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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Thermal dynamics of cold atoms in171Yb optical lattice clocks

Chengyin Han, Min Zhou, Xiaohang Zhang, Qi Gao, Yilin Xu, Peng Xu, Shangyan Li, Shuang Zhang, Xinye Xu

Published May 1, 2016• 0 citations
Publisher

Abstract

The ultracold atoms served as quantum references is a key part of optical lattice clocks, and the thermal dynamics of the atoms in the optical lattice affects the uncertainty and stability of the lattice clocks. Since the carrier spectrum of clock transition in lattices reflects the thermal dynamics of cold atoms, we can obtain the temperature of atoms by measuring the carrier spectrum in171Yb optical lattice clocks. Furthermore we find that the temperatures obtained from the carrier spectra are in a good agreement with the results obtained by using methods of time-of-flight and thermometry based on sideband spectrum. In addition, we study the heating effects caused by the lattice laser in ytterbium optical lattice clocks.

DOI
10.1109/fcs.2016.7563584

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The ultracold atoms served as quantum references is a key part of optical lattice clocks, and the thermal dynamics of the atoms in the optical lattice affects the uncertainty and stability of the lattice clocks. Since the carrier spectrum of clock transition in lattices reflects the thermal dynamics of cold atoms, we can obtain the temperature of atoms by measuring the carrier spectrum in171Yb optical lattice clocks. Furthermore we find that the temperatures obtained from the carrier spectra are in a good agreement with the results obtained by using methods of time-of-flight and thermometry based on sideband spectrum. In addition, we study the heating effects caused by the lattice laser in ytterbium optical lattice clocks.
The ultracold atomic gas serving as the quantum reference is a key part of an optical lattice clock, and the temperature of atoms in the optical lattice affects the uncertainty and instability of the optical lattice clocks. Since the carrier spectrum of the clock transition in the lattices reflects the thermal dynamics of cold atoms, the temperature of atoms can be extracted from the carrier spectrum in a non-magic wavelength lattice of ytterbium optical clocks. Furthermore, the temperatures obtained from the carrier spectra are in good agreement with the results obtained by the time-of-flight method and thermometry based on the sideband spectrum. In addition, the heating effects caused by the lattice laser are studied on the basis of the sample temperatures.
For atoms or molecules in optical lattices, conventional thermometry methods are often unsuitable due to low particle numbers or a lack of cycling transitions. However, a differential spectroscopic light shift can map temperature onto the line shape with a low sensitivity to trap anharmonicity. We study narrow molecular transitions to demonstrate precise frequency-based lattice thermometry, as well as carrier cooling. This approach should be applicable down to nanokelvin temperatures. We also discuss how the thermal light shift can affect the accuracy of optical lattice clocks.
Accurate measurement of atomic temperature is fundamental for a wide range of applications, from quantum sensing to precision metrology. In optical lattice clocks, precise characterization of atomic temperature is required to minimize systematic uncertainties at the 10−1810^{-18}10−18 level. In this work, we investigate atomic temperature measurements in the ytterbium optical clock developed at INRIM, IT-Yb1, employing sideband and transverse Doppler spectroscopy across a wide range of trapping conditions in a one-dimensional optical lattice. By implementing clock-line-mediated Sisyphus cooling [Phys. Rev. Lett. 133, 053401 (2024)], we reduce the atomic temperature and enable operation at shallower lattice depths down to D=50 ERD = 50\,E_\text{R}D=50ER​, where ERE_\text{R}ER​ is the photon recoil energy. We compare temperature estimates obtained from the perturbed harmonic oscillator model with those derived using a Born-Oppenheimer model [Phys. Rev. A 101, 053416 (2020)]. Discrepancies of up to a factor of four are observed between the temperature estimates obtained with the two approximations. We assess the impact of these modeling differences on the evaluation of lattice light shifts and find relative frequency deviations of up to 8×10−178\times10^{-17}8×10−17. Although sufficiently low atomic temperatures and the consistent use of theoretical models reduce these discrepancies to the low 10−1810^{-18}10−18 level, these results motivate further investigations that will be relevant to state-of-the-art optical lattice clocks.
We have investigated the frequency shifts caused by inhomogeneous excitation in a 171Yb optical lattice clock. The dependences of the inhomogeneity on the temperature of the cold ytterbium atoms and the misaligning angle between the lattice laser and the clock laser are analyzed by numerical calculations. The dependence of the fractional collisional frequency shift on the ground state fraction under different cold atom temperatures, atom numbers, lattice trap depths and unequal transverse and longitudinal temperatures are also shown. The results show that the uncertainty of the ytterbium clocks, contributed by the inhomogeneous excitation, can be reduced to be 10−19 or even lower with certain conditions.
An optical lattice clock probes a spectrally narrow electronic transition in an ensemble of optically trapped, laser-cooled atoms, for use as a time and frequency standard. To date, several lattice clocks have been demonstrated with superior stability and accuracy compared to primary frequency standards based on microwave transitions. Yet, the question of which atomic system (including the element and isotope) performs best as a lattice clock remains unsettled. This thesis describes the first detailed investigation of an optical lattice clock using a spin-1/2 isotope of the ytterbium atom. A spin-1/2 system possesses several advantages over higher-spin systems, including a simplified level structure (allowing for straightforward manipulation of the nuclear spin state) and the absence of any tensor light shift from the confining optical lattice. Moreover, the ytterbium atom (Yb) stands among the leading lattice clock candidates, offering a high-performance optical clock with some degree of experimental simplicity. The frequency stability of the Yb clock is highlighted by resolving an ultra-narrow clock spectrum with a full-width at half-maximum of 1 Hz, corresponding to a record quality factor Q = ν0/Δν = 5 × 1014. Moreover, this system can be highly accurate, which is demonstrated by characterizing the Yb clock frequency at the 3 × 10-16 level of fractional uncertainty, with further progress toward a ten-fold improvement also presented. To reach this low level of uncertainty required careful consideration of important systematic errors, including the identification of the Stark-canceling wavelength, where the clock’s sensitivity to the lattice intensity is minimized, a precise determination of the static polarizability of the clock transition, and the measurement and control of atom-atom collisions.
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.