Session Q09: Cold and Ultracold Molecules II

An Ultracold Ensemble of NaCs Ground State Molecules

We report on the creation of sodium-cesium (NaCs) molecules in their rovibrational ground state, assembled from ultracold clouds of Na and Cs atoms [1]. We present one- and two-photon studies of electronically excited molecular states. We use one of these states to transfer NaCs Feshbach molecules [2] to the ground state via stimulated Raman adiabatic passage (STIRAP) and observe 70% transfer efficiency.  In the ground state, we measure the two-body loss rate, which approaches the unitary limit. In addition, we measure the DC Stark shift of the ground state with fields up to 2.1 kV/cm, corresponding to dipole moments of 2.6 Debye for NaCs.    

Microwave Studies of Ultracold NaCs Ground State Molecules

We report on microwave studies on sodium-cesium (NaCs) molecules in their rovibrational ground state. We perform microwave spectroscopy and identify transitions to the first rotationally excited state.  The coupling between rotational motion and hyperfine angular momentum is weak in NaCs, leading to a clean rotational structure. We also observe exceptionally strong Rabi coupling, making NaCs an attractive candidate for microwave shielding against collisional losses. This may allow for direct evaporation of NaCs molecules and provide a path to a Bose-Einstein condensate of dipolar ground state molecules.   

Towards magnetoassociation of the ultracold open-shell RbSr molecule

Ultracold dipolar molecules offer an ideal platform for investigations in the fields of quantum simulation, precision measurement and quantum chemistry. The range of possibilities offered by ultracold molecules could be substantially extended by employing the previously unexplored class of open-shell molecules like RbSr. Thanks to its unpaired valence electron RbSr possesses both a magnetic and an electric dipole moment offering an additional level of control with new exciting possibilities for quantum simulations.

Here we present our novel quantum-engineering approach towards the production of ultracold open-shell RbSr molecules that utilizes an ultra-narrow magnetic Feshbach resonance at 1311 G. Due to its extremely low relative width on the order of 10^-6, this is a serious experimental challenge.

Our state-of-the-art dual stabilization approach to the control of magnetic fields now enables us to achieve a relative magnetic field stability at the ppm level at fields beyond 1000 G. As a real-life benchmark of the capabilities of our system, we efficiently produce Rb Feshbach molecules on an ultranarrow 1.3-mG-wide resonance at 911.7 G. This is the first time that Feshbach molecule association is successful for such a narrow relative width and it is a crucial step towards the ultimate goal of magnetoassociating RbSr molecules.   

Coherent synthesis of molecules in atomic Bose-Einstein condensates Zhendong Zhang, Kai-Xuan Yao, Shu Nagata, Cheng Chin

Chemical reactions in the quantum degenerate regime can be drastically different from that in a normal thermal gas. Quantum statistics and collective behavior can dominate the reaction kinetics once the reactants are prepared close to their many-body ground state. Here we report coherent molecule formation processes in atomic Bose-Einstein condensates (BECs) near a g-wave Feshbach resonance. The molecule formation rates sharply transition from the values determined by thermal collisions between atoms to those in the degeneracy regime where the wave nature of atoms dominates, as we reduce temperature to below the critical temperature. Starting from an almost pure atomic BEC, the number of produced molecules shows coherent oscillatory evolution with the oscillation frequency determined by both the molecular binding energy and the atomic density. We further enhance the amplitude of the molecule number oscillation by periodically modulating the molecular binding energy. Our observation demonstrates collective chemical reactions in a strongly interacting atomic BEC.   

Detection of long-lived complexes in ultracold atom-molecule collisions

Though intermediate complexes formed through molecular collisions are integral to understanding chemical reactions and making stable molecular gasses, their short lifetimes at even cryogenic temperatures preclude detailed study. In the ultracold regime, however, such complexes have been observed to live substantially longer, providing new opportunities to study molecular scattering and reactions. Here, we investigate ultracold collisions between ^{40}K^{87}Rb molecules and ^{87}Rb atoms, a collision for which chemical reactions are energetically forbidden. The KRb_2^* intermediate complexes formed through atom-molecule collisions are detected directly via ionization imaging, and we show that a 1064 nm laser source used for optical trapping of the sample can efficiently deplete the complex population via photo-excitation. Our measured complex lifetime is ~10^5 times longer than recent estimates based on Rice-Ramsperger-Kassel-Marcus statistical theory. These experimental results call for new insight to explain such a dramatic discrepancy.   

