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High Precision Spectroscopy of Strontium in an Optical Lattice: Towards a New Standard for Frequency and Time

TitleHigh Precision Spectroscopy of Strontium in an Optical Lattice: Towards a New Standard for Frequency and Time
Publication TypeThesis
Year of Publication2007
AuthorsBoyd, MM
Abstract

Clocks based on optical transitions in ions and atoms are quickly moving to the forefront of the frequency standards field mainly because of the high spectral resolution, and therefore the potential stability and accuracy, which can be achieved. In this thesis a new optical clock based on neutral atoms trapped in an optical lattice is presented, demonstrating spectroscopy of the clock transition free of any lineshape or accuracy degradation due to atomic motion. The system simulates a single trapped ion clock, but allows use of thousands of atoms for improved signal to noise ratio, and clock stability. High accuracy can also be achieved as the lattice can be designed to shift the energy of the two atomic clock states equally, such that the transition frequency is unchanged. Strontium is a natural candidate for such a clock as it offers an extremely narrow optical transition (1 mHz) based on atomic states which are very insensitive to external fields. The strontium level structure allows efficient laser cooling to 1 μK with diode laser sources, and a convenient wavelength of 813 nm for the zero-differential Stark shift optical lattice. The strontium lattice clock system has allowed observation of high signal-to-noise spectral features with the largest line quality factor ever observed in coherent spectroscopy (Q > 2×1014), attesting to the stability of the clock. The effects of nuclear spin in the Sr isotope used are explored in how it pertains to the potential accuracy of the clock. The clock accuracy is evaluated at a fractional level of 9 × 10−16, representing the first time a neutral atom optical clock has reached an accuracy comparable to the primary Cs fountains. The optical frequency is then measured using a fs-comb referenced to the NIST Cs standard via a calibrated hydrogen maser. The final frequency value of 429,228,004,229,874.0(1.1) Hz is in excellent agreement with other measurements from labs around the world, and represents one of the most accurate optical frequency measurements to date.