New frequency ratios with a PHz-scale atomic clock

Atomic clocks are a tour de force when it comes to rigorous tests of measurement. The ultimate validation of one's careful assessments is to find agreement on a given parameter with a completely independent laboratory. Frequency ratios between clock transitions of different atomic species make for quintessential tests of measurement precision. Tyumenev et al (2016 New J. Phys. 18 113002) report on frequency ratio measurements between a ¹⁹⁹Hg optical lattice clock and three other atomic frequency standards: ¹³³Cs, ⁸⁷Rb and ⁸⁷Sr, two of which are unprecedented in accuracy. Most notable is the level of agreement for the ¹⁹⁹Hg/⁸⁷Sr frequency ratio found between two independent laboratories at further indication that optical lattice clocks are fulfilling their early expectations.