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arXiv:1806.02260 · IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control

Dual-Mode Operation of an Optical Lattice Clock Using Strontium and Ytterbium Atoms

Daisuke Akamatsu, Takumi Kobayashi, Yusuke Hisai, Takehiko Tanabe, Kazumoto Hosaka, Masami Yasuda, Feng-Lei Hong

Published Mar 26, 2018• 17 citations
arXivHTMLPDF

Abstract

We have developed an optical lattice clock that can operate in dual modes: a strontium (Sr) clock mode and an ytterbium (Yb) clock mode. Dual-mode operation of the Sr-Yb optical lattice clock is achieved by alternately cooling and trapping87Sr and171Yb atoms inside the vacuum chamber of the clock. Optical lattices for Sr and Yb atoms were arranged with horizontal and vertical configurations, respectively, resulting in a small distance of the order of 100 μm between the trapped Sr and Yb atoms. The1S0-3P0clock transitions in the trapped atoms were interrogated in turn and the clock lasers were stabilized to the transitions. We demonstrated the frequency ratio measurement of the Sr and Yb clock transitions by using the dual-mode operation of the Sr-Yb optical lattice clock. The dual-mode operation can reduce the uncertainty of the blackbody radiation shift in the frequency ratio measurement, because both Sr and Yb atoms share the same blackbody radiation.

DOI
10.1109/tuffc.2018.2819888

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Chinese Physics Letters•Oct 1, 2016•9 citations
We have developed an optical lattice clock that can operate in dual modes: a strontium (Sr) clock mode and an ytterbium (Yb) clock mode. Dual-mode operation of the Sr-Yb optical lattice clock is achieved by alternately cooling and trapping87Sr and171Yb atoms inside the vacuum chamber of the clock. Optical lattices for Sr and Yb atoms were arranged with horizontal and vertical configurations, respectively, resulting in a small distance of the order of 100 μm between the trapped Sr and Yb atoms. The1S0-3P0clock transitions in the trapped atoms were interrogated in turn and the clock lasers were stabilized to the transitions. We demonstrated the frequency ratio measurement of the Sr and Yb clock transitions by using the dual-mode operation of the Sr-Yb optical lattice clock. The dual-mode operation can reduce the uncertainty of the blackbody radiation shift in the frequency ratio measurement, because both Sr and Yb atoms share the same blackbody radiation.
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.
Laser frequency stabilization and atom manipulation are important techniques for optical frequency metrology. We report on the development of an ytterbium (171Yb) and a strontium (87Sr) optical lattice clock at the National Metrology Institute of Japan (NMIJ). The result of absolute frequency measurement of the171Yb lattice clock is shown. We also report on narrow-linewidth fiber-based frequency combs that are used not only to measure the absolute frequency of the optical lattice clocks but also to servo control the frequency of the lasers for cooling and trapping Yb and Sr atoms.
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.
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.
We report on the magic wavelength measurement of our optical lattice clock based on fermion strontium atoms at the National Institute of Metrology (NIM). A Ti:sapphire solid state laser locked to a reference cavity inside a temperature-stabilized vacuum chamber is employed to generate the optical lattice. The laser frequency is measured by an erbium fiber frequency comb. The trap depth is modulated by varying the lattice laser power via an acousto-optic modulator. We obtain the frequency shift coefficient at this lattice wavelength by measuring the differential frequency shift of the clock transition of the strontium atoms at different trap depths, and the frequency shift coefficient at this lattice wavelength is obtained. We measure the frequency shift coefficients at different lattice frequencies around the magic wavelength and linearly fit the measurement data, and the magic wavelength is calculated to be 368554672(44) MHz.