Abstract

The ¹⁷¹Yb⁺ ion possesses two transitions that are applicable as references for optical frequency standards and several general features make this ion attractive: All relevant radiations for cooling and excitation can be generated from diode lasers. A nuclear spin of 1/2 ensures a simple sublevel structure. Long storage times of Yb⁺ in ion traps are easily obtained. The ²S_1/2(F = 0) → ²D_3/2(F = 2,m_F = 0) electric quadrupole transition at 436 nm with a natural linewidth of 3.1 Hz provides a quantum noise limited instability of a single-ion frequency standard of about 2 × 10⁻¹⁵ in 1 s. The electric octupole transition ²S_1/2(F = 0)→²F_7/2(F = 3,m_F = 0) at 467 nm with a natural linewidth in the nHz range will allow to surpass that instability level. It also possesses a lower sensitivity to field-induced frequency shifts [1]. In this system the Fourier-limited interrogation linewidth can be adapted to that of the best available laser sources, which are presently at the level below 1 Hz. Both transitions have been recommended as secondary representations of the SI second.

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