We present a practical roadmap to achieve optical cycling and laser cooling of asymmetric top molecules (ATMs).Our theoretical analysis describes how reduced molecular symmetry, as compared to diatomic and symmetric nonlinear molecules, plays a role in photon scattering.We present methods to circumvent limitations on rapid photon cycling in these systems.We calculate vibrational branching ratios for a diverse set of asymmetric top molecules and find that many species within a broad class of molecules can be effectively cooled with a manageable number of lasers.We also describe methods to achieve rotationally closed optical cycles in ATMs.Despite significant structural complexity, laser cooling can be made effective by using extensions of the current techniques for linear molecules.Potential scientific impacts of laser-cooled ATMs span frontiers in controlled chemistry, quantum simulation, and searches for physics beyond the Standard Model.
We present a practical roadmap to achieve optical cycling and laser cooling of asymmetric top molecules (ATMs).Our theoretical analysis describes how reduced molecular symmetry, as compared to diatomic and symmetric nonlinear molecules, plays a role in photon scattering.We present methods to circumvent limitations on rapid photon cycling in these systems.We calculate vibrational branching ratios for a diverse set of asymmetric top molecules and find that many species within a broad class of molecules can be effectively cooled with a manageable number of lasers.We also describe methods to achieve rotationally closed optical cycles in ATMs.Despite significant structural complexity, laser cooling can be made effective by using extensions of the current techniques for linear molecules.Potential scientific impacts of laser-cooled ATMs span frontiers in controlled chemistry, quantum simulation, and searches for physics beyond the Standard Model.
Rapid and repeated photon cycling has enabled precision metrology and the development of quantum information systems using atoms and simple molecules. Extending optical cycling to structurally complex molecules would provide new capabilities in these areas, as well as in ultracold chemistry. Increased molecular complexity, however, makes realizing closed optical transitions more difficult. Building on already established strong optical cycling of diatomic, linear triatomic, and symmetric top molecules, recent work has pointed the way to cycling of larger molecules, including phenoxides. The paradigm for these systems is an optical cycling center bonded to a molecular ligand. Theory has suggested that cycling may be extended to even larger ligands, like naphthalene, pyrene, and coronene. Herein, we study optical excitation and fluorescent vibrational branching of CaO-[Formula: see text], SrO-[Formula: see text], and CaO-[Formula: see text] and find only weak decay to excited vibrational states, indicating a promising path to full quantum control and laser cooling of large arene-based molecules.
We realize two-dimensional magnetically-assisted Sisyphus laser cooling of an asymmetric top molecule (ATM), calcium monoamide (CaNH2). Vibrational state closure is achieved with 41.1±6.3 photons scatters using optical pumping of the X[31] state. Photon-cycling measurements show good agreement with branching ratios determined by dispersed fluorescence spectroscopy. Rotational closure is maintained by driving the X[111]→A[000] transition. The observed absence of additional state leakage channels broadens the scope of molecular laser cooling to include ATMs, which are the most general geometric class of molecules and possess the richest internal structure. Future applications of quantum controlled ATMs include new quantum information platforms and searches for physics beyond the Standard Model.
In this dissertation, I present studies of molecules for uses in quantum science ranging from quantum computing and ultracold collisions to controlling organic-inspired molecular species. Starting with a diatomic molecule, calcium monofluoride, we developed an optical tweezer platform for use in quantum computing and demonstrated rotational coherence times significantly longer than measured dipole-enabled gate times. Using the Tweezer platform, we studied ultracold collisions of exactly two molecules and exerted full quantum control over the internal structure of the molecules. The collisions resulted in a rapid loss from the tweezers, leading us to develop a microwave shield to prevent these lossy collisions. The shielding scheme enhanced the rate of elastic collisions enabling the demonstration of forced evaporative cooling. To explore the possible use of larger molecules for quantum science, we studied the rotational structure of an asymmetric top and aromatic molecule, calcium monophenoxide, with the intent of identifying a path toward laser cooling and trapping. We cycled several photons, but theory predicted many more. After eliminating several decay paths, the key loss remained unknown. Applying the same methods to CaNH2, a smaller asymmetric top molecule with similar spatial symmetry, we performed laser cooling. Using CaNH2, we demonstrated photon cycling, and then observed Sisyphus cooling and heating features for the first time using an asymmetric top molecule. This work lays the foundation for future laser-cooling of organic-inspired molecules in an optical tweezer array for applications ranging from quantum computing to quantum chemistry, and precision measurement.
