Bright beams of cold molecules provide valuable quantum sensors for measuring permanent electric dipole moments that violate fundamental symmetries, which enables these tabletop experiments to probe energy scales exceeding those available with current or near-future particle colliders. The precision of such experiments generally scales with the number of molecules that are detected per unit time, typically a few meters downstream of the place where the molecular beam is created. So, improving the precision -and thereby probing new physics at higher and higher energy scales- requires increasing the downstream flux of molecules. This can be achieved by collimating the molecular beam to maintain a high number density. These molecular experiments can borrow techniques for control and manipulation from the extremely well-developed field of atomic physics. One such technique is laser cooling, which is a powerful method to achieve this boost in brightness, whose implementation however remains challenging due to the complex energy level structure of molecules. In this work, we demonstrate and analyze laser cooling of a focused cryogenic beam of BaF molecules. We conclude that with some follow-up steps this method will increase the beam brightness by a factor 100, a crucial milestone towards a high precision measurement of the electron electric dipole moment (eEDM).
Bright beams of cold molecules provide valuable quantum sensors for measuring permanent electric dipole moments that violate fundamental symmetries, which enables these tabletop experiments to probe energy scales exceeding those available with current or near-future particle colliders. The precision of such experiments generally scales with the number of molecules that are detected per unit time, typically a few meters downstream of the place where the molecular beam is created. So, improving the precision -and thereby probing new physics at higher and higher energy scales- requires increasing the downstream flux of molecules. This can be achieved by collimating the molecular beam to maintain a high number density. These molecular experiments can borrow techniques for control and manipulation from the extremely well-developed field of atomic physics. One such technique is laser cooling, which is a powerful method to achieve this boost in brightness, whose implementation however remains challenging due to the complex energy level structure of molecules. In this work, we demonstrate and analyze laser cooling of a focused cryogenic beam of BaF molecules. We conclude that with some follow-up steps this method will increase the beam brightness by a factor 100, a crucial milestone towards a high precision measurement of the electron electric dipole moment (eEDM).
Laser cooling of molecules is a powerful technique for producing cold, slow beams for precision measurements and quantum control, yet its implementation remains challenging due to molecular complexity. Here, we combine a cryogenic buffer gas beam, an electrostatic hexapole lens, and 2D transverse Doppler laser cooling to produce a bright beam of barium monofluoride (138Ba19F) molecules. We study both numerically and experimentally the laser cooling effect as a function of laser detuning, laser power, laser alignment, and interaction time. We find a scattering rate of 6.1(1.4) × 105 s−1 on the laser cooling transition (14% of the expected maximum) and identify suboptimal dark Zeeman state remixing, suboptimal laser sideband powers and detunings, and a lack of vibrational repump laser intensity as possible causes of such a low rate. Using 3 tuneable lasers with appropriate sidebands and detuning, each molecule scatters approximately 400 photons during 2D laser cooling, limited by the interaction time and scattering rate. Leaks to dark states are less than 10%. Finally, we use the experimental results to benchmark the trajectory simulations to predict the achievable flux 3.5 m downstream for a planned eEDM experiment. Laser cooling is a powerful technique that enables precision measurements and quantum control, yet its implementation in molecules remains challenging due to their complex structures. Here, the authors apply 2D transverse laser cooling to a focused beam of cold barium monofluoride (138Ba19F) molecules to significantly increase beam brightness.
Permanent electric dipole moments (EDMs) are signatures of time-reversal and parity violation and act as sensitive probes for physics beyond the Standard Model (BSM). The work in this thesis has been done as part of the NL-eEDM collaboration, which plans to measure the electron EDM using a beam of BaF molecules. The sensitivity of such a table-top precision experiment benefits from utilizing an intense, slow beam of molecules. We plan to achieve that through a combination of Stark deceleration and transverse laser cooling of a cryogenic beam of BaF. The work presented in this thesis has primarily been done to produce intense, slow molecular beams for an electron EDM measurement. We constructed a supersonic beam source of SrF and BaF molecules. The source produces high intensity beams with low translational and rotational temperatures. Time-resolved detection of laser-induced fluorescence from pulsed excitation of electronic states in BaF molecules was performed to determine the lifetimes of the A^{2}Pi_{1/2} and A^{2}Pi_{3/2} states. The lifetimes of these states were determined to be 57.1(3) ns and 47.9(7) ns, respectively. The transition between the A^{2}Pi_{1/2} and X^{2}Sigma^{+} states is the main laser cooling transition in BaF. We decelerated a beam of SrF molecules from a cryogenic source to a standstill inside a traveling-wave Stark decelerator, with the number of decelerated molecules on the order of 1000. BaF molecules will be decelerated with the same technique to perform an electron EDM measurement.
