While cold molecule experiments are rapidly moving towards their promised benefits of precision spectroscopy, controllable chemistry, and novel condensed phases, heretofore the field has been greatly limited by a lack of methods to cool and compress chemically diverse species to temperatures below ten millikelvin. While in atomic physics these needs are fulfilled by laser cooling, magneto-optical trapping, and evaporative cooling, until now none of these techniques have been applicable to molecules. In this thesis, two major breakthroughs are reported. The first is the observation of evaporative cooling in magnetically trapped hydroxyl (OH) radicals, which potentially opens a path all the way to Bose-Einstein condensation of dipolar radicals, as well as allowing cold- and ultracold-chemistry studies of fundamental reaction mechanisms. Through the combination of an extremely high gradient magnetic quadrupole trap and the use of the OH Λ-doublet transition to enable highly selective forced evaporation, cooling by an order of magnitude in temperature was achieved and yielded a final temperature no higher than 5mK. The second breakthrough is the successful application of laser cooling and magneto-optical trapping to molecules. Motivated by a proposal in this thesis, laser cooling of molecules is now known to be technically feasible in a select but substantial pool of diatomic molecules. The demonstration of not only Doppler cooling but also two-dimensional magneto-optical trapping in yttrium (II) oxide, YO, is expected to enable rapid growth in the availability of ultracold molecules--just as the invention of the atomic magneto-optical trap stimulated atomic physics twenty-five years ago.
While cold molecule experiments are rapidly moving towards their promised benefits of precision spectroscopy, controllable chemistry, and novel condensed phases, heretofore the field has been greatly limited by a lack of methods to cool and compress chemically diverse species to temperatures below ten millikelvin. While in atomic physics these needs are fulfilled by laser cooling, magneto-optical trapping, and evaporative cooling, until now none of these techniques have been applicable to molecules. In this thesis, two major breakthroughs are reported. The first is the observation of evaporative cooling in magnetically trapped hydroxyl (OH) radicals, which potentially opens a path all the way to Bose-Einstein condensation of dipolar radicals, as well as allowing cold- and ultracold-chemistry studies of fundamental reaction mechanisms. Through the combination of an extremely high gradient magnetic quadrupole trap and the use of the OH Λ-doublet transition to enable highly selective forced evaporation, cooling by an order of magnitude in temperature was achieved and yielded a final temperature no higher than 5mK. The second breakthrough is the successful application of laser cooling and magneto-optical trapping to molecules. Motivated by a proposal in this thesis, laser cooling of molecules is now known to be technically feasible in a select but substantial pool of diatomic molecules. The demonstration of not only Doppler cooling but also two-dimensional magneto-optical trapping in yttrium (II) oxide, YO, is expected to enable rapid growth in the availability of ultracold molecules--just as the invention of the atomic magneto-optical trap stimulated atomic physics twenty-five years ago.
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.
From studies of fundamental physics to quantum technologies the production of ultracold molecules will have a huge impact across a range of applications. For many years laser cooling, which became an invaluable tool in cold atomic physics, was deemed to be too impractical for application to molecules. Nevertheless, laser cooling has now been demonstrated for a few molecular species. Using a frequency-chirped laser slowing technique, the velocity distribution of a pulse of CaF molecules is compressed and slowed from 180 m/s to about 10 m/s. These slow molecules are then captured in a magneto-optical trap. I present measurements that show how the number of molecules, the photon scattering rate, the oscillation frequency, damping constant, temperature, cloud size and lifetime depend on the key parameters of the magneto-optical trap, especially the intensity and detuning of the main cooling laser. The trap contains up to 2*10^4 molecules, the maximum photon scattering rate is 2.5*10^6 s-1 per molecule, the maximum oscillation frequency is 100 Hz, the maximum damping constant is 500 s^-1, and the minimum rms radius of the trapped cloud is 1.5 mm. A minimum temperature of 730 microkelvin is obtained by ramping down the laser intensity to lower values. To reach lower temperatures, the cloud is loaded into a blue-detuned optical molasses, which cools the molecules to 55 microkelvin, well below the Doppler-limiting temperature. I characterise the cooling process and suggest the sub-Doppler mechanisms responsible. These ultracold molecules are the optically pumped into a single quantum state, and coherently transferred between selected hyperfine components of the ground and first-excited rotational states. Finally, the ultracold, state-selected molecules are loaded into a magnetic trap that has a lifetime of about 1 s.
