Absolute frequency measurement of the 40 Ca+ clock transition using a GNSS link to the SI second

We report the absolute frequency measurement of the 4s squaredS 1/2 3d squaredD 5/2 40 Ca+ clock transition with respect to the SI second. To perform this measurement, a link between our laboratory in Innsbruck and the clocks realizing the Coordinated Universal Time at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig was installed and characterized using the Global Navigation Satellite System GNSS. The comparison between our clock and the ones at PTB was done using the Precise Point Positioning technique. After the evaluation of the systematic shifts, the measured transition frequency is 411 042 129 776 401.2 plus or minus 0.6 Hz with a fractional uncertainty of 1.5 times 10-15. The stability of the clock measurements was also corroborated by comparing two different calcium ion clock experiments, which share the clock laser source at our institute. Furthermore, after careful evaluation of the trap-drive induced ac magnetic fields, we estimate ac Zeeman shifts on the D 5/2 sublevels and reevaluate the Landé g-factor of the 3d squaredD 5/2 level to be g 5/2= 1.200329(1).