Session S04: Cooling Techniques for Molecules and Atoms

Progress towards a cold, optically trapped cloud of CaH molecules

Recent progress in the field of laser cooling and trapping of molecules has led to 3D magneto-optical traps (MOTs) of SrF, CaF, YO, and CaOH, and Sisyphus cooling of SrOH, YbOH, BaH, and CaOCH3. Here we present our progress towards laser cooling and trapping of another alkaline-earth monohydride species, CaH. First, we present experimental results on transverse Sisyphus cooling of a cold beam of CaH, and establish that a scattering rate of ~10⁶ s^-1 is achievable. We obtain good agreement with state-of-the-art optical Bloch equations and Monte Carlo simulations for both Doppler and Sisyphus-like forces. Next, we present our progress towards longitudinal white-light slowing of the molecular beam. With the application of the first three vibrational repumps, we expect to slow the molecular beam forward velocity to within the capture range of a MOT without significant loss to higher vibrational states. The ultracold CaH molecules created in this way could serve as a precursor to ultracold hydrogen. We will discuss potential dissociation pathways, including one-photon photodissociation and STIRAP. This work sets the stage for a MOT of the first alkaline-earth monohydride, and, eventually, a dilute cloud of trapped atomic hydrogen.   

Laser-Cooling and Trapping of CaF in a Dynamical Repulsive Optical Trap

In this talk, we present results on trapping and laser-cooling of CaF molecules in a dynamically tunable repulsive ring-shaped optical trap. We characterize the dependencies of molecular loss rates on temperature and trap barrier height and find strongly suppressed off-resonant photon scattering, indicating suppressed AC Stark shifts within the ring trap. In addition, we explore in-trap Λ-cooling and find it to be as effective as that in free space. By combining in-trap laser cooing with dynamical compression of the trap, we demonstrate rapid and efficient transfer from a MOT into a conventional attractive optical trap.  This method to enhance transfer efficiencies could be applicable to a wide class of molecules, and could be used to increase the loading probabilities of molecular optical tweezer arrays.   

Chemistry-driven buffer gas cooling

This work presents a first principle study of the role of chemical reactions in buffer gas cooling efficiency towards generating more efficient cold and intense molecular beams. In particular, we study the reactions producing AlF/CaF from two typical fluoride-donor gases: NF₃ and SF₆, based on ab initio molecular dynamics (AIMD) simulations. In addition, reaction models are constructed to analyze the intermediate steps and main branching processes. Our results will help establish the basis for more precise control and design of buffer gas sources, which are the cornerstone for having a high dense ensemble of ultracold molecules.   

Trapped ions in an ultracold gas: buffer gas cooling and chemical reactions

Hybrid ion-atom systems combine the benefits of a single, well-controlled ion with those of a many-body quantum gas, offering prospects for quantum simulation, ultracold chemistry, and charged impurity physics. In this talk I will present recent experiments, where we observed collisions between Li₂ dimers and a single Yb+ ion, leading to the formation of a LiYb+ molecular ion [1]. Our results show a new method for molecular ion formation and for probing an atom cloud for small quantities of dimers. Furthermore, I will present numerical simulations for cooling our system further into the quantum regime [2]. Ion trap parameter optimization will allow buffer gas cooling to energies competitive with some sub-Doppler laser cooling techniques, reaching atom-ion collision energies twice as cold as previous experiments [3].

[1] H. Hirzler et al. arXiv:2110.14475 (2021)

[2] E. Trimby et al. arXiv:2109.15195 (2021)

[3] T. Feldker et al. Nat. Phys. 4, 16 (2020)   

Towards single-laser cooling and trapping of transition metal elements using ablation beam sources

Quantum degenerate gases of transition metals are a promising platform for quantum simulation and precision measurement. Previous work suggests many of these atoms can be laser cooled out of a metastable excited state. We demonstrate a method for producing atomic beams of metastable Ti without optical pumping via laser ablation of a Ti sample into a room temperature buffer gas. We measure the inelastic collision rate between the metastable Ti atoms and buffer gas atoms to be on the order of 5000 times smaller than the elastic collision rate, enabling extraction of a metastable atomic beam from a buffer gas source. This lays out a path for laser cooling and trapping of several transition metal elements requiring only an ablation laser and a single laser for Doppler cooling. We measure metastable atom densities in our ablation cell of approximately 1e8 atoms/cm^3. Based simulations of a ablation-fed MOT, we expect to load on the order of 1e11 atom/sec in a 3D MOT.   