Ultracold polar molecules have emerged as promising platforms for quantum simulation, quantum information science, precision measurements of fundamental symmetry violations, and quantum chemistry. Over the past decade, rapid progress in direct laser cooling of molecules has enabled new capabilities such as magneto-optical traps (MOTs), sub-Doppler cooling, and optical dipole traps (ODTs), expanding the quantum toolbox with ultracold molecules of complex internal structures and strong dipolar interactions. This thesis presents the design, construction, and operation of an experiment on ultracold strontium monofluoride (88Sr19F) molecules, demonstrating state-of-the-art techniques for molecular laser cooling and trapping. The apparatus integrates a broad range of technical sub-systems, including cryogenics, ultra-high vacuum, optics, electronics, and automated laboratory control software. SrF molecules produced in a slowed cryogenic buffer gas beam are captured into a dual-frequency MOT. Subsequent Λ-enhanced gray molasses cooling reduces the molecule temperature to as low as 5 μK. A novel blue-detuned MOT is also implemented to further compress the molecular cloud, increasing the density by a factor of ∼ 400. At this high density, SrF molecules are efficiently loaded into an ODT, a tight, conservative trap with long lifetimes. Population transfer between different rotational levels is achieved via optical pumping and coherent microwave control, yielding ∼ 0.8 ms coherence time in Rabi oscillation and ∼ 90 % transfer efficiency. This thesis also describes the development of an ultracold rubidium (87Rb) atom experiment, in which a loading rate of ∼ 10^10 atoms/s into a radio-frequency (RF) MOT is demonstrated. A modular two-dimensional MOT serves as the Rb source and can be seamlessly integrated into the SrF apparatus, opening the possibility for future studies of co-trapping and sympathetic cooling of SrF molecules with atoms. The results presented in this thesis establish an experimental foundation for exploring new frontiers in ultracold molecular physics.
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.
Laser cooling and trapping are central to modern atomic physics. It has been roughly three decades since laser cooling techniques produced ultracold atoms, leading to rapid advances in a vast array of fields and a number of Nobel prizes. Prior to the work presented in this thesis, laser cooling had not yet been extended to molecules because of their complex internal structure. However, this complexity makes molecules potentially useful for a wide range of applications. The first direct laser cooling of a molecule and further results we present here provide a new route to ultracold temperatures for molecules. In particular, these methods bridge the gap between ultracold temperatures and the approximately 1 kelvin temperatures attainable with directly cooled molecules (e.g. with cryogenic buffer gas cooling or decelerated supersonic beams). Using the carefully chosen molecule strontium monofluoride (SrF), decays to unwanted vibrational states are suppressed. Driving a transition with rotational quantum number R=1 to an excited state with R'=0 eliminates decays to unwanted rotational states. The dark ground-state Zeeman sublevels present in this specific scheme are remixed via a static magnetic field. Using three lasers for this scheme, a given molecule should undergo an average of approximately 100,000 photon absorption/emission cycles before being lost via unwanted decays. This number of cycles should be sufficient to load a magneto-optical trap (MOT) of molecules. In this thesis, we demonstrate transverse cooling of an SrF beam, in both Doppler and a Sisyphus-type cooling regimes. We also realize longitudinal slowing of an SrF beam. Finally, we detail current progress towards trapping SrF in a MOT. Ultimately, this technique should enable the production of large samples of molecules at ultracold temperatures for molecules chemically distinct from competing methods.
Laser cooling and magneto-optical trapping of neutral atoms has been a driving force in physics over the last few decades by providing an efficient method to produce cold and dense samples of cold atoms. Polar molecules which can have strong, long-range dipolar interactions and possess more rich internal structure promise a wide range of new physics and chemistry studies if they can be controlled efficiently. Until recently, the only way to achieve sub-milliKelvin level temperatures in molecules was to associate two precooled atomic species, which while very effective, has a firm bound on the chemical diversity accessible. While laser cooling of molecules relies on fortuitous molecular properties, it will significantly increase the choices of species available for molecular cooling.In this thesis I present the improved optical deceleration of the molecule yttrium monoxide (YO) and the loading of molecules into a 3-D magneto-optical trap (MOT). By making an improved choice of repumping scheme from our groups’ previous work, our photon scattering rate has been increased. By also improving our buffer gas cell source, we produce a much larger flux (2000 times higher) of slowed molecules at 5 m/s to reach the trapping region. I have designed and implemented 3-D MOT field coils operating at 5 MHz, and demonstrated a successful operation of a 3-D MOT. This is the first 3-D laser trapping of an oxide and the first of a molecule with an electronic state intermediate to the main cycling transition states.I have also performed a study of the transition from the ground state to the intermediate electronic state, which has a narrower linewidth of 150 kHz, in contrast to the main cycling transition’s linewidth of 5 MHz. This transition is well suited to second-stage narrow line cooling after loading molecules with the main MOT transition, which will allow direct laser cooling of a molecule to the 10 uK regime.