Transportable strontium lattice clock with 4 × 10-19 blackbody radiation shift uncertainty

Abstract We describe a transportable optical lattice clock based on the 1 S 0 → 3 P 0 transition of lattice-trapped 87 Sr atoms with a total systematic uncertainty of 2.1 × 10 -18 . The blackbody radiation shift, which is the leading systematic effect in many strontium lattice clocks, is controlled at the level of 4.0 × 10 -19 , as the atoms are interrogated inside a well-characterised, cold thermal shield. Using a transportable clock laser, the clock reaches a frequency instability of about 5 × 10 -16 /√(τ/s), which enables fast reevaluations of systematic effects. By comparing this clock to the primary caesium fountain clocks CSF1 and CSF2 at Physikalisch-Technische Bundesanstalt, we measure the clock transition frequency with a fractional uncertainty of 1.9 × 10 -16 , in agreement with previous results. The clock was successfully transported and operated at different locations. It holds the potential to be used for geodetic measurements with centimetre-level or better height resolution and for accurate inter-institute frequency comparisons.