A High Intensity Cold Atomic Beam

Continuous, high intensity, cold atomic beams are important for precision measurement and atom optics applications. Laser cooling and buffer gas-based methods are already well developed, but new methods that could potentially provide brighter beams are still of interest. We present our work on a new approach based on continuous post-nozzle injection of lithium atoms into a supersonic helium jet. We reduce the jet velocity to 210 m/s by cryogenically cooling the helium nozzle, and extract the lithium atoms from the jet with magnetic focusing. The focused lithium beam has a width of approximately 5 mm, a peak intensity of  8×10¹² cm^-2 s^-1 , and a temperature of less than 20 mK in the moving frame. Ongoing efforts to increase the beam brightness will also be discussed.   

Simultaneous Sub-Doppler laser cooling of 6Li and 7Li isotopes

Ultracold atoms provide an ideal platform for probing mixtures of quantum gases. A successful example of such systems is the Bose-Fermi superfluid mixture of Lithium 6 and Lithium 7 isotopes [1, 2, 3]. However, the preparation of such systems comes with a significant technical overhead. Here, we propose a simplification of the preparation protocol by demonstrating the simultaneous sub-Doppler cooling of Lithium 6 and Lithium 7. The method is based on gray molasses and operates with positive detuning on the D1 atomic transition for the respective isotopes. We circumvent the proximity of the D1 transition of Lithium 7 to the D2 transition of Lithium 6, by applying an alternating laser beam pulse sequence on both isotopes. We obtain temperatures of 190 µK and 70 µK and atom numbers of 5×10⁷ and 1×10⁸ for Lithium 6 and Lithium 7, respectively. This Bose-Fermi mixture is a good starting point for  loading into an optical dipole trap and evaporative cooling to quantum degeneracy.

 

[1]          I. Ferrier-Barbut et al., Science 345, 1035 (2014)

[2]          M. Delahaye et al., PRL 115, 265303 (2015)

[3]          S. Laurent et al., PRL 118, 103403 (2017)   

Optical deceleration of atomic hydrogen

We will discuss a method for laser slowing of atomic hydrogen.  The technique proceeds by first exciting a cryogenic beam of atomic hydrogen to the metastable 2S state.  Atoms from this beam are then loaded into an optical lattice which is tuned near the 2S-4P transition.  The lattice is moving at approximately the mean velocity of the beam and is capable of trapping atoms with velocities +-10 m/s of this velocity.  The lattice is then rapidly decelerated using electro-optics to slow the trapped atoms.  With this technique, we have observed accelerations of -10^9 m/s^2, and total velocity changes of about -15 m/s.   

Laser cooling of indium

To date nearly all quantum degenerate gas experiments have been based on alkali, alkaline earth, or lanthanide atoms. Meanwhile, most of the periodic table remains unexplored in the quantum degenerate regime. One such unexplored class of atoms are the triels (periodic table Main Group III). Like alkaline earths, some triel atoms have narrow-linewidth electronic transitions at wavelengths amenable to stable laser technology; however, unlike alkaline earths, triels also have alkali-like ground state magnetic Feshbach resonances. Therefore, triel atoms could be probed with optical clock resolution while offering the many-body control of alkali atoms.

In this talk we report the first Zeeman slower of a triel atom, namely indium. Using a cycling transition for cooling that driven from a metastable state, we observe a bright slowed beam at 70 m/s. We also present our progress towards magneto-optical trapping of indium.   

Withdrawn

Feedback cooling based on real-time quantum measurements could be used to create Bose-Einstein condensates (BECs) with larger atom numbers compared with traditional evaporative cooling. Previous theoretical works have shown that for an almost pure condensate, heating caused by a continuous dispersive optical measurement can be overcome via feedback to the BEC’s confining trapping potentials, enabling cooling close to the ground state [Phys. Rev. A 82, 043632, 2010]. Here we extend this model and improve upon this feedback protocol by (1) considering a stroboscopic measurement scheme and (2) adaptively choosing measurement parameters, with the aim of decreasing measurement backaction (compared with the continuous, fixed measurement case) whilst still obtaining the information needed for efficient real-time feedback cooling. We will present the details of the adaptive, stroboscopic measurement-based feedback control and show it delivers improved cooling performance over a continuous measurement protocol with fixed parameters. Our results demonstrate the importance of optimizing the information-backaction trade-off for efficient feedback control, both within the particular context of feedback cooling a BEC and for quantum systems more generally.