Improved Frequency Measurement of the 1S0–3P0 Clock Transition in 87Sr Using a Cs Fountain Clock as a Transfer Oscillator

We performed an absolute frequency measurement of the ¹S₀-³P₀ transition in ⁸⁷Sr with a fractional uncertainty of 1.2 × 10⁻¹⁵, which is less than one third that of our previous measurement. A caesium fountain atomic clock was used as a transfer oscillator to reduce the uncertainty of the link between a strontium optical lattice clock and the SI second. The absolute value of the transition frequency is 429 228 004 229 873.56(49) Hz.