Observation of magnetically tunable resonances in ultracold 40K87Rb + 87Rb mixtures

Scattering resonances between ultracold atoms and molecules offer new opportunities to study ultracold collisions and chemistry. In this work, we search for magnetically tunable Feshbach resonances in ultracold mixtures of ⁴⁰K⁸⁷Rb + ⁸⁷Rb across a broad range of magnetic fields with KRb molecules prepared in four different hyperfine levels of the rovibrational ground state. We observe various magnetic field-dependent enhanced loss features, some of which are sharp while others are broad and may reflect a near continuum of resonances. By combining measurements of the loss rate with direct detection of the intermediate complexes formed in these collisions, our experimental results open new questions in the loss mechanism of ultracold collisions involving molecules and atoms.   

Highly polar molecules consisting of a silver atom interacting with an alkali-metal or alkaline-earth-metal atom

We theoretically investigated the properties of highly polar diatomic molecules containing ²S-state transition-metal atoms and proposed these molecules possessing unprecedentedly large permanent electric dipole moments in their ground states for a new generation of ultracold physics and chemistry experiments. We calculated potential energy curves, permanent electric dipole moments, spectroscopic constants, and leading long-range dispersion-interaction coefficients for molecules consisting of either a Cu and Ag atom interacting with an alkali-metal (Li, Na, K, Rb, Cs, Fr) or alkaline-earth-metal (Be, Mg, Ca, Sr, Ba, Ra, Yb) atom. We used ab initio electronic structure methods, such as the coupled cluster and configuration interaction ones, with large Gaussian basis sets and small-core relativistic energy-consistent pseudopotentials. We predicted that the studied molecules in the ground electronic state are strongly bound with highly polarized covalent or ionic bonds resulting in very large permanent electric dipole moments. We found that highly excited vibrational levels have maximal electric dipole moments, e.g., exceeding 13 debye for CsAg and 6 debye for BaAg. The studied molecules may find application in ultracold dipolar many-body physics, controlled chemistry, or precision measurement experiments, which are planned in Warsaw.   

Magneto-optical trapping and sub-Doppler cooling of a polyatomic molecule

We demonstrate magneto-optical trapping (MOT) of a polyatomic molecule, calcium monohydroxide (CaOH). Compared to atoms and diatomic molecules, polyatomic molecules have unique rotational and vibrational degrees of freedom that, while appealing for applications in quantum science, complicate the task of forming a photon cycling scheme sufficient to perform laser cooling. By addressing these degrees of freedom with 11 repumping lasers, we scatter >10,000 photons on average per molecule before loss to dark states occurs. This enables radiative slowing of a buffer gas-cooled beam of CaOH, followed by trapping, cooling, and compression in a radio-frequency (rf) MOT. We further cool the CaOH molecular cloud with a blue-detuned molasses to temperatures near 100 μK. These results represent a starting point for optical trapping of CaOH, e.g., in arrays of optical tweezers.   

Collectively Enhanced Molecule Formation in a Cavity Using Dissipative Effects

Ultracold molecules provide a controlled environment to study quantum many-body effects in physics and chemistry but creating samples with high phase-space densities has proven challenging. We propose a mechanism to realize high-yield formation of ultracold ground state molecules. We consider an ensemble of atom pairs trapped inside an optical cavity. The atom pairs are continuously excited by a laser, followed by a collective decay into the molecular ground state induced by a coupling to the lossy cavity mode. We adiabatically eliminate the excited states and the cavity mode and derive a master equation which describes purely dissipative population transfer of initial atom pairs to the molecular ground state. The ground state yield can be improved by simply increasing the number of initial atom pairs, however at the cost of a slowdown of the transfer. We identify polariton formation as the source of this slowdown, and discuss how to mitigate it by tuning in resonance with the polaritons. We study realistic experimental setups, where our method can overcome efficiencies of state-of-the-art association schemes. This opens up collective light matter interactions as a tool for quantum state engineering, enhanced molecule formation, collective dynamics, and cavity mediated chemistry.   

Metal-ligand engineering for molecular optical cycling

State-controlled polyatomic molecules are promising tools for quantum science, metrology, and studies of fundamental physics and chemistry. The experimental power of polyatomic systems arises primarily from their rich internal structure, which can be leveraged to realize features such as high polarizability and large dipole moments for robust long-range interactions and high-sensitivity metrology, co-magnetometer states and tunable EM sensitivity for error protection, as well as quasi-closed photon cycling channels for enhanced optical control and cooling. Recently, polyatomic molecules of increasing geometric complexity have been found to be compatible with optical cycling and control, and several species have been proposed for use in precision measurement and quantum science. In this abstract, we will discuss efforts towards rational design of new polyatomic optical cycling centers (OCC) using simple bonding and chemical concepts. We will present ab initio results for designing optical cycling properties using novel metal-ligand motifs, such as systems with multi-electron OCCs, as well as higher-than-single OCC-ligand bond order. Our results point to diverse possibilities for designing new, optically controlled polyatomic quantum sensors and bits in previously unexplored chemical space.