Session H05: Cold and Ultracold Molecules I

Functionalized Aromatic Molecules for Laser Cooling and Trapping

Recent experimental and theoretical work has demonstrated that direct laser cooling can be applied to many classes of polyatomic molecules, including derivatives of aromatic molecules. These complex polyatomic molecules are appealing targets because they can be "decorated" with useful quantum control centers while maintaining the ability to be laser cooled and optically detected. In this talk, we summarize ongoing work to laser cool and trap a functionalized aromatic molecule composed of a Ca-based optical cycling center bonded to a phenyl ring. High-resolution laser excitation spectra are presented as part of a rotational analysis of these species. Dispersed fluorescence spectra from rotationally-resolved excited states allow us to identify the transitions that must be addressed to scatter thousands of photons per molecule. These measurements provide a practical roadmap for direct laser cooling and trapping of complex polyatomic molecules.   

Fast optical transport of ultracold molecules over long distance

Optically trapped laser cooled polar molecules are an attractive platform for quantum information and quantum simulation. Long molecular lifetimes and large numerical aperture optical access is needed, but can be difficult to achieve in a typical MOT apparatus with a cryogenic buffer gas beam source. Long distance transport places molecules into a chamber separate from the MOT, easing the difficulty of fulfilling these requirements. We present a fast transport method for ultracold molecules based on an electronically focus-tunable lens. The high transport speed is achieved by employing 1D red-detuned optical lattice generated by interference of a focus-tunable laser beam and a focus-fixed laser beam. Transport of ultracold CaF molecules over 45 cm distance is realized in less than 50 ms with no significant heating. Fluctuation in the final position of the transported molecular cloud is small, allowing stable loading into an optical tweezer array.   

Development of a rotationally magic trap for ultracold RbCs molecules

Ultracold polar molecules offer many exciting opportunities in the fields of quantum computation, quantum simulation and fundamental studies of quantum matter. Many of these applications utilise the rotational states of the molecule and rely on long rotational coherence times. Achieving this in experiments has so far proved challenging, however, owing to the presence of large differential light shifts between rotational levels as a result of the anisotropic molecular polarizability. The solution is to construct a magic-wavelength trap, where the polarizabilities are identical for two (or more) rotational states. Here we report the development of such a magic trap at a wavelength of 1146nm. This wavelength lies between the X¹Σ→b³Π vibronic transitions, allowing the anisotropic component of the polarizability to be tuned to zero. We present spectroscopy of the X¹Σ→b³Π transitions and show that the differential shift of the N=0→N=1 rotational transition can be tuned to be zero in the magic trap for a detuning of approximately 220 GHz from the X¹Σ (v=0,N=0)→b³Π(v'=0,N=1) transition. Using Ramsey spectroscopy we show the absence of dephasing in the magic trap. Based upon reasonable parameters for the laser frequency stability and intensity, we estimate coherence times greater than 10s should be easily achievable in a magic 3D optical lattice.   

Evaporation of microwave-shielded polar molecules to quantum degeneracy

Collisional loss at short range has so far prevented the cooling of interacting polar molecules to quantum degeneracy in three dimensions (3D). Here, we demonstrate evaporative cooling of a 3D gas of fermionic NaK molecules to well below the Fermi temperature using microwave shielding. The molecules are protected from reaching short range with a repulsive barrier engineered by coupling rotational states with a blue-detuned circularly polarized microwave. The microwave dressing induces strong tunable dipolar interactions between the molecules, leading to high elastic collision rates that can exceed the inelastic ones by at least a factor of 460. This large elastic-to-inelastic collision ratio allows us to cool the molecular gas down to 21 nK, corresponding to 0.36 times the Fermi temperature.

arXiv:2201.05143   

Pathway to magneto-optical trapping of SrOH for beyond-Standard Model searches

The complex structure of polyatomic molecules offers powerful features that can be exploited for applications in quantum simulation and precision measurement. One of these features is a plethora of ro-vibrational states. In the linear triatomic molecule SrOH, two vibrational states have a near-degeneracy that can be probed with microwaves (in contrast to typical vibrational splittings of ∼10 THz). Because the energies of these states depend differently on the proton-to-electron mass ratio, μ, a microwave resonance between them is highly sensitive to changes in μ over time. We report on high-resolution vibrational branching ratio measurements of four excited vibronic states. We are now able to design a laser-cooling scheme with over 10,000 photon scatters before loss to an unaddressed vibrational state, more than enough to form a MOT and load an optical trap. Only 8 lasers are required for this scheme, fewer than for any other known polyatomic molecule cooling cycle. We describe progress towards laser cooling of SrOH and a high-sensitivity measurement of μ variation using ultracold SrOH molecules.   

