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 CaNH$_2$, a smaller asymmetric top molecule with similar spatial symmetry, we performed laser cooling. Using CaNH$_2$, 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.
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.
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.
Potential wide-ranging scientific applications, spanning fundamental physics to quantum engineering, has led to significant efforts in controlling molecules at the quantum level. The rich internal structure of molecules, that gives rise to these desirable properties also complicates the task of controlling such species. Over the past decade, our ability to produce, control, and detect molecules has advanced tremendously with direct laser-cooling and magneto-optical trapping of diatomic molecules realized by several groups around the world. In this thesis, we describe the creation of the first RF MOT of CaF, which at the time of the writing of this thesis, remains the largest and densest molecular MOT. New laser cooling techniques for molecules are demonstrated, allowing laser cooling of molecules to 4 uK, 50 times colder than the Doppler limit. Optical trapping of these molecules is also achieved, which, in combination with in-trap laser cooling increased the density of trapped molecules by five orders of magnitude compared to the density in the MOT. We developed methods to cool and detect single molecules with high fidelity, which aided us in the creation of an optical tweezer array of single, ultracold CaF molecules. The densities of molecules (10^12 cm^-3) attained inside the tweezer traps also enabled us to observe both light-assisted and ground state collisions of laser cooled molecules, for the first time. The phase space density within the merged tweezers reached 5x10^-4, 10 orders of magnitude larger than the initial MOT. We implemented internal quantum state control of the molecules and dynamical control of the tweezers to build a platform for exploring state-selective ultracold quantum chemistry.
Ultracold molecules are a promising platform for a wide range of applications in quantum science, including searches for new particles (beyond the Standard Model), quantum computing and simulation, and studies of ultracold collisions and chemistry. Over the last decade, tremendous progress has been made in developing techniques to trap diatomic molecules (molecules containing only two atoms) and cool them to ultracold temperatures using laser cooling. Polyatomic molecules (molecules containing more than two atoms) provide new, unique scientific opportunities not afforded by their diatomic counterparts and represent the next frontier in laser cooling and quantum science with molecules. In this thesis, we present our work on extending laser cooling and quantum control to polyatomic molecules. We describe our demonstration of laser cooling and the creation of a magneto-optical trap (MOT) of a polyatomic molecule, calcium monohydroxide (CaOH). We further demonstrate loading of the trapped molecules into an optical dipole trap (ODT) and loading of individual CaOH molecules into the conservative potential an optical tweezer array. We also describe our realization of a new type of MOT developed for molecules, a "conveyor belt MOT", which provides increased compression compared to traditional molecular MOTs and was discovered using numerical simulations of the CaOH MOT that were developed as part of this thesis. The increased MOT density and ODT loading afforded by the conveyor belt MOT provides the density required to perform studies of collisions between ultracold CaOH molecules. We also describe our work on the laser cooling the symmetric top molecule CaOCH₃, opening the possibility for 3-D laser cooling of even larger, more complex molecules into the ultracold regime. With the ability to laser cool and trap CaOH molecules, our most recent work has focused on developing tools for quantum control and on initial explorations of some of the new features afforded by polyatomic molecules. We describe a novel protocol established to perform a measurement of the electron's electric dipole moment ("eEDM"). Looking towards quantum information processing with polyatomic molecules, we create a new type of qubit using parity-doublet states in the bending mode of CaOH and characterize its coherence properties. Finally, we provide a brief outlook towards the future of laser cooling of polyatomic molecules for quantum science and provide a few suggestions for new directions opened up by the work presented in this thesis.
The rich internal structure of polyatomic molecules has inspired a number of theoretical proposals exploring a myriad of unique scientific and engineering frontiers, including quantum simulation and next-generation searches for physics beyond the standard model. Establishing full control over these complex quantum objects is likely necessary to harness their full potential; however, the same rich internal structure that gives rise to the immense scientific potential of polyatomic molecules also makes manipulating them a daunting task. In the past decade there has been considerable progress in the production, cooling, and control of diatomic species using direct laser cooling and magneto-optical trapping. This dissertation focuses on extending direct laser cooling and trapping to polyatomic molecules. Direct laser cooling of triatomic radical calcium monohydroxide (CaOH) is demonstrated. This molecules is a prototypical example of a broad class of polyatomic molecules that includes symmetric and asymmetric tops. Transverse magneto-optical trapping is demonstrated, establishing the ability to scatter an unprecedented number of photons in a polyatomic species. An understanding of previously unappreciated intricacies of molecular structure is developed, that explains observations of novel vibrational decay pathways. High resolution spectroscopy of hybrid vibrational modes is performed. A laser cooling scheme is constructed that will support future efforts to use the full range of laser cooling and high-fidelity detection techniques required to implement an optical tweezer array of polyatomic molecules. Our experimental efforts demonstrate the feasibility of direct laser cooling applied to CaOH, up ending long-standing assumptions of the inextricable complexity of polyatomic molecules.
