Historically, the global standard for keeping time has been based on an average of signals from cesium microwave atomic clocks from around the world. Now, as the precision and stability of optical clocks steadily improve, there is growing momentum to redefine the International System of Units’ second to use optical clocks instead. Current optical clocks are 100x times more accurate than the best cesium clocks and can measure time so accurately that they lose or gain less than one second over billions of years. However, the use of optical clocks for international timekeeping requires a comparison of data between optical clocks to verify that they are performing as expected. In an effort to establish a global optical time scale using optical clock precision timekeeping, a multi-institutional group of researchers compared 10 different optical clocks, simultaneously, across six countries. According to the researchers, the results of the optical clock network demonstration could help to improve the performance of next-generation optical clocks and chart a course to new applications. Optical clocks use lasers to excite atoms in a controlled way that causes the atoms to shift between specific energy levels. These shifts happen at very precise frequencies which serves as the “ticks” of the clock. Realizing the full potential of these precision timekeepers requires comparing them across long distances since these clocks come in various forms, each using different atoms to keep time. Laser light with an ultra-stable frequency in the ytterbium ion optical clock at the National Physical Laboratory in the U.K. Courtesy of National Physical Laboratory. The researchers reported results from 38 comparisons, four of which they conducted directly for the first time, and for many of which they obtained measurements with much greater accuracy than previous experiments. To carry out the measurements, the researchers linked frequency outputs from the different optical clock systems. They did this using radio signals from satellites and laser lights travelling through optical fibers. The satellite method used GPS signals from the satellite navigation system, which was available to all clocks in the study. However, this linking technique enabled only limited precision due to measurement uncertainties caused by factors like signal noise or instrument limits. The researchers also used customized optical fiber links; the use of these links allowed measurements 100x greater than the satellite technique. However, these stable, high-precision connections could only be used in certain locations during the international comparison. The researchers conducted local comparisons, in the U.K. and Germany, where multiple clocks were located at the same institute, with short optical fibers. The use of these short fibers reduced uncertainty even more. Rachel Godun coordinated a multi-institutional group of researchers who measured 38 frequency ratios simultaneously for ten different optical clocks. Courtesy of the National Physical Laboratory. According to the researchers, the experiment identified some areas in which more work is needed: For example, to confirm that all clocks are performing as expected, measurement uncertainties must be reduced to match the precision of the clocks themselves. Repeated measurements will then be needed to confirm the reliable operation necessary to build confidence in the clocks and links. Several other criteria must be met before redefining the second, including proving that optical clocks can contribute regularly and consistently to international time scales, the researchers said. This research was published in Optica (www.doi.org.0.1364/OPTICA.561754).
