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Setup of a Transportable Yb Optical Lattice Clock

C. Abou-Jaoudeh

Published Jan 1, 2016• 0 citations
OpenAlex

Abstract

In this thesis, a transportable setup of an Yb atomic source for a lattice optical clock is built and characterized. All three laser setups for cooling and trapping the Yb atoms, together with the vacuum system, are contained on a transportable optical table. The preparation of the atomic sample before doing spectroscopy on the clock transition starts by slowing the atoms in the Zeeman slower. This is followed by trapping the atoms in the pre-cooling and the post-cooling MOTs, before loading the atoms into the optical lattice. Slowing and pre-cooling the atoms is done using the strong transition at 399 nm. This cools the atoms down to around a mK. To successfully load the atoms in the optical lattice, we have to cool the atoms further to beyond 50 μK. This is done by exciting the weak transition at 556 nm. The atoms that are transferred into the post-cooling stage are cooled down to a few tens of μK. The atoms are then ready to be loaded into the optical lattice at the magic wavelength. The amplified laser light at 760 nm inside the 1-D intra-vacuum cavity results in a lattice trapping potential of approximately 50 μK. A transfer efficiency, from the MOT into the optical lattice, of more than 25% was measured. The spectroscopy on the clock transition was done for 171Yb. A step-by-step mechanism was adopted, starting by exciting the forbidden transition at 578 nm inside the post-cooling stage MOT, by using the ”chopped-MOT” method. The 300 kHz Doppler-broadened line is then narrowed down to 3 kHz by switching on the light at 760 nm, while keeping the MOT on. The so-called ”chopped-lattice” technique helped suppress the Doppler-broadening; however the other effects due to the light and the magnetic fields of the MOT were still present. By switching off the MOT’s light and magnetic fields, the Yb atoms are completely trapped in the optical lattice. As a result, the FWHM of the spectroscopy line was reduced by a factor of 10.

