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An ultracold molecular beam for measuring the electric dipole moment of the electron

M. A. Trigatzis

Published Mar 1, 2020• 1 citation
Publisher

Abstract

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.

DOI
10.25560/82294

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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.
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.
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.
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.
We propose an experiment to measure the electric dipole moment (EDM) of the electron using ultracold YbF molecules. The molecules are produced as a thermal beam by a cryogenic buffer gas source, and brought to rest in an optical molasses that cools them to the Doppler limit or below. The molecular cloud is then thrown upward to form a fountain in which the EDM of the electron is measured. A non-zero result would be unambiguous proof of new elementary particle interactions, beyond the standard model.
Abstract We use two-dimensional transverse laser cooling to produce an ultracold beam of YbF molecules. Through experiments and numerical simulations, we study how the cooling is influenced by the polarization configuration, laser intensity, laser detuning and applied magnetic field. The ultracold part of the beam contains more than 2 × 105 molecules per shot and has a temperature below 200 μK, and the cooling yields a 300-fold increase in the brightness of the beam. The method can improve the precision of experiments that use molecules to test fundamental physics. In particular, the beam is suitable for measuring the electron electric dipole moment with a statistical precision better than 10−30 e cm.