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Chinese Physics B

Effective sideband cooling in an ytterbium optical lattice clock

Jin-Qi Wang, Ang Zhang, Cong-Cong Tian, Ni Yin, Qiang Zhu, Bing Wang, Zhuan-Xian Xiong, Lingxiang He, Bao-Long Lv

Published Feb 10, 2022• 3 citations
Publisher

Abstract

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.

DOI
10.1088/1674-1056/ac5392

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Chinese Physics B•Aug 1, 2017•2 citations
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.
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.
Abstract Effcient population transfer and motional state control are important for optical lattice clocks and precision experiments with lattice-trapped neutral atoms. Here we experimentally investigate adiabatic rapid passage (ARP) on the clock transition of a one-dimensional 87 Sr optical lattice clock and develop a mode-resolved model for ARP on motional sidebands. On the carrier transition, ARP enables robust internal-state transfer with a maximum excitation fraction of 0.98(2). On the blue motional sideband, ARP increases the maximum excitation fraction from 0.47(2) to 0.55(3), and the measured dynamics are consistently reproduced by the mode-resolved model. The model further predicts that the sideband transfer effciency can exceed 0.88 when the carrier Rabi frequency is increased to 15 kHz. Applying ARP to the red-sideband cooling reduces the axial mean vibrational occupation to approximately 0.038, corresponding to a motional ground-state population of 0.96. These results provide a quantitative basis for designing ARP excitation on motional sidebands and for optimizing sideband cooling in lattice-trapped atoms, with direct relevance to coherent state preparation in optical lattice clocks and related precision-measurement experiments in neutral atoms.
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
Efficient cooling of trapped ions is a prerequisite for various applications of the ions in precision spectroscopy, quantum information, and coherence control. Raman sideband cooling is an effective method to cool the ions to their motional ground state. We investigate both numerically and experimentally the optimization of Raman sideband cooling strategies and propose an efficient one, which can simplify the experimental setup as well as reduce the number of cooling pulses. Several cooling schemes are tested and compared through numerical simulations. The simulation result shows that the fixed-width pulses and varied-width pulses have almost the same efficiency for both the first-order and the second-order Raman sideband cooling. The optimized strategy is verified experimentally. A single 25Mg+{^{25}\mathrm{Mg}}^{+}25Mg+ ion is trapped in a linear Paul trap and Raman sideband cooled, and the achieved average vibrational quantum numbers under different cooling strategies are evaluated. A good agreement between the experimental result and the simulation result is obtained.
Efficient high-fidelity ground-state cooling of motional degrees of freedom is crucial for applications in quantum simulation, computing, and metrology. Here, we demonstrate direct ground-state cooling of fermionic Yb 171 and bosonic Yb 174 atoms in two- and three-dimensional magic-wavelength optical lattices on the ultranarrow clock transition. Its high spectral resolution offers the potential for reaching extremely low temperatures. To ensure efficient cooling, we develop a chirped sideband cooling scheme, where we sweep the clock-laser frequency to mitigate the effects of spatial trap inhomogeneities. We further generalize the theoretical description of sideband spectra to higher-dimensional lattices for precise thermometry. We achieve 2D ground-state fractions of 97 % for Yb 171 with an average motional occupation of n ¯ ≃ 0.015 and provide a direct comparison with Yb 174 , reaching similar cooling performance. Applying the same scheme in 3D results in n ¯ ≃ 0.15 limited by layer-to-layer inhomogeneities in the vertical direction. These results demonstrate efficient motional ground-state cooling in optical lattices, especially for bosonic alkaline-earth(-like) atoms, where other methods are not applicable, opening the door to novel protocols for quantum science applications with neutral atoms.
We develop a simple and practical scheme to apply sideband cooling to a cloud of rubidium atoms. A sample containing 4 × 10 70 87 Rb is trapped in a far red detuned optical lattice. Through optimizing the relevant parameters, i.e., laser detuning, magnetic field, polarization, and duration time, a temperature around 1.5 μK and phase space density close to 1/500 are achieved. Compared with polarization gradient cooling, the temperature decreases by around one order of magnitude. This technique could be used in high precision measurement such as atomic clocks and atom interferometer. It could also serve as a precooling means before evaporation cooling in a dipole trap, and may be a promising method of achieving quantum degeneracy with purely optical means.