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.
We propose and theoretically analyze methods for quantum many-body control through geometric reshaping of molecular tweezer arrays. Dynamic rearrangement during entanglement is readily available due to the extended coherence times of molecular rotational qubits. We show how motional dephasing can be suppressed and enhanced spin squeezing can be achieved in an actively rearranged short-range XY model. We also analyze in detail a specific static geometry that significantly suppresses decoherence. These general methods as applied to programmable quantum systems offer robust control modalities that are well suited to molecules.
Ultracold molecules are attractive candidates for various applications including ultracold chemistry, quantum information processing, quantum simulation and searches for physics beyond the standard model. Combined with optical tweezer techniques, an optical tweezer array of ultracold polar molecules holds promise for versatile quantum applications. Their long-lived molecular rotational states form robust qubits, while the long-range dipolar interaction between molecules provides quantum entanglement. In this thesis, we build a new generation experiment of optical tweezer array of ultracold CaF molecules. We optically transport the laser cooled and optically trapped molecules into a science glass cell using a hybrid method, where high numerical aperture optical access allows for tighter optical tweezers. In an array of these tight optical tweezers, we demonstrate dipolar spin-exchange interactions between single CaF molecules for the first time. We realize the spin-
21 quantum XY model by encoding an effective spin-
21 system into the rotational states of the molecules, and use it to generate a Bell state through an iSWAP operation. Conditioned on the verified existence of molecules in both tweezers at the end of the measurement, we obtain a Bell state fidelity of 0.89(6). Employing interleaved tweezer arrays, we demonstrate single site molecular addressability. To further improve the platform, cooling of the molecules to near the motional ground state is crucial for reducing various dephasings. We demonstrate Raman sideband cooling (RSC) of CaF molecules in optical tweezers to near their 3-D motional ground state, with a 3-D motional ground state probability of
54±18% of the molecules that survive the RSC. This paves the way to increase molecular coherence times in optical tweezers for robust quantum information processing and simulation applications in the near future.
Polar molecules are a promising platform for quantum-enhanced sensing and precision tests of fundamental physics, owing to their strong long-range dipolar interactions, broad sensitivity to electromagnetic fields, and sensitivity to potential physics beyond the Standard Model. However, the creation of metrologically useful entangled states in molecular systems has remained elusive. Here, we report the first observation of a class of metrologically useful entangled states - spin-squeezed states - in polar CaF molecules trapped in an optical tweezer array. The spin degree of freedom is encoded in rotational levels which are directly coupled by dipolar exchange interactions. By harnessing appropriate dynamical decoupling schemes we observe up to 3.0(3)dB of metrological gain, (2.2(3)dB without measurement correction) from direct exchange interactions. Using Floquet engineering, we further realize richer Hamiltonians that preserve spin squeezing while enabling the development of longer-range quantum correlations. Using site- and spin-resolved measurements we demonstrate that these entangled states enhance sensitivity to both homogeneous and spatially varying fields, and reveal strong non-classical correlations, including bipartite entanglement and Einstein-Podolsky-Rosen steering. Finally, we transfer the spin-squeezed states into long-lived and non-interacting hyperfine states, where the metrological enhancement persists for up to 100ms. Our results establish molecular optical tweezer arrays as a scalable platform for generating, controlling, characterizing, and storing entangled states of molecules, opening new opportunities for quantum-enhanced sensing and precision tests of fundamental physics.
Abstract Realizing quantum control and entanglement of particles is crucial for advancing both quantum technologies and fundamental science. Substantial developments in this domain have been achieved in a variety of systems 1–5 . In this context, ultracold polar molecules offer new and unique opportunities because of their more complex internal structure associated with vibration and rotation, coupled with the existence of long-range interactions 6,7 . However, the same properties make molecules highly sensitive to their environment 8–10 , affecting their coherence and utility in some applications. Here we show that by engineering an exceptionally controlled environment using rotationally magic 11,12 optical tweezers, we can achieve long-lived entanglement between pairs of molecules using detectable hertz-scale interactions. We prepare two-molecule Bell states with fidelity
0.924−0.016+0.013 0.92 4 − 0.016 + 0.013 , limited by detectable leakage errors. When correcting for these errors, the fidelity is
0.976−0.016+0.014 0.97 6 − 0.016 + 0.014 . We show that the second-scale entanglement lifetimes are limited solely by these errors, providing opportunities for research in quantum-enhanced metrology 7,13 , ultracold chemistry 14 and the use of rotational states in quantum simulation, quantum computation and as quantum memories. The extension of precise quantum control to complex molecular systems will enable their additional degrees of freedom to be exploited across many domains of quantum science 15–17 .
Polar molecules trapped in optical tweezer arrays offer a promising platform for quantum information processing, providing precise control and long-range interactions that enable high-fidelity gate operations. We investigate quantum error correction in this system and show the influence of underlying physical noise. A mapping is constructed from a molecular tweezer array onto a rotated surface code in which a single specification of the array, namely, which code qubits share a molecule and where those molecules are located, determines both the correlated erasure structure and the dipolar exchange graph. We compare molecular encodings with rotational qudit dimensions (D), two and four under heralded molecular loss, coherent dipolar exchange, and imperfect heralding. It is found that a D= 4 encoding with spatially dispersed pairing exhibits a finite distance crossing at approximately the same per molecule loss rate as D= 2, while using 48-49 % fewer molecules, at the cost of a 6-10 % increase in sub-threshold logical error. Fixing the encoding and varying only the spatial embedding produces substantially larger effects: pairing the two co-located qubits along a lattice direction yields a logical-sector asymmetry of approximately fifty times. Over the simulated distances (d = 5, 7, 9), the disfavoured sector shows little or no suppression of logical error with increasing code distance, whereas the favoured sector improves by a factor of 1.5-2.3. We also find that Pauli twirl of the exchange interaction overestimates the logical error rate, which we attribute to the excitation-conserving structure of the interaction. These results reveal that the spatial embedding of correlated loss units is an important design parameter for molecular architectures and that single-sector benchmarks may be insufficient when correlated loss has directional structure.