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In this thesis, a transportable setup of an Yb atomic source for a lattice optical clock is built and characterized. All three laser setups for cooling and trapping the Yb atoms, together with the vacuum system, are contained on a transportable optical table.
The preparation of the atomic sample before doing spectroscopy on the clock transition starts by slowing the atoms in the Zeeman slower. This is followed by trapping the atoms in the pre-cooling and the post-cooling MOTs, before loading the atoms into the optical lattice. Slowing and pre-cooling the atoms is done using the strong transition at 399 nm. This cools the atoms down to around a mK. To successfully load the atoms in the optical lattice, we have to cool the atoms further to beyond 50 μK. This is done by exciting the weak transition at 556 nm. The atoms that are transferred into the post-cooling stage are cooled down to a few tens of μK. The atoms are then ready to be loaded into the optical lattice at the magic wavelength. The amplified laser light at 760 nm inside the 1-D intra-vacuum cavity results in a lattice trapping potential of approximately 50 μK. A transfer efficiency, from the MOT into the optical lattice, of more than 25% was measured.
The spectroscopy on the clock transition was done for 171Yb. A step-by-step mechanism was adopted, starting by exciting the forbidden transition at 578 nm inside the post-cooling stage MOT, by using the ”chopped-MOT” method. The 300 kHz Doppler-broadened line is then narrowed down to 3 kHz by switching on the light at 760 nm, while keeping the MOT on. The so-called ”chopped-lattice” technique helped suppress the Doppler-broadening; however the other effects due to the light and the magnetic fields of the MOT were still present. By switching off the MOT’s light and magnetic fields, the Yb atoms are completely trapped in the optical lattice. As a result, the FWHM of the spectroscopy line was reduced by a factor of 10.
We present an optical lattice clock based on ytterbium 171Yb atoms developed in the laboratories of INRIM. In the experiment, we cool and trap ytterbium atoms in a two stage magneto-optical trap (MOT) (at 399nm and 556nm for the first and second stage, respectively). Atoms are then transferred in a horizontal, one-dimensional optical lattice at the magic wavelength (759 nm). Here the clock transition at 578nm is probed by a laser stabilized on an ultra-stable cavity. We describe the generation of all the laser sources, the physic package and the operation of the clock. Lasers at 399 nm, 556nm and 578nm are obtained, with different techniques, using non-linear crystals starting from infrared sources. The clock laser is stabilized using a high finesse notched ULE cavity. The lattice is made with a titanium-sapphire laser. The aluminum vacuum chamber is designed for wide optical access and its temperature is measured by 8 thermistors for blackbody shift evaluation. Our system allows for fast loading of the lattice with 1 × 104atoms trapped in the lattice in 250 ms. We obtained preliminary spectroscopy results and we locked the clock laser to the atomic line. Future perspectives are discussed.
We present an upgraded setup for cooling and trapping of ytterbium atoms in a optical clock experiment. The experiment aims to cool and trap ytterbium atoms in a two stage magneto-optical trap (MOT) (at 399nm and 556nm for the first and second stage, respectively) and to probe the narrow-line clock transition at 578 nm in an optical lattice at the magic wavelength (759 nm). We describe here the generation of all the laser sources and the design of a new physic package, including the vacuum chamber and the atomic source.
A compact laser cooling system for transportable optical lattice clocks based on cold 171 Yb atoms has been presented. The system size has been reduced by eliminating the first cooling stage and using compact configurations for the main subsystems of the laser cooling system. A Zeeman slower has been modeled and developed, reducing the velocity of atoms from a thermal source to a level sufficient for their direct capture in a second-stage magneto-optical trap, avoiding the first cooling stage. A method for estimating the number of atoms and their temperature in the magneto-optical trap is presented. The performance of the compact laser cooling system with the developed Zeeman slower has been experimentally confirmed: ~10 6 171 Yb atoms with a temperature of ~35 μK have been collected in ~1 s. The achieved parameters allow for loading of ~4000 atoms into the optical lattice.
Optical lattice clocks based on ytterbium (Yb) and strontium (Sr) are emerging as key technologies in precision timekeeping and metrology. Recent advancements in miniaturization and transportability are making them increasingly accessible for applications outside traditional laboratory settings. However, the degree of miniaturization is limited by the atomic source that is used to provide laser-cooled atoms for the clock spectroscopy. Conventional atomic sources for optical lattice clocks consist of a high-power oven and a magneto-optical trap (MOT) that requires six incident laser beams for three-dimensional trapping and cooling. Here, we present technologies that allow a significant miniaturization of the required experimental setups to realize a MOT for Yb and Sr. These are a chip-based microstructured atomic oven and in-vacuum optical elements that require only a single incident laser beam to generate all of the required beams for three-dimensional trapping and cooling. The oven was manufactured with LIDE® (laser induced deep etching). It was paired with an aluminum pyramid reflector to load Yb atoms into a MOT. The oven was placed directly at the outside of the reflector, forming a compact setup that is used for atom evaporation and direct trapping and cooling. In this configuration, MOT loading rates above 10^8 atoms/s were measured for oven heating powers below 250 mW. Furthermore, the microstructured oven was paired with a grating MOT chip inside a highly compact vacuum chamber. The chamber was additively manufactured from titanium and the assembled setup spans a volume of less than 750 ml. In this setup, up to 10^5 Sr atoms were trapped in the MOT.
Atomic clocks are crucial for precise time dissemination, supporting industrial applications for navigation, telecommunication, the financial system, and the energy industry by ensuring accurate timekeeping and synchronization. At present, the best atomic clocks are based on optical transitions. Optical lattice clocks based on Yb or Sr achieve fractional frequency uncertainties in the low 10^(-18) regime. This remarkable accuracy allows resolving the gravitational redshift on the centimeter scale. This progress has driven the development of compact transportable optical lattice clocks, which are crucial for field- or even space applications. Especially for space operation, there is a critical need to minimize size, weight and power consumption of key components, particularly the source of laser-cooled atoms. Conventional atom sources for optical lattice clocks consist of an atomic oven with a high power consumption and a six-beam magneto-optical trap (MOT) that requires a large and complex optical setup. This thesis addresses the technical challenges associated with the development of compact sources of laser-cooled alkaline-earth-like atoms by demonstrating a miniaturized atomic oven and a miniaturized MOT for Yb and Sr. The MOT generation is tested with three types of in-vacuum optical elements that generate all the required laser beams for three-dimensional trapping and cooling with only one single input beam. A quasi-monolithic aluminum structure generating a conventional MOT beam geometry that is called pyramid reflector is compared to a quasi-planar optical element called Fresnel reflector. Both are characterized and compared using the same high-flux Yb source. Particularly, two-stage cooling of a fermionic alkaline-earth-like atom in a non-conventional MOT configuration is performed for the first time, demonstrating the applicability of planar MOT optics in optical lattice clocks. Furthermore, a planar diffraction grating is combined with the miniaturized atomic oven inside a highly compact vacuum chamber. The atomic oven is based on a circular fused silica chip and is tested by loading an Yb MOT generated with the pyramid reflector and a Sr MOT generated with the diffraction grating. It requires less than 250 mW of electrical heating power to generate a MOT loading rate above 10^8 atoms/s with the pyramid reflector. The presented components form a compact source of laser-cooled Yb or Sr atoms, and, when combined with a compact optical lattice setup, will serve as the basic building blocks of a future transportable optical lattice clock.
An optical atomic clock with 171 Yb atoms is devised and tested. By using a two-stage Doppler cooling technique, the 171 Yb atoms are cooled down to a temperature of 6±3 μK, which is close to the Doppler limit. Then, the cold 171 Yb atoms are loaded into a one-dimensional optical lattice with a wavelength of 759 nm in the Lamb—Dicke regime. Furthermore, these cold 171 Yb atoms are excited from the ground-state 1 S 0 to the excited-state 3 P 0 by a clock laser with a wavelength of 578 nm. Finally, the 1 S 0 – 3 P 0 clock-transition spectrum of these 171 Yb atoms is obtained by measuring the dependence of the population of the ground-state 1 S 0 upon the clock-laser detuning.