Ultracold molecules can be used for a diverse set of exciting applications including controlled quantum chemistry, probes of physics beyond the standard model, simulation of many-body quantum systems and quantum information processing. However their rich internal structure makes the required control of their motional and internal states difficult. In the last decade, the field has made rapid progress on developing techniques for producing, cooling and trapping molecules and, whilst challenges remain, many of these applications are now being realised. This thesis presents my work on laser cooling and coherent control of calcium monofluoride (CaF) molecules. We load molecules into a three-dimensional magneto-optical trap (MOT), the first such trap for CaF and the second for any molecule. The MOT contains around 2×10⁴ molecules with an initial temperature of 12 mK. Lowering the laser intensity brings the temperature of the MOT down to 960 μK. To further cool the molecules we load them into a blue-detuned optical molasses, cooling them to 5.4(7) μK. This temperature is ~40 times below the Doppler limit and corresponds to a typical molecule moving at only a few times the single-photon recoil velocity. We optically pump the molecules into a single Zeeman sub-level and coherently transfer them between rotational states using microwave radiation. We load the molecules into a magnetic trap where they remain confined for about 5 s, the lifetime being limited by vibrational excitation due to room-temperature blackbody radiation. We show how to prepare the trapped molecules in a coherent superposition of two rotational states, and how to maintain this coherence for long periods, up to 6.4(8) ms. Detailed simulations of the remaining decoherence mechanisms suggest that it is possible to extend this to more than a second by compressing the sample to a smaller size, and using a magnetic trap geometry where the direction of the field is nearly uniform across the trapping volume. Finally we explore theoretically how Raman sideband cooling can be applied to molecules in optical tweezer traps, potentially providing a route to full quantum control.
Molecules controlled at the single-quantum-state level offer wide-ranging applications spanning many of the frontiers of modern physics. Within the past decade, simple diatomic molecules have been brought under full quantum control both via assembly from pre-cooled atoms and by direct laser cooling. Polyatomic molecules\footnote{Molecules that contain three or more atoms.} have attracted focus as novel quantum resources providing distinct advantages (and challenges) compared to both atoms and diatomic molecules. For example, nearly all polyatomic molecules have long-lived states that can be fully polarized at low applied electric fields. These and other features generic to polyatomic molecules can be applied to quantum simulation, quantum chemistry, and tests of fundamental physics beyond the Standard Model. While these structural features of polyatomic molecules are highly promising for applications, they complicate the process of bringing molecules under full quantum control. In this thesis, we describe a series of experimental and theoretical advances toward the production of ultracold complex polyatomic molecules. We demonstrate the first laser cooling of the polyatomic radical ytterbium monohydroxide (YbOH), a promising candidate to search for particles and interactions beyond the Standard Model of particle physics. We develop a novel deceleration technique (``Zeeman-Sisyphus deceleration") capable of slowing molecular beams to trappable velocities while scattering fewer than 10 optical photons. This deceleration method is demonstrated using CaOH molecules, a lighter analog of YbOH. In order to determine a viable pathway to full three-dimensional cooling and trapping of YbOH, we develop a spectroscopic technique to measure vibrational branching ratios with relative intensity sensitivity around 1 part in 105, approximately two orders of magnitude beyond the previous state of the art. We also perform the first spectroscopic observations and characterizations of the related molecule ytterbium monomethoxide, YbOCH3, a promising species to probe beyond-the-Standard-Model physics. In addition, we show theoretically that an entire class of asymmetric top molecules can be laser cooled using essentially the same techniques used for much simpler species. Measurements of the vibrational branching ratios for a few prototypical asymmetric to molecules (CaSH and CaNH2) are presented to confirm these predictions.