Ultrastable laser with average fractional frequency drift rate below 5 × 10−19_s

Cryogenic single-crystal optical cavities have the potential to provide high dimensional stability. We have investigated the long-term performance of an ultrastable laser system that is stabilized to a single-crystal silicon cavity operated at 124 K. Utilizing a frequency comb, the laser is compared to a hydrogen maser that is referenced to a primary caesium fountain standard and to the Sr87 optical lattice clock at Physikalisch-Technische Bundesanstalt (PTB). With fractional frequency instabilities of σy(τ)≤2×10−16 for averaging times of τ=60  s to 1000 s and σy(1  d)≤2×10−15 the stability of this laser, without any aid from an atomic reference, surpasses the best known microwave standards for short averaging times and is competitive with the best known hydrogen masers for longer times of 1 day. The comparison of modeled thermal response of the cavity with measured data indicates an average fractional frequency drift below 5×10−19/s, which we do not expect to be a fundamental limit.