 50 × 10 3 ultracold Li Cr molecules at peak phase-space densities exceeding 0.1, prepared within the least bound rotationless level of the Li Cr electronic ground state X 6 Σ + . By also developing new probing methods, we thoroughly characterize the molecular gas, demonstrating the paramagnetic nature of Li Cr dimers and the precise control of their quantum state. We investigate their stability against inelastic processes and identify a parameter region where pure Li Cr samples exhibit lifetimes exceeding 0.2 s. Parallel to this, we employ state-of-the-art quantum chemical calculations to accurately predict the properties of Li Cr ground and excited electronic states. This model, able to reproduce the experimental Li - Cr high-spin, scattering length, allows us to identify both efficient paths to coherently transfer weakly bound Li Cr dimers to their absolute ground state, and suitable transitions for their subsequent optical manipulation. Our studies establish Li - Cr as a prime candidate to realize ultracold gases of doubly polar molecules with significant electric (3.3 D) and magnetic ( 5 μ B ) dipole moments. Published by the American Physical Society 2024
We investigate ultracold collisions in a novel mixture of ^{6}Li and ^{53}Cr fermionic atoms, discovering more than 50 interspecies Feshbach resonances via loss spectroscopy. Building a full coupled-channel model, we unambiguously characterize the ^{6}Li-^{53}Cr scattering properties and yield predictions for other isotopic pairs. In particular, we identify various Feshbach resonances that enable the controlled tuning of elastic s- and p-wave ^{6}Li-^{53}Cr interactions. Our studies thus make lithium-chromium mixtures emerge as optimally suited platforms for the experimental search of elusive few- and many-body regimes of highly correlated fermionic matter, and for the realization of a new class of ultracold polar molecules possessing both electric and magnetic dipole moments.
Ultracold polar molecular gases promise new directions and exciting appli-cations in precision measurements, ultracold chemistry, electric-field controlled collisions, dipolar quantum gases, and quantum information sciences. This the-sis presents experimental realization of a near quantum degenerate gas of polar molecules, where the phase-space density of the gas achieved is more than 10 or-ders of magnitude higher than previous results. The near quantum degenerate gas of polar molecules is created using two coherent steps. First, atoms in an ul-tracold gas mixture are converted into extremely weakly bound molecules near a Fano-Feshbach resonance. Second, the weakly bound molecules are transferred to the ro-vibronic ground state using a coherent two-photon Raman technique. The fact that these ground-state molecules are polar is confirmed with a spectroscopic measurement of the permanent electric dipole moment. Finally, manipulation of the molecular hyperfine state is demonstrated; this allows molecules to be pop-ulated in a single quantum state, in particular, the lowest energy state. With an ultracold gas of molecules, full control of molecular internal state, and electric field as a new handle, ultracold molecular collisions, including ultracold chemical reactions and dipolar collisions, are studied.
Ultracold bialkali polar molecules play a leading part at the frontline of quantum physics. They recently attract a lot of attention in the field of ultracold quantum chemistry, quantum many-body physics and quantum simulations. The key for their success is the rich internal level structure with rotational and vibrational degrees of freedom and their large electric dipole moments. Still, only a handful of molecular species are available at ultracold temperatures until now, although it is highly desirable to produce new molecular species to further expand the range of applications. Besides direct laser cooling methods for molecules, the assembly of heteronuclear ground-state molecules from ultracold atomic mixtures is the most promising approach for the creation of polar molecules. It includes the formation of weakly bound Feshbach molecules from the diatomic mixture and the subsequent two-photon stimulated Raman adiabatic passage (STIRAP) transfer to the rovibrational ground state. This creation strategy has been successfully demonstrated for the first time in the pioneering experiments at JILA with ultracold 40K87Rb molecules. Since then, only a few more molecular species from different alkali atoms have been created, namely 6Li23Na, 23Na40K, 23Na87Rb and 87Rb133Cs. In this thesis, I report the successful creation of a new species of ultracold polar ground-state molecules: 23Na39K. Starting from an ultracold mixture of bosonic 23Na and 39K atoms, weakly bound molecules are created. For this purpose, a Feshbach resonance in a high angular momentum scattering channel is chosen, experimentally identified and characterized. Close to this resonance the weakly bound Feshbach molecules are formed using resonant radio frequency radiation. For the two-photon ground-state transfer, a unique, highly specialized two-color laser system is designed and realized. It is used for one- and two-photon spectroscopy to identify the relevant transitions for the ground-state transfer. Based on the obtained data, a local model of the singlet-triplet mixed excited state manifolds is developed, with which the hyperfine structure and the magnetic field dependence is predicted with high accuracy. According to these findings, a suitable pathway to a single hyperfine ground state is chosen considering selection rules and experimental conditions such as laser polarization and beam alignment. To precisely determine the two-photon resonance condition for STIRAP, electromagnetically induced transparency measurements are performed. The ground-state transfer is then performed using STIRAP. The experimental findings regarding the STIRAP are successfully supported theoretically by a model based on a five-level master equation. The pure molecular gas shows evidence for two-body dominated loss mechanisms, such as sticky four-body collisions. The molecule-atom mixture of 23Na39K+39K reveals an unexpectedly low loss rate coefficient although sticky three-body collisions are assumed to occur. This behavior demands further investigations regarding different hyperfine collisional chan-nels and magnetic field regions.
