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Absolute frequency measurement of the 1S0–3P0 transition of 171Yb with a link to international atomic time
Metrologia, Volume 57, Number 3
https://iopscience.iop.org/article/10...

We report the absolute frequency measurement of the unperturbed optical clock transition 1S0–3P0 in 171Yb performed with an optical lattice frequency standard. Traceability to the International System of Units is provided by a link to International Atomic Time. The measurement result is 518 295 836 590 863.61(13) Hz with a relative standard uncertainty of , obtained operating our 171Yb optical frequency standard intermittently for 5 months. The 171Yb optical frequency standard contributes with a systematic uncertainty of .
1. Introduction
Cesium fountains are the best realization of the second in the International System of Units (SI) [1]. Frequency standards based on optical transitions of several ions and atoms can outperform Cs standards in accuracy and stability [2–4] and a redefinition of the SI second based on an optical transition is anticipated [5]. In preparation, eight optical transitions are recommended as secondary representation of the SI second [6], with uncertainties comparable to those of Cs standards. Their values are calculated by a least square fit of absolute frequency measurements and frequency ratios involving optical frequency standards [7]. New ratio and frequency measurements are fundamental to improve the uncertainty of secondary representations of the second and to check the consistency of optical frequency standards.

Absolute frequency measurements are usually performed relative to Cs fountains that provide the local realization of the SI second. When a local Cs fountain is unavailable, absolute frequency measurements of optical standards are possible via international atomic time (TAI) [8–19]. TAI is a timescale maintained by the International Bureau of Weights and Measures (BIPM) from the satellite-based comparison of frequency standards in about 85 world-wide laboratories [20]. The BIPM computes TAI in 5-day intervals and publishes monthly its frequency deviation from the SI second in the Circular T bulletin [21]. The BIPM also disseminates coordinated universal time (UTC), the international timescale recommended for civil use, which differs from TAI only by an integer number of leap seconds [20].

Among the optical secondary representations of the second is the frequency of the forbidden 1S0–3P0 transition in 171Yb. Here we present a measurement of this frequency obtained operating a 171Yb optical frequency standard for 5 months where traceability to the SI is provided by a link to TAI. During this period, the 171Yb optical frequency standard was operated only intermittently for a few hours at a time. Without a continuous measurement of the optical frequency [24], we used flywheels based on a hydrogen maser and TAI to account for the correction and uncertainty introduced by dead times in the operation of the optical frequency standard [11, 25, 26].

2. Experiment overview
Our 171Yb optical lattice frequency standard has been described previously in [27]. A beam of Yb atoms is produced in an ultra-high-vacuum chamber from an atomic oven. Atoms from the beam are trapped and cooled in a two-stage magneto-optical trap, first using the 1S0–1P1 transition at 399 nm and then using the weaker 1S0–3P1 transition at 556 nm. A slower beam at 399 nm counter-propagating the atomic beam is used to increase the number of trapped atoms. Atoms are then loaded in a horizontal, one-dimensional optical lattice. Approximately 1000 atoms are trapped in about 1200 lattice sites with a temperature of 10 ${\rm \mu}$ K. Atoms are prepared in either single-spin ground state ($m_F = \pm 1/2$ ) with 98% efficiency by optical pumping on the 1S0–3P1 transition. The clock laser at 578 nm is obtained by second-harmonic generation of a diode laser at 1156 nm. It is stabilized on a horizontal ultrastable cavity made in ultra-low-expansion glass with a length of 10 cm [30].

We achieved traceability of the 171Yb frequency to the SI through the chain shown in figure 1. The 1156 nm cavity-stabilized laser is sent to a fibre frequency comb [31] by a noise-compensated fibre link [32]. The comb has a repetition rate of 250 MHz and is referenced to the 10 MHz output of a hydrogen maser. Fibre links and acousto-optic modulators are referenced to the hydrogen maser as well. The beatnote between the laser and the comb is redundantly measured to detect and remove cycle slips. The frequency ratio between the 171Yb transition and the hydrogen maser frequency is calculated from the comb measurement, accounting for the acousto-optic modulator used for steering and the second-harmonic generation stage.