Ytterbium optical clock. Physicist working on an ytterbium optical clock, also known as a frequency standard. Inside it is an ion trap, where ytterbium ions oscillate between two energy levels in response to the light of a 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 believed to be more precise than the caesium clock as higher numbers of oscillations in a given time provide a more accurate measurement. Photographed at the National Physical Laboratory in Teddington, UK.