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Ultrastable lasers are essential tools in optical frequency metrology enabling unprecedented measurement precision that impacts on fields such as atomic timekeeping, tests of fundamental physics, and geodesy. To characterise an ultrastable laser it needs to be compared with a laser of similar performance, but a suitable system may not be available locally. Here, we report a comparison of two geographically separated lasers, over the longest ever reported metrological optical fibre link network, measuring 2220 km in length, at a state-of-the-art fractional-frequency instability of 7 × 10−17 for averaging times between 30 s and 200 s. The measurements also allow the short-term instability of the complete optical fibre link network to be directly observed without using a loop-back fibre. Based on the characterisation of the noise in the lasers and optical fibre link network over different timescales, we investigate the potential for disseminating ultrastable light to improve the performance of remote optical clocks. Precision measurement plays an important role in frequency metrology and optical communications. Here the authors compare two geographically separate ultrastable lasers at 7 × 10−17 fractional frequency instability over a 2220 km optical fibre link and these measurements can be useful for dissemination of ultrastable light to distant optical clocks. [ABSTRACT FROM AUTHOR]

We demonstrate a method for accurately locking the frequency of a continuous-wave laser to an optical frequency comb under conditions where the signal-to-noise ratio is low, too low to accommodate other methods. Our method is typically orders of magnitude more accurate than conventional wavemeters and can considerably extend the usable wavelength range of a given optical frequency comb. We illustrate our method by applying it to the frequency control of a dipole lattice trap for an optical lattice clock, a representative case where our method provides significantly better accuracy than other methods. [ABSTRACT FROM AUTHOR]

In this paper, we review several characteristics of optical lattice clocks as candidates for a future redefinition of the International System of Units (SI) second using an atomic transition in the optical domain, focusing on experiments performed at SYRTE using one mercury (Hg) and two strontium (Sr) optical lattice clocks. Beyond the technical aspects such as the stability and systematic frequency-shift assessments of the clocks, practical aspects have to be considered, such as the careful determination of the optical frequency with respect to the cesium primary standard and the worldwide reproducibility of the clock frequency by local and remote clock comparisons. [ABSTRACT FROM PUBLISHER]

The article discusses the use of quantum sensor networks as optical atomic clocks for the detection of dark matter coupling. According to the author the fine-structure constant susceptibilities in parts of ytterbium and strontium optical atomic clock enable the search of dark matter. Topics discussed include use of global sensor network, measurements of clocks, and search for topological defect and massive scalar field by optical clocks.