Hyperpolarizability effects and accuracy evaluation of a 87Sr optical lattice clock

The authors report the observation of the hyperpolarizability frequency shift due to the trapping field in a ⁸⁷Sr optical lattice clock. The authors show that at the magic wavelength of the lattice, where the first order term cancels, this higher order shift will not constitute a limitation to the fractional accuracy of the clock down to the 10 _-18 level. This result is achieved by operating the clock at very high trapping intensity up to 400 kW/cm² and by a specific study of the effect of the two two-photon transitions near the magic wavelength. The authors also report an accurate frequency measurement of the clock transition. The frequency is determined to be v_1S0-³P₀ = 429 228 004 229 879 (5) Hz with a fractional uncertainty that is comparable to state-of-the-art optical clocks with neutral atoms in free fall