Progress towards a two-photon clock based on laser-cooled calcium

The remarkable and steady improvements in the accuracy of optical clocks have greatly advanced the state of the art in time and frequency metrology. In addition to the accuracy of these clocks, the high precision available from them can open up new classes of measurements. In particular, optical clocks with high signal-to-noise ratios (SNR) can be used as components of sensitive antennae for micro-hertz-band gravitational waves [1], [2]. Such clocks could also be useful for field applications such as geodesy [3], and as transportable secondary frequency standards to enable comparisons between distant primary standards [4].