We report the experimental realization of a conveyor-belt magneto-optical trap for calcium monofluoride (CaF) molecules. The obtained highly-compressed cloud has a mean radius of 64(5)
μm and a peak number density of
3.6(5)×1010 cm
−3, a 600-fold increase over the conventional red-detuned MOTs of CaF, and the densest molecular MOT observed to date. Subsequent loading of these molecules into an optical dipole trap yields up to
2.6×104 trapped molecules at a temperature of 14(2)
μK with a peak phase-space density of
∼2.4×10−6. This opens new possibilities for a range of applications utilizing high-density, optically trapped ultracold molecules.
Abstract Laser cooling is used to produce ultracold atoms and molecules for quantum science and precision measurement applications. Molecules are more challenging to cool than atoms due to their vibrational and rotational internal degrees of freedom. Molecular rotations lead to the use of type-II transitions ( F ⩾ F ′ ) for magneto-optical trapping (MOT). When typical red detuned light frequencies are applied to these transitions, sub-Doppler heating is induced, resulting in higher temperatures and larger molecular cloud sizes than realized with the type-I MOTs most often used with atoms. To improve type-II MOTs, Jarvis et al (2018 Phys. Rev. Lett. 120 083201) proposed a blue-detuned MOT to be applied after initial cooling and capture with a red-detuned MOT. This was successfully implemented (Burau et al 2023 Phys. Rev. Lett. 130 193401; Jorapur et al 2024 Phys. Rev. Lett. 132 163403; Li et al 2024 Phys. Rev. Lett. 132 233402), realizing colder and denser molecular samples. Very recently, Hallas et al (2024 arXiv:2404.03636) demonstrated a blue-detuned MOT with a ‘1+2’ configuration that resulted in even stronger compression of the molecular cloud. Here, we describe and characterize theoretically the conveyor-belt mechanism that underlies this observed enhanced compression. We perform numerical simulations of the conveyor-belt mechanism using both stochastic Schrödinger equation and optical Bloch equation approaches. We investigate the conveyor-belt MOT characteristics in relation to laser parameters, g-factors and the structure of the molecule, and find that conveyor-belt trapping should be applicable to a wide range of laser-coolable molecules.
We report the realization of a blue-detuned magneto-optical trap (BDM) of BaF molecules. The (1 + 1) type BDM and (1 + 2) type conveyor-belt MOT are explored. While the (1+1) BDM provides only weak trapping force, the conveyor-belt MOT significantly compresses the molecular cloud, achieving a radius of 320(20) μm, a temperature of 240(60) μK, and a peak density of 1.3{\times}10^7 cm^{-3}, representing a significant improvement over the red MOT. Interestingly, the conveyor-belt MOT of BaF exhibits a large capture velocity, and the loading efficiency from red MOT reaches near unity even without gray molasses. We confirm this by directly loading slowed molecules into the conveyor-belt MOT.
We report the realization of a blue-detuned magneto-optical trap (BDM) of BaF molecules. The (1+1) type BDM and (1+2) type conveyor-belt MOT (CB-MOT) are explored. While the (1+1) BDM provides only a weak trapping force, the CB-MOT significantly compresses the molecular cloud, achieving a Gaussian rms width of 316 μm, a temperature of 240 μK, and a peak number density of 2.4×10^{7} cm^{-3}. This performance marks two orders of magnitude improvement over red-detuned MOTs. More importantly, we demonstrate the efficient direct loading of the CB-MOT from a slowed molecular beam, surpassing the loading efficiency of the standard dual-frequency MOT. This surprising result is attributed to the dipole-force-dominated trapping mechanism, wherein the capture velocity increases continuously with laser power, unimpeded by the saturation limit of photon scattering. This method is particularly advantageous for laser cooling and trapping challenging molecular species with large mass, small photon recoil, and narrow linewidths.
