Work package leader: Dr Ian Hill, NPL Other participants: INRIM, LUH, OBSPARIS, PTB, UMK The aim of this work package is to develop and apply interrogation methods that boost clock accuracy and stability, and, together with the advancements made throughout the other technical work packages, firmly establish the operation of optical clocks at the 10-18 level by direct comparison of co-located clock systems. Hyper-Ramsey pulse sequencefrom PTB Presently, optical clocks generally use a simple implementation of Rabi spectroscopy, but more sophisticated interrogation schemes have been proposed. We aim to implement modified hyper-Ramsey schemes to be able to control probe-induced shifts to below 1 x 10‑18. We also plan to explore interrogation methods that deal better with local oscillator noise. In particular, optical lattice clocks can benefit greatly from a reduced dead-time interrogation, which in the limit of uninterrupted operation would eliminate noise due to stroboscopic sampling effects of the local oscillator noise (Dick effect). To verify the performance of optical clocks at a level of 10‑18 the project will go beyond estimated systematic uncertainties and make direct measurements of the clock outputs. This requires two clocks to be built at this level and then compared.
