Ytterbium optical clock laser. This laser beam is use in a frequency standard consisting of an ion trap where ytterbium ions oscillate between two energy levels in response to the light of the laser beam. Counting these oscillations is the basis for the standard second. The current basis for the international definition of time is the caesium atomic clock, where one second is about 9193 million oscillations of caesium-133 atoms. Optical clocks are more precise than the caesium clock as higher numbers of oscillations in a given time provide a more accurate measurement. Photographed in 2014, at the National Physical Laboratory, Teddington, UK.