The present thesis focuses on developing a general method to trap and to cool molecules to ultralow temperatures (<1mK). The implemented opto-electrical Sisyphus cooling uses state-dependent potentials combined with optical pumping to achieve this goal. In a first experiment, a temperature reduction by a factor of 5 was achieved. A key part of the experiment is the realization of a novel design for an electric trap for molecules. This allowed molecules to be trapped with a lifetime of almost 30s.
The present thesis focuses on developing a general method to trap and to cool molecules to ultralow temperatures (<1mK). The implemented opto-electrical Sisyphus cooling uses state-dependent potentials combined with optical pumping to achieve this goal. In a first experiment, a temperature reduction by a factor of 5 was achieved. A key part of the experiment is the realization of a novel design for an electric trap for molecules. This allowed molecules to be trapped with a lifetime of almost 30s.
We present a study of Sisyphus cooling of molecules: the scattering of a single-photon remove a substantial amount of the molecular kinetic energy and an optical pumping step allow to repeat the process. A review of the produced cold molecules so far indicates that the method can be implemented for most of them, making it a promising method able to produce a large sample of molecules at sub-mK temperature. Considerations of the required experimental parameters, for instance the laser power and linewidth or the trap anisotropy and dimensionality, are given. Rate equations, as well as scattering and dipolar forces, are solved using Kinetic Monte Carlo methods for several lasers and several levels. For NH molecules, such detailed simulation predicts a 1000-fold temperature reduction and an increase of the phase space density by a factor of 10^7 . Even in the case of molecules with both low Franck-Condon coefficients and a non-closed pumping scheme, 60% of trapped molecules can be cooled from 100 mK to sub-mK temperature in few seconds. Additionally, these methods can be applied to continuously decelerate and cool a molecular beam
In this thesis, direct cooling of gaseous formaldehyde (H2CO) to the ultracold regime (T < 1 mK) is experimentally demonstrated. Employing optoelectrical Sisyphus cooling, a dissipative cooling method applicable to electrically trapped dipolar molecules, an ensemble of about 300,000 molecules with a temperature of around 400 µK is generated. More than 80% of these molecules populate a single rotational state. These results are used to implement a technique for high-resolution spectroscopy on electrically trapped molecules.
We present a study of Sisyphus cooling of molecules: The scattering of a single photon removes a substantial amount of the molecular kinetic energy and an optical pumping step allows one to repeat the process. A review of the produced cold molecules so far indicates that the method can be implemented for most of them, making it a promising method able to produce a large sample of molecules at sub-mK temperatures. Considerations of the required experimental parameters, for instance the laser power and linewidth or the trap anisotropy and dimensionality, are given. Rate equations, as well as scattering and dipolar forces, are solved using kinetic Monte Carlo methods for several lasers and several levels. For NH molecules, such detailed simulation predicts a 1000-fold temperature reduction and an increase of the phase-space density by a factor of 107. Even in the case of molecules with both low Franck-Condon coefficients and a nonclosed pumping scheme, 60% of trapped molecules can be cooled from 100 mK to sub-mK temperatures in a few seconds. Additionally, these methods can be applied to continuously decelerate and cool a molecular beam.
Rapid progress in cooling and trapping of molecules has enabled first experiments on high-resolution spectroscopy of trapped diatomic molecules, promising unprecedented precision. Extending this work to polyatomic molecules provides unique opportunities due to more complex geometries and additional internal degrees of freedom. Here, this is achieved by combining a homogeneous-field microstructured electric trap, rotational transitions with minimal Stark broadening at a"magic" offset electric field, and optoelectrical Sisyphus cooling of molecules to the low millikelvin temperature regime. We thereby reduce Stark broadening on the J=5←4 (K=3) transition of formaldehyde at 364 GHz to well below 1 kHz, observe Doppler-limited linewidths down to 3.8 kHz, and determine the magic-field line position with an uncertainty below 100 Hz. Our approach opens a multitude of possibilities for investigating diverse polyatomic molecule species.
Polyatomic molecules offer many applications in quantum science once the wealth of internal and external degrees of freedom can be controlled. A widely applicable approach for controlling the rotational states of trapped polyatomic molecules is presented in this thesis. It includes the development of a rotational-state detection method, rotational-state cooling via optical pumping, and the preparation of a motionally cold (~30 mK) ensemble of 10 6 trapped CH 3 F molecules in a single rotational state.
Polyatomic molecules contain diverse structures – including rotational and vibrational degrees of freedom, nuclear and electronic spins, and electric dipole moments – that make them promising for a range of quantum science applications. These include quantum information science, quantum simulation, studies of ultracold collisions and ultracold chemistry, and precision searches for physics beyond the Standard Model. However, maximizing the potential of polyatomic molecules for these applications requires them to be cooled to ultracold temperatures ( mK), trapped in three dimensions, and controlled at the single quantum state level. Significant progress has been made with diatomic molecules over the last fifteen years, but the increased complexity of polyatomic species (defined as molecules containing more than two atoms) makes them more challenging to control at the same level. In this thesis, we describe our work bringing a linear triatomic molecule, calcium monohydroxide (CaOH), into the quantum regime. We demonstrate laser cooling of CaOH molecules to microkelvin temperatures, confinement in a magneto-optical trap (MOT), and loading of conservative optical dipole traps and optical tweezer arrays. We characterize the lifetime of low-lying vibrational states of CaOH in optical traps, including the vibrational bending mode, whose parity-doublet states are a key resource for many applications. Next, we develop techniques for preparing CaOH molecules in a single internal quantum state and for coherently manipulating the internal state with microwave and radio-frequency fields. We also show that we can nondestructively and state-selectively detect trapped CaOH molecules, including single molecules in optical tweezers. We demonstrate the applicability of these tools to applications including precision measurements, quantum information science, and ultracold collisions. We establish a method for future CP-violating physics searches with optically trapped polyatomic molecules, by identifying states that are sensitive to the electron electric dipole moment (eEDM) and showing that these states have long coherence times in CaOH. We identify potential qubit states in the CaOH bending mode and show that they can be coherently controlled at the single-molecule level in an optical tweezer array. Finally, we observe and characterize single quantum-state-controlled collisions between ultracold CaOH molecules, and identify states in the bending mode that could potentially be used for evaporative cooling to quantum degeneracy. Compared to linear triatomic molecules, nonlinear molecules contain even more structures that can be harnessed for quantum science applications, at the cost of making them even more challenging to cool and control. Towards this goal, in this thesis we describe work demonstrating 1D laser cooling of a beam of CaOCH3, a symmetric top molecule. This contributes to a growing body of evidence that complex polyatomic molecules could soon be cooled, trapped, and controlled at ultracold temperatures.