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 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.
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.
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 constitute a fundamental tool for time and frequency metrology and their application is widespread in many technological fields. In particular, the International System of units (SI) defines the second on a microwave transition of Caesium atoms. The realization is made by clocks reaching uncertainties of few parts in 10^−16, making the second to be the quantity realized with the smallest uncertainty in the SI. However, a new generation of atomic clocks, called optical clocks, have already demonstrated to surpass Caesium standards both in accuracy and stability. The research performed during my PhD activity has been focused on the development and characterization of Ytterbium (Yb) optical lattice clocks. These systems operate with a large number of ultra-cold neutral atoms having a clock transition in the visible region of the electromagnetic spectrum. The atomic sample is trapped in an periodical optical potential called optical lattice that gives the advantage to interrogate many quantum absorbers for an extended time, with small perturbations, allowing to achieve an unprecedented stability and accuracy. The main experimental work has been carried out in the laboratories of the Istituto Nazionale di Ricerca Metrologica (INRIM) in Turin, where several atomic clocks are present including the Italian primary frequency standard, the Caesium fountain ITCsF2, and where an Yb optical lattice clock is currently being developed. This thesis describes the functioning of the system along with the full characterization of systematic effects, the complete uncertainty budget and its first absolute frequency measurement against the primary frequency standard. The clock exhibited an accuracy of 1.6 × 10^−16 and the comparison with the Cs fountain resulted in a frequency of f171Yb = 518 295 836 590 863.59(31) Hz, limited by the fountain uncertainty. This measurement is in agreement with the ytterbium frequency recommended as a secondary representation of the second in the SI and constitutes the first measurement of a Yb clock in Europe and the second one in the world against a primary frequency standard. Several upgrades have been applied after the absolute measurement. In particular, the design and realization a system capable to frequency stabilize several lasers on a single optical cavity is illustrated. This cavity has been implemented to lock the lasers used to cool and trap the atomic sample at 399, 556 and 759nm using the offset sideband locking technique, a modified version of the Pound–Drever–Hall method that gives an extended frequency tunability. The system proved to be an easy-to-use and reliable tool for the experimental activity showing a linewidth below 300 Hz at 556nm, which is the wavelength that requires the most stringent performance, and a long term drift below 20 kHz per day at 759nm. That is suitable for operating the lattice laser with a light shift uncertainty below 1 × 10−18. During my PhD I have been guest researcher at the National Institute of Standards and Technology (NIST) of Boulder, Colorado, for nine months in 2016. In these laboratories two Yb optical lattice clocks are operative. I worked on the instability measurement of a composite system exploiting the two clocks to suppress the Dick effect, called zero-dead-time (ZDT) clock, which demonstrated a fractional instability of 3 × 10−17 at 1s. The two clocks can also be operated to extend the interrogation time obtaining a spectroscopic feature after 4 s of 120(20)mHz corresponding to a quality factor Q > 4 × 10^15. I also worked on the characterization of several systematic shifts that allowed to complete the uncertainty budget of the clocks at 1.6 × 10^−18. In particular, I contributed to the characterization of lattice light shifts considering the effect of atomic sample temperature and the identification of a metrological regime called operational magic frequency where frequency shifts are insensitive to changes in trap depth.
Experiments on trapping ytterbium atoms in various optical lattices are presented. After the two-stage cooling, first in a blue magneto—optical trap and then in a green magneto—optical trap, the ultracold 171 Yb atoms are successfully loaded into one-, two-, and three-dimensional optical lattices operating at the Stark-free wavelength, respectively. The temperature, number, and lifetime of cold 171 Yb atoms in one-dimensional lattice are measured. After optimization, the one-dimensional lattice with cold 171 Yb atoms is used for developing an ytterbium optical clock.
Atoms confined in the optical lattice can be interrogated with Doppler- and recoil-free operation. However, if not properly controlled, the optical lattice may limit clock accuracy. To improve the lattice-light-shift uncertainty, the cooling and trapping lasers' frequency stability is optimized, and the atom's signal stability is enhanced. A ring-cavity Ti:sapphire laser is locked to the optical frequency comb, which is referenced to a 578 nm ultra-stable laser, and the beat note's stability is on the order of 10−16. Using a 10 cm Fabry–Pérot cavity referenced to the Ti:sapphire laser, the optical frequency stability is transferred to the 399 nm cooling laser, creating favorable conditions for evaluating the lattice-light-shift accurately. We reevaluate lattice-light-shift in our 171Yb optical lattice clock with an uncertainty of 8.1 × 10−18, which is an order lower than our previous result, and the magic frequency is determined to be 394 798 266.6(1.3) MHz.