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Trapping and Laser Cooling an Ensemble of Ytterbium-171 Atoms for use in an Atomic Clock

Velez, Gustavo A.

Published May 1, 2025• 0 citations
OpenAlex

Abstract

Optical lattice clocks require careful preparation of atomic ensembles in order to ensure homogeneous interactions with the clock laser. We demonstrate loading and laser cooling of an ensemble of ytterbium-171 atoms in a 2D optical dipole trap created by an optical cavity. Our loading method ensures that all atoms are located in the intersection of 2 perpendicular dipole traps as verified through absorption imaging. Raman sideband cooling was used to cool the atomic ensemble from 15.7 uK to 6.3 uK as measured through optical sideband spectroscopy on the 578 nm clock transition. Together, these steps improved the transfer of atoms during a Rabi oscillation from the ground to the clock state from approximately 45 percent excitation fraction to 80 percent excitation fraction. The final atomic ensemble preparation is now sufficient for running an atomic clock.

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Optical lattice clocks require careful preparation of atomic ensembles in order to ensure homogeneous interactions with the clock laser. We demonstrate loading and laser cooling of an ensemble of ytterbium-171 atoms in a 2D optical dipole trap created by an optical cavity. Our loading method ensures that all atoms are located in the intersection of 2 perpendicular dipole traps as verified through absorption imaging. Raman sideband cooling was used to cool the atomic ensemble from 15.7 uK to 6.3 uK as measured through optical sideband spectroscopy on the 578 nm clock transition. Together, these steps improved the transfer of atoms during a Rabi oscillation from the ground to the clock state from approximately 45 percent excitation fraction to 80 percent excitation fraction. The final atomic ensemble preparation is now sufficient for running an atomic clock.
Sideband cooling is a key technique for improving the performance of optical atomic clocks by preparing cold atoms and single ions into the ground vibrational state. In this work, we demonstrate detailed experimental research on pulsed Raman sideband cooling in a 171Yb optical lattice clock. A sequence comprised of interleaved 578 nm cooling pulses resonant on the 1st-order red sideband and 1388 nm repumping pulses is carried out to transfer atoms into the motional ground state. We successfully decrease the axial temperature of atoms in the lattice from 6.5 μK to less than 0.8 μK in the trap depth of 24 μK, corresponding to an average axial motional quantum number 〈 nz 〉 < 0.03. Rabi oscillation spectroscopy is measured to evaluate the effect of sideband cooling on inhomogeneous excitation. The maximum excitation fraction is increased from 0.8 to 0.86, indicating an enhancement in the quantum coherence of the ensemble. Our work will contribute to improving the instability and uncertainty of Yb lattice clocks.
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.
The strong coupling of atoms to optical cavities can improve optical lattice clocks as the cavity enables metrologically useful collective atomic entanglement and high-fidelity measurement. To this end, it is necessary to cool the ensemble to suppress motional broadening, and advantageous to maximize and homogenize the atom-cavity coupling. We demonstrate resolved Raman sideband cooling via the cavity as a method that can simultaneously achieve both goals. In 200 ms of Raman sideband cooling, we cool Yb 171 atoms to an average vibration number 〈 n x 〉 = 0.23 ( 7 ) in the tightly binding direction, resulting in 93 % optical π -pulse fidelity on the clock transition S 0 1 → P 0 3 . During cooling, the atoms self-organize into locations with maximal atom-cavity coupling, which will improve quantum metrology applications. Published by the American Physical Society 2024
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.
Because of its simple structure, the hydrogen atom is often used as a testbed for quantum electrodynamics. Spectroscopy of trapped atomic samples can greatly improve the accuracy of these tests. Trapping atomic hydrogen in an optical dipole trap or an optical lattice has never been achieved. Only trapping in magnetic fields that lead to large Zeeman shifts has been demonstrated. Standard techniques of atomic physics are difficult to apply to atomic hydrogen. The small mass of the atom and the large photon energy of the 1-2 cooling transition significantly complicate Doppler cooling. This proposal introduces a photon recoil-assisted loading scheme that uses these properties to our advantage to load atomic hydrogen into an optical dipole trap without laser cooling. The magic wavelength (515 nm) for the 1-2 clock transition (1.3-Hz natural linewidth) is easily accessible with current laser technology. Since the 1-2 clock transition can be driven Doppler free, we do not require a very low temperature. Besides improving spectroscopy for fundamental science, such a system can also be used as a “computable” atomic clock that may one day justify the redefinition of the SI second in terms of the Rydberg constant.
We report the realization of ytterbium magneto-optical trap (MOT) operating on the dipole-allowed 1 S 0 — 1 P 1 transition at 398.9 nm. The MOT is loaded by a slowed atomic beam produced by a Zeeman slower. All seven stable isotopes of Yb atoms could be trapped separately at different laser detuning values. Over 10 7 174 Yb atoms are collected in the MOT, whereas the atom number of fermionic isotope 171 Yb is roughly 2.3 × 10 6 due to a lower abundance. Without the Zeeman slower, the trapped atom numbers are one order of magnitude lower. Both the even and odd isotopes are recognized as excellent candidates of optical clock transition, so the cooling and trapping of ytterbium atoms by the blue MOT is an important step for building an optical clock.