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The Magnetic Fields and their Noise in the Next Generation Ultracold Strontium Optical Lattice Clock

Jacob Scott

Published Jan 1, 2019• 0 citations
OpenAlex

Abstract

The first generation Ultracold Strontium Optical Lattice Clock has achieved an uncertainty on the order of 10^-18 using a 1D optical lattice. The next generation clock is now being constructed with the goal of achieving uncertainties on the 10^-19 level. To achieve this goal, the system will be upgraded to a 3D optical lattice and new apparatus is being built with lower noise and better compatibility with this new system. This thesis discusses the design and construction of the current driving system for the magnetic fields in the experiment. It also presents the design, construction, and testing of the magnetic fields of the Magneto-Optical Trap. Lastly, it analyzes the noise of the generated magnetic fields in the experiment to confirm they commit to the low noise goal of the next generation experiment.

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The first generation Ultracold Strontium Optical Lattice Clock has achieved an uncertainty on the order of 10^-18 using a 1D optical lattice. The next generation clock is now being constructed with the goal of achieving uncertainties on the 10^-19 level. To achieve this goal, the system will be upgraded to a 3D optical lattice and new apparatus is being built with lower noise and better compatibility with this new system. This thesis discusses the design and construction of the current driving system for the magnetic fields in the experiment. It also presents the design, construction, and testing of the magnetic fields of the Magneto-Optical Trap. Lastly, it analyzes the noise of the generated magnetic fields in the experiment to confirm they commit to the low noise goal of the next generation experiment.
Optical atomic clocks have proven to be the most stable clocks with the lowest systematic uncertainty which has now reached a record level of (9.4 X10/(^{19}/)). Thus optical clocks now surpass the best fountain Cs atomic clocks by three orders of magnitude which opens the prospect of using optical clocks as frequency standards for timekeeping in the near future. \n \nThis thesis describes the progress that has been made towards a stationary optical lattice clock based on strontium atoms, and reports the results of an ongoing characterization. \n \nAn experimental apparatus was designed and built to efficiently slow, cool, and trap the strontium atoms which represent the heart of the strontium optical lattice clock discussed in this thesis. The novel aspects of the experimental development described in the thesis comprise a frequency doubling cavity for producing 461 nm laser light employed for cooling and trapping the strontium atoms in the first stage cooling. This cavity provided sufficient power for cooling and trapping the strontium atoms for up to 300 mW of 461 nm laser light. Furthermore, a new design of Zeeman slower based on spherical permanent magnets was produced, built and characterized. \n \nThe slower is compact, tunable, easy to assemble and maintain, and cost-effective as the permanent magnets do not require any power supply or water cooling. It is suggested that this kind of Zeeman slower constitutes an ideal solution for portable optical clock systems. A blue magneto-optical trap was realized for the first time with such a Zeeman slower. With about 30 mW of a 461 nm laser, up to 6X10/(^8/) atoms were captured.
Atomic Caesium clocks have made an astonishing development in the past few decades, with them gaining an order of magnitude in precision every decade. A new generation of atomic optical clocks is now emerging, which is moving to the forefront of timekeeping. By going from microwave to optical frequencies these clocks are now reaching fractional uncertainty levels in the 10\\(^-\\)\\(^1\\)\\(^8\\), opening a whole new range of exciting applications. This thesis presents the design and implementation of an experiment for laser cooling of strontium atoms, focusing on the mobility of the setup, opening a range of new applications as a mobile clock. The experimental apparatus applies a range of new techniques for strontium, such as the use of a two dimensional magneto-optical trap for pre cooling atoms, allowing for a compact and transportable system. A mobile laser system for laser cooling strontium atoms was realized. This includes a frequency doubling system for the main cooling transition. The constructed monolithic frequency doubling cavity has generated 300 mW of blue light and proven to be a stable setup suitable for a mobile system. The 3D-MOT was able to capture 4 * 10\\(^5\\) atoms with a long lifetime of 5.5 s, with further improvements for a significant increase in atom number currently being implemented.
We present the advancement toward the realization of a new optical clock apparatus based on ultracold strontium atoms. The atomic source employs a novel design based on two dimensional magneto optical trapping which is advantageous because of the suppression of thermal atoms in the interrogation volume, and the elimination of hot BBR in the view of the science cell. Compact frequency stabilization of cooling and trapping lasers is achieved via single PDH lock to a three-color ultrastable optical resonator. We also study different lattice topologies to operate the ultracold atomic ensemble in the high-cooperativity regime toward sub-quantum projection noise instability and millihertz-linewidth frequency stabilization by cavity QED.
Optical atomic clocks with precisions and accuracies in the 10−^-−1^118^88 level [1] are now the most advanced man-made timekeeping devices. They outperform the microwave cesium atomic clocks that realize the SI definition of the second. Scaling down the size of optical atomic clocks may open the door to a range of industrial and space applications. In this thesis, the design and preliminary results of a compact strontium cooling system are presented. In the first cooling stage, the high power 461 nm laser with 300 mW output features a modular design, while smaller laser sources for demonstrating a strontium magneto-optical trap have also been investigated. An innovative design that couples a spectroscopy cell directly into the scientific chamber reduces the overall size and power consumption of the system. Additionally, using strontium oxide as a source of strontium atoms suitable for optical clocks has achieved initial success. For the first time, a single-beam MOT configuration is applied to strontium. In this novel apparatus, the blue MOT and red MOT broadband cooling phases are able to trap 5x106^66 and 1000 atoms, respectively. This work shows promising progress towards developing a functional miniaturized strontium optical lattice clock.
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 spec-tral 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 simu-lates 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 con-venient 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
Accurate time keeping is critical to the function and advancement of many human endeavours. Atomic clocks based on a microwave transition in atomic Cs have served as primary frequency standards to realise the SI second since their formal adoption in 1967. State-of-the-art, laser cooled, Cs fountains now routinely operate with fractional frequency uncertainties below the 10⁻¹⁵ level in laboratories throughout the world. Over the last decade, efforts to build frequency standards based on optical transitions have come to fruition and potential inaccuracies below the 10⁻¹⁷ level have been demonstrated. Such standards point to the future of time keeping and much development is being pursued to realise the potential of these 'next generation' optical clocks. One such approach, on which this work focusses, relies on a 'magic' wavelength optical lattice trap to provide (almost) Doppler- and recoil-free confinement of atoms during clock interrogation. At this 'magic' wavelength, light induced perturbation of the clock states is equal and so cancels precisely. This thesis describes the design and implementation of an optical lattice frequency standard apparatus based on 88Sr held in a 1D optical lattice. A review of the development of atomic frequency metrology is given from a historical viewpoint and the technology and limitations of current methods discussed. To tackle the blackbody radiation shift, which currently presents the greatest limitation to the Sr lattice clock, imposing at the 10⁻¹⁶ level, a scheme to measure the blackbody radiation shift coefficient at the 10⁻¹⁷ level is developed. Techniques to laser cool and trap atomic strontium are described and the implementation detailed. All laser systems required for clock operation are described including the design and implementation of extended-cavity diode lasers for precision spectroscopy. Details relevant to the eventual operation of the apparatus are discussed.