Cold and ultracold molecules are highly desirable for a diverse range of applications in physics and chemistry such as precision measurements, tests of fundamental physics, quantum simulation and information processing, quantum chemistry, and the physics of strongly correlated quantum matter. Laser cooling is usually infeasible in molecules because their rotational and vibrational transitions make is difficult to come up with a closed scattering cycle. Recently, a narrow range of diatomic molecules, one of which is CaF, has been shown to possess a convenient electronic structure and a highly-diagonal Franck-Condon matrix and thus be amenable to laser cooling. This thesis describes experiments on laser cooling of CaF radicals produced in a supersonic source. We first investigate the increased fluorescence when multi-frequency resonant light excites the molecules from the four hyperfine levels of the ground X²∑+(N = 1, v = 0) state to the first excited A²π½ (J’ = 1=2; v’ = 0) state. The number of photons scattered per molecule increases significantly from one or two in the single frequency case to more than 50 before the molecules get pumped into the X²∑+(N = 1; v = 1) state. We demonstrate laser cooling and slowing of CaF using counter-propagating laser light which causes the molecules to scatter more than a thousand photons on the X (N = 1, v = 0, 1) <->A (J’ = 1=2; v’ = 0) transition. The effect of the laser cooling is to slow a group of molecules from 600 ms-1 to about 580 ms-1 and to narrow their velocity distribution from an initial temperature of 3 K down to 300 mK. In addition, chirping the frequency of the cooling light to keep it on resonance with the decelerating molecules doubles the deceleration and further compresses the velocity distribution. The effect of the laser cooling is limited by the optical pumping of molecules in the X (N = 1, v = 2) state.
Abstract The extension of laser cooling and trapping techniques from atoms to molecules is currently the subject of intense and growing interest. Molecular laser cooling and trapping offers a general technique applicable to a variety of molecules with different properties and internal structures and is well-suited to the growing list of proposed applications for ultracold molecules. The recent development of optical cycling schemes for specific molecular species now allow each molecule to scatter up to ∼10 6 photons. This has proven sufficient for multiple groups to laser cool and slow molecular beams and, notably, capture molecules in magneto-optical traps (MOTs) and cool them to ultracold temperatures. MOTs are the workhorse technique for atomic physics in the ultracold regime and today’s molecular MOTs represent a powerful starting point for the study and manipulation of ultracold molecular gases. This Topical Review presents a summary of the progress made in this new and emerging field.
Laser cycling of resonances can remove entropy from a system via spontaneously emitted photons, with electronic resonances providing the fastest cooling timescales because of their rapid relaxation rates. Although atoms are routinely laser cooled, even simple molecules pose two interrelated challenges for cooling: every populated rotational-vibrational state requires a different laser frequency, and electronic relaxation generally excites vibrations. Here, we cool trapped AlH+ molecules to their ground rotational-vibrational quantum state using an electronically-exciting broadband laser to simultaneously drive cooling resonances from many different rotational levels. Undesired vibrational excitation is avoided because of vibrational-electronic decoupling in AlH+. We demonstrate rotational cooling on the 140(20) ms timescale from room temperature to 3.8(+0.9/-0.3) K, with the ground state population increasing from ~3% to 95.4(+1.3/-2.0) %. This cooling technique could be applied to several other neutral and charged molecular species useful for quantum information processing, ultracold chemistry applications, and precision tests of fundamental symmetries.