We report on the realization of a degenerate mixture of ultracold fermionic lithium and chromium atoms. Based on an all-optical approach, with an overall duty cycle of about 13 s, we produce large and degenerate samples of more than $2\ifmmode\times\else\texttimes\fi{}{10}^{5}{\phantom{\rule{4pt}{0ex}}}^{6}\mathrm{Li}$ atoms and ${10}^{5}^{53}\mathrm{Cr}$ atoms, with both species exhibiting normalized temperatures of about T/TF=0.25. Additionally, through the exploitation of a crossed bichromatic optical dipole trap, we can controllably vary the density and degree of degeneracy of the two components almost independently and widely tune the lithium-to-chromium density ratio. Our $^{6}\mathrm{Li}\text{\ensuremath{-}}^{53}\mathrm{Cr}$ Fermi mixture opens the way to the investigation of a variety of exotic few- and many-body regimes of quantum matter and it appears as an optimally suited system to realize ultracold paramagnetic polar molecules, characterized by both electric and magnetic dipole moments. Ultimately, our strategy also provides an efficient pathway to produce dipolar Fermi gases, or spin mixtures, of ultracold 53Cr atoms.
Ultracold quantum gases are versatile model systems for exploring quantum physics or for the simulation of solid state materials. Meanwhile, they have been created from various atomic species - from alkali metals over alkaline earths to rare earth elements. The latest addition are quantum gases of different kinds of polar molecules. Expectations for quantum gases of heteronuclear molecules are high: Due to their large electric dipole moments, these molecules can interact with each other via long-range interactions - not just via contact interactions as is the case for most atoms. Additionally, they have vibrational and rotational degrees of freedom, with open up new possibilities for quantum simulation. But the same degrees of freedom also pose some challenges. For example, they make the preparation of the quantum gas more difficult, which is typically produced with a combination of laser cooling and evaporative cooling in the atomic case. Molecules mostly lack closed transitions in their spectra, which are required for laser cooling. Therefore, we create our molecular quantum gas from a mixture of two atomic quantum gases. In this work, such an experimental method was developed for fermionic NaK molecules, which is based on the two-photon process Stimulated Raman Adiabatic Passage (STIRAP). Within STIRAP, the hyperfine structure of the chosen intermediate state plays an important role. Experimentally, and with the help of a theoretical model describing the whole process, we find that we produce the most molecules when we use a large one-photon detuning, if the hyperfine structure of the intermediate state is unresolved. In another project, we explored the rotational level structure of the molecular ground state populated by STIRAP. Rotation is closely linked to the electric dipole moment. The superposition of the ground state with the first excited rotational state, for example, has a transition dipole moment of almost 60% of the permanent electric dipole moment. Unfortunately, coherence times of such superpositions are typically short, as the different polarizabilities of the rotational states lead to dephasing in optical traps. However, using a special polarization angle and a small, dc electric field, we can compensate these differences and realize a spin-decoupled magic trap. With this new technique we obtain record coherence times, at least for small molecular densities. For larger densities we observe first indications for dipolar interactions in a bulk gas of polar molecules, which we also model using the moving-average cluster expansion (MACE).
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.