Campbell, Gretchen K.2; Ludlow, Andrew D.2; Blatt, Sebastian2; Thomsen, Jan W.3; Martin, Michael J.2; de Miranda, Marcio H. G.2; Zelevinsky, Tanya2; Boyd, Martin M.2; Ye, Jun2; Diddams, Scott A.2; Heavner, Thomas P.2; Parker, Thomas E.2; Jefferts, Steven R.2
1 Quantop, The Niels Bohr Institute, Faculty of Science, Københavns Universitet2 unknown3 Quantop, The Niels Bohr Institute, Faculty of Science, Københavns Universitet
The absolute frequency of the 1S0–3P0 clock transition of 87Sr has been measured to be 429 228 004 229 873.65 (37) Hz using lattice-confined atoms, where the fractional uncertainty of 8.6 × 10-16 represents one of the most accurate measurements of an atomic transition frequency to date. After a detailed study of systematic effects, which reduced the total systematic uncertainty of the Sr lattice clock to 1.5 × 10-16, the clock frequency is measured against a hydrogen maser which is simultaneously calibrated to the US primary frequency standard, the NIST Cs fountain clock, NIST-F1. The comparison is made possible using a femtosecond laser based optical frequency comb to phase coherently connect the optical and microwave spectral regions and by a 3.5 km fibre transfer scheme to compare the remotely located clock signals.