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Scientific Reports

Laser controlled atom source for optical clocks

Ole Kock, Wei Jian He, Dariusz Świerad, Lyndsie Smith, Joshua Hughes, Kai Bongs, Yeshpal Singh

Published Nov 18, 2016• 34 citations
Publisher

Abstract

Precision timekeeping has been a driving force in innovation, from defining agricultural seasons to atomic clocks enabling satellite navigation, broadband communication and high-speed trading. We are on the verge of a revolution in atomic timekeeping, where optical clocks promise an over thousand-fold improvement in stability and accuracy. However, complex setups and sensitivity to thermal radiation pose limitations to progress. Here we report on an atom source for a strontium optical lattice clock which circumvents these limitations. We demonstrate fast (sub 100 ms), cold and controlled emission of strontium atomic vapours from bulk strontium oxide irradiated by a simple low power diode laser. Our results demonstrate that millions of strontium atoms from the vapour can be captured in a magneto-optical trap (MOT). Our method enables over an order of magnitude reduction in scale of the apparatus. Future applications range from satellite clocks testing general relativity to portable clocks for inertial navigation systems and relativistic geodesy.

DOI
10.1038/srep37321

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Journal of the Physical Society of Japan•Nov 28, 2017•13 citations
Precision timekeeping has been a driving force in innovation, from defining agricultural seasons to atomic clocks enabling satellite navigation, broadband communication and high-speed trading. We are on the verge of a revolution in atomic timekeeping, where optical clocks promise an over thousand-fold improvement in stability and accuracy. However, complex setups and sensitivity to thermal radiation pose limitations to progress. Here we report on an atom source for a strontium optical lattice clock which circumvents these limitations. We demonstrate fast (sub 100 ms), cold and controlled emission of strontium atomic vapours from bulk strontium oxide irradiated by a simple low power diode laser. Our results demonstrate that millions of strontium atoms from the vapour can be captured in a magneto-optical trap (MOT). Our method enables over an order of magnitude reduction in scale of the apparatus. Future applications range from satellite clocks testing general relativity to portable clocks for inertial navigation systems and relativistic geodesy.
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.
Atomic clocks have been in common use since the 1950s and over this period the clock community has had two main goals: increasing the performance of the clocks and increasing their portability. At Birmingham we aim to compress a strontium optical lattice clock, a clock with an extremely high accuracy, into a portable package. Many novel approaches need to be employed to allow such a clock to operate with a favorable SWaP while maintaining the low instability and uncertainty that makes optical clocks a desirable frequency standard. The focus of this work is to use laser activation of a strontium compound as the atomic source for the clock. Previously work published by this group [1] demonstrated that it is possible to release the strontium atoms from strontium oxide successfully load a magneto-optical trap (MOT).
Optical atomic clocks provide exceptionally accurate and precise signals for timekeeping and precision measurements, but they require high-power, free-space laser configurations that limit scalability. We introduce and explore a scalable infrastructure for strontium (Sr) optical-lattice clocks that incorporates co-design of atomic-beam slowing and a magneto-optical trap (MOT) from an effusion source, generation of complex, three-dimensional free-space laser configurations with a photonic integrated circuit (PIC) and metasurface (MS) optics, and laser stabilization to a frequency-comb supercontinuum generated with integrated nonlinear photonics. With these elements, we realize MOTs of all stable strontium isotopes (84^{84}84Sr, 86^{86}86Sr, 87^{87}87Sr, 88^{88}88Sr) with populations commensurate with natural abundances, demonstrating precise beam control and robustness. Access to laser-cooled alkaline-earth atoms with scalable integrated photonics enables system engineering for optical clocks, quantum sensing, and quantum information, and our experiments demonstrate extensible technologies that advance toward a Sr optical clock largely free of bulk optics.
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.