Magneto-Optical Trapping of Strontium Atoms for Optical Atomic Clock

Abstract

Optical lattice clocks, leveraging their remarkable precision, have the capability to measure time with unprecedented accuracy. This precision opens up possibilities for the utilization of lattice clocks in various domains, such as probing new physics beyond the standard model, advancing geodesy and navigation technologies, and re-definition of the SI second in the future. The meticulous process of measuring time, known as frequency metrology, emerges as a pivotal field of scientific inquiry within the SI system. It serves as the foundation for defining other fundamental units. Currently, the SI second is defined by the duration of 9,192,631,770 periods of radiation corresponding to a specific transition in the 133Cs atom’s ground state, within the microwave spectrum. However, the transition of an optical frequencies with high quality factor (Q) values presents an opportunity to increase measurement precision. Optical frequency clocks have demonstrated operational capabilities with fractional uncertainties as small as 10 18. Such unprecedented precision not only holds the potential to revolutionize global time keeping but also offers insights into fundamental constants of nature, thus enabling rigorous tests of fundamental theories. To achieve the objective of developing an optical atomic clock based on the singlet to triplet intercombination line of strontium (Sr), this thesis outlines the design and implementation of an experimental apparatus for producing cold Sr atoms in a magneto-optical trap (MOT). The entire setup is constructed on a 1.5m ⇥ 3m optical table, incorporating both laser optical components and vacuum systems for the generation, deceleration, and trapping of Sr atoms, all isotopes of Sr. The MOT is loaded through a high-intensity Sr atom beam emitted by an oven and subsequently decelerated by a zero-field crossing Zeeman slower. The oven design addresses the challenge of clogging over long-term operation and provides thermal isolation between the vacuum chamber and the high-temperature oven region, simplifying the setup. A home-built injection lock module is used for the generation of the first stage cooling laser, utilizing the 461 nm laser to prepare a blue MOT for all Sr isotopes, with temperatures reaching a few millikelvins. Subsequently, atoms are transferred to the second stage where stringent requirements for the lasers arise due to the narrow linewidth of the transition. Using a high-finesse optical cavity, we reduce the line width of the 689 nm laser, while an isolation chamber for the optical cavity enhances its stability, aiding in achieving a single-frequency red MOT with a final temperature of approximately 10µK. The clock frequency reference atomic transition is susceptible to various external influences, contributing to the total uncertainty budget of the clock transition. These uncertainties, including AC Stark shift, Zeeman shift, black body radiation shift, and background gas collision-induced shift, must be systematically evaluated to exploit the potential of the ultra-narrow clock transition fully. Our investigation focuses on the 483nm transition originating from the clock state to a higher excited state, characterizing its properties. Moreover, this transition serves as a potential repumper transition in conjunction with other single-frequency repumpers such as 707 nm, 481 nm, and 405 nm.