A key method to produce trapped and laser-cooled molecules is the magneto-optical trap (MOT), which is conventionally created using light red detuned from an optical transition. In this work, we report a MOT for CaF molecules created using blue-detuned light. The blue-detuned MOT (BDM) achieves temperatures well below the Doppler limit and provides the highest densities and phase-space densities reported to date in CaF MOTs. Our results suggest that BDMs are likely achievable in many relatively light molecules including polyatomic ones, but our measurements suggest that BDMs will be challenging to realize in substantially heavier molecules due to sub-mK trap depths. In addition to record temperatures and densities, we find that the BDM substantially simplifies and enhances the loading of molecules into optical tweezer arrays, which are a promising platform for quantum simulation and quantum information processing. Notably, the BDM reduces molecular number requirements ninefold compared to a conventional red-detuned MOT, while not requiring additional hardware. Our work therefore substantially simplifies preparing large-scale molecular tweezer arrays, which are a novel platform for simulation of quantum many-body dynamics and quantum information processing with molecular qubits.
Ultracold polar molecules provide a powerful platform for quantum science because they combine long-lived rotational states, rich internal structure, and strong electric dipole-dipole interactions. Optical tweezer arrays provide a pristine way of controlling these molecules at single-particle level, in which molecules can be individually trapped, rearranged into programmable geometries, coherently manipulated by optical and microwave fields, and entangled through their native dipolar interactions. This dissertation describes the development of geometric control as a resource for improving interaction coherence and high fidelity of quantum entanglement between directly laser-cooled calcium monofluoride (CaF) molecules in optical tweezers. A central challenge in molecular tweezer systems is that dipolar interactions are intrinsically sensitive to the relative position of the molecules. Thermal motion in the traps, especially along the weakly confined axial direction, causes molecules to sample a distribution of intermolecular separations, degrading dipolar coherence and limiting entanglement fidelity. We show in this work that the interaction itself can be made robust by controlling its geometry. The experimental work begins with improvements to the production and optical trapping of dense CaF samples. We realize a blue-detuned ``conveyor-belt" magneto-optical trap (MOT) that compresses laser-cooled CaF to a mean cloud radius of
64(5) μm and a peak density of
3.6(5)×1010 cm−3, a 600-fold density increase over conventional red-detuned CaF MOTs. Loading from this compressed source into an optical dipole trap yields up to
2.6×104 trapped molecules at
14(2) μK, with a peak phase-space density of approximately
2.4×10−6. This high-density source improves the transfer of laser-cooled molecules into conservative optical traps and provides the starting point for loading and manipulating single molecules in a hybrid SLM-AOD optical tweezer array. Using this platform, we prepare CaF molecules in selected rotational and hyperfine states to encode a rotational qubiyt. We show how to preserve the single-qubit coherence with a combination of techniques, including ``magic" polarization condition and microwave dynamical decoupling, as well as reduce motional quantum state of the trapped molecule with Raman sideband cooling. We then characterize coherent dipolar spin exchange between pairs of molecules and identify thermal motion and relative trap-center offsets as dominant sources of interaction dephasing. To suppress these effects, we engineer the three-dimensional interaction geometry between two individually trapped molecules. More specifically, we show that one can tune the \textit{static} geometric of the dipolar interaction to configurations where sensitivity to thermal fluctuations is strongly reduced while retaining a finite interaction strength. Furthermore, we show that a molecule can be moved during the interaction while preserving the coherence of ongoing entangling operations. We leverage programmable molecular motion to refocus dephasing arising from relative positional jitter of the optical tweezers. Combining static geometric insensitivity, programmable molecular motion, and partial Raman sideband cooling, we generate two-molecule entanglement with a Bell-state fidelity of
F=0.976−0.011+0.008 in directly laser-cooled molecules. Crucially, the high-fidelity molecular entanglement here is performed without ground-state preparation. Rather than relying on stringent cooling or species-specific trapping conditions, as in prior work, the methods demonstrated here are instead based on dynamic and static geometric control of interacting molecules and are therefore broadly applicable across molecular platforms. These results extend moving optical tweezers beyond assembly and rearrangement, opening routes to robust two-qubit gates, many-body dynamics, and dynamically programmable networks of interaction.