Cold molecules are attractive for a wide range of scientific applications, including quantum computation, the study of chemical reactions, and tests of fundamental physics. Laser cooling has proved to be an invaluable technique in the cooling of atoms. This technique was once thought to be infeasible for molecules, because it is difficult to find a closed cycling transition due to their vibrational structure. Recently, laser cooling of several diatomic species has been demonstrated. These molecules possess electronic transitions with highly diagonal Franck-Condon matrices, which make it possible to drive a quasi-closed cycling transition. Ytterbium fluoride (YbF) molecules are amenable to laser cooling and are especially interesting because they are used to measure the electron's electric dipole moment (eEDM). Measurements of the eEDM test the prediction of theories that extend the Standard Model of particle physics. The sensitivity of an eEDM experiment could be greatly increased by using ultracold molecules produced by direct laser cooling. This thesis presents work done towards producing a laser-cooled beam of YbF for an eEDM experiment. This work includes the construction of the cooling experiment, a novel method for efficiently combining laser beams of very similar frequencies, results of spectroscopic measurements to find the required transitions for laser cooling, the results of initial optical cycling experiments, and the first laser cooling results of YbF. Using a one-dimensional optical molasses, a beam of molecules is Doppler cooled in one transverse direction to a temperature of approximately 70 mK. Preliminary evidence of cooling to lower temperatures through a Sisyphus mechanism is also presented. Finally, paths towards improving the laser cooling are suggested. The work opens the door to improved measurements of the eEDM using ultracold YbF molecules.
That the universe contains a disproportionate amount of matter compared to antimatter cannot be explained by the standard model of particle physics; this is because it contains too little charge-parity (CP) symmetry violation. Low energy experiments with atoms and molecules can be extremely sensitive probes for many types of new physics. An active area of research is the search for electric dipole moments (EDMs) of fundamental particles or nuclei; a nonzero EDM would be evidence for new sources of CP violation. Ytterbium monofluoride (YbF) molecules in a supersonic beam are currently being used to measure the electric dipole moment of the electron (eEDM). This thesis presents progress towards a new, more sensitive eEDM experiment that will use a slower YbF beam source and transverse laser cooling to collimate this beam. We have demonstrated laser cooling of YbF in one transverse dimension to less than 100 μK, which is below the Doppler cooling limit. This was done using a Sisyphus-type mechanism called magnetic field induced laser cooling. We also demonstrate polarisation gradient cooling of YbF. After rebuilding and upgrading much of the experiment, we cooled the beam in both transverse dimensions. By scanning experimental parameters, we have gained insight into the cooling mechanisms, and the optimum conditions for laser cooling. We placed an upper limit of a few mK on the transverse temperature of the molecules, although I argue that the true temperature must be very much lower. The number of ultracold molecules was carefully quantified and found to be 2.0(4)x10^5 per pulse. These experiments lay the foundations for an eEDM sensitivity improvement by up to a factor of 100.
Abstract Over the past century, the molecular beam methods pioneered by Otto Stern have advanced our knowledge and understanding of the world enormously. Stern and his colleagues used these new techniques to measure the magnetic dipole moments of fundamental particles with results that challenged the prevailing ideas in fundamental physics at that time. Similarly, recent measurements of fundamental electric dipole moments challenge our present day theories of what lies beyond the Standard Model of particle physics. Measurements of the electron’s electric dipole moment (eEDM) rely on the techniques invented by Stern and later developed by Rabi and Ramsey. We give a brief review of this historical development and the current status of eEDM measurements. These experiments, and many others, are likely to benefit from ultracold molecules produced by laser cooling. We explain how laser cooling can be applied to molecules, review recent progress in this field, and outline some eagerly anticipated applications.
Abstract Measurements of the electron’s electric dipole moment (eEDM) are demanding tests of physics beyond the standard model. We describe how ultracold YbF molecules could be used to improve the precision of eEDM measurements by two to three orders of magnitude. Using numerical simulations, we show how the combination of magnetic focussing, two-dimensional transverse laser cooling, and frequency-chirped laser slowing, can produce an intense, slow, highly-collimated molecular beam. We show how to make a magneto-optical trap of YbF molecules and how the molecules could be loaded into an optical lattice. eEDM measurements could be made using the slow molecular beam or using molecules trapped in the lattice. We estimate the statistical sensitivity that could be reached in each case and consider how sources of noise can be reduced so that the shot-noise limit of sensitivity can be reached. We also consider systematic effects due to magnetic fields and vector light shifts and how they could be controlled.