Remote frequency measurement of the 1S 0→ 3P 1 transition in laser-cooled 24Mg

We perform Ramsey-Bordé spectroscopy on laser-cooled magnesium atoms in free fall to measure the 1S 0→ 3P 1 intercombination transition frequency. The measured value of 655 659 923 839 730 (48) Hz is consistent with our former atomic beam measurement (Friebe et al 2008 Phys. Rev. A 78 033830). We improve upon the fractional accuracy of the previous measurement by more than an order of magnitude to 7×10 -14. The magnesium frequency standard was referenced to a fountain clock of the Physikalisch Technische Bundesanstalt (PTB) via a phase-stabilized telecom fiber link and its stability was characterized for interrogation times up to 8000 s. The high temperature of the atomic ensemble leads to a systematic shift due to the motion of atoms across the spectroscopy beams. In our regime, this leads to a counterintuitive reduction of residual Doppler shift with increasing resolution. Our theoretical model of the atom-light interaction is in agreement with the observed effect and allows us to quantify its contribution in the uncertainty budget. DFG/EXC/QUEST