quantum state control of chiral molecules

Recently, the enantiomer-specific state transfer (ESST) method [1] was demonstrated using tailored microwave fields. This method allows to populate or depopulate a rotational state of a chosen enantiomer, providing a way of quantum-controlled chiral separation. Thus far, the transfer efficiency of ESST has been limited by thermal population of the energy levels participating in ESST [1,2] and by M_J degeneracy [3]. To address these prior limitations, we developed a new experimental scheme which increases the efficiency of ESST by over a factor of ten compared to previously reported values [4]. This scheme enables a quantitative comparison between experiment and theory for the transfer efficiency in what is the simplest ESST triangle for any chiral molecule, that is, the one involving the absolute ground state level. Starting with a racemic mixture, a straightforward extension of this scheme should be able to create a molecular beam with an enantiomer-pure rotational level, holding great prospects for future spectroscopic and scattering studies.

References

[1] S. Eibenberger, et al., Phys. Rev. Lett. 118, 123002 (2017)

[2] P. Cristóbal, et al., Angew. Chem. Int. Ed. 56, 12512 (2017)

[3] M. Leibscher, et al., arXiv:2010.09296 (2020)

[4] J.H. Lee, et al., arXiv:2112.09058 (2021)   

Nonadiabatic decay in Rydberg molecules

Rydberg molecules have become an active research area within Rydberg-atom physics. Among a variety of unusual properties of Rydberg molecules, their decay mechanisms present a rich ground for both theoretical and experimental explorations. In this talk, the effect of nonadiabatic processes on Rydberg-molecule lifetimes will be discussed. An analysis for incorporating these effects using the Born-Huang representation will be reviewed. The method is employed to determine the non-adiabatic lifetimes of Rydberg-ion-molecules, which were recently predicted and observed. The obtained results, perspectives for other types of Rydberg molecules, as well as implications for future experiments will be discussed.   

Electronic spectroscopy of ~A-~X transitions of jet-cooled calcium monoalkoxide radicals: spin-ro-vibronic structure of nonlinear polyatomic molecules as candidates for direct laser cooling

Alkaline earth monoalkoxide free radicals are promising candidates for laser cooling. Here we report a combined experimental and computational investigation of the spin-vibronic structure of the lowest electronic states of calcium methoxide [CaOCH₃ (C_3v)], ethoxide [CaOC₂H₅ (C_s)], and isopropoxide [CaOCH(CH₃)₂ (C_s)]. Laser-induced fluorescence/dispersed fluorescence (LIF/DF) and cavity ring-down spectra of the ^~A-^~X transitions were recorded under jet-cooled conditions. The ~A₂-^~A₁ energy separation of vibronic levels is attributed to the spin-orbit (SO) interaction and, in the case of the two asymmetric tops, the difference potential between the A’ and A’’ states, both of which affect the rotational and fine structure of the nearly degenerate ^~A₁/ ^~A ₂ states. A spin-vibronic Hamiltonian has been developed in a quasi-diabatic basis for the spectral simulation, aided by ab initio calculations. The (pseudo-)Jahn-Teller and SO interactions induce off-diagonal Franck-Condon matrix elements, leading to vibronic transitions that are forbidden or negligible under the Born-Oppenheimer approximation.   

Towards direct laser cooling of barium monofluoride

Cold molecular gases are the starting point for many novel and interdisciplinary applications ranging from few- and many-body physics to cold chemistry and precision measurements. However, while there has recently been significant progress in the direct cooling of molecules, the preparation of a new molecular species in the cold temperature regime still requires a careful optimization of the available cooling techniques. We have performed vibrational spectroscopy of monofluoride (BaF), to determine the cooling and repumping transitions of this molecule with an accuracy of better than 100 MHz. Together with a detailed modeling of the cooling processes, this brings laser cooling of this species within reach.   

Zeeman-Sisyphus Deceleration of Polyatomic Molecules

Recent efforts to laser cool molecules have generally relied upon radiative slowing. This technique has been effective in loading MOTs of molecular species that can efficiently optical cycle; that is, species that can scatter ~10,000 photons. A method that uses many fewer photon scatters would enable slowing of a much larger class of molecules. We present here a Zeeman-Sisyphus (Z-S) slower that uses large magnetic fields and only a few scattered photons. CaOH is slowed and efficiently accumulated at velocities below 10 m/s, while using an average of ~7 scattered photons per molecule. This approach can be applied to essentially any paramagnetic molecule. We also report progress on Z-S slowing YbOH, a species of interest for precision measurement of the permanent electron electric dipole moment.