Strontium optical clock. Inside this strontium clock, also known as a frequency standard, is an ion trap (centre), where strontium 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. The strontium optical clock has 100,000 more oscillations than the caesium clock and