Comparison of optical frequency standards is essential for verifying that the clocks both locally and remotely at other national metrological institutes all agree to within their quoted uncertainties. These quoted uncertainties can be as low as ~1e-18, and so agreement between multiple clocks all operating at this level is required to verify that uncertainty. Remote clock comparisons at NPL, or between NPL and other European NMIs, are only possible at this uncertainty using optical frequency combs and telecoms optical fibre links to compare and transmit frequencies over long distances.

By using an optical frequency comb with the full flexibility to cover the 500 nm to 2000 nm wavelength range, the infrastructure will be in place to enable any of the likely candidates for an optical representation of the SI second to be compared over a short cross-site fibre link. The optical frequency comb will also require dedicated branches to link with the optical clocks and oscillators already available at NPL, these include the Sr+ at 674 nm, the Sr lattice at 698 nm, the Yb+ clocks at both 871 nm and 934 nm, and an ultrastable laser at 1064 nm.

An optical frequency comb for comparison of optical clocks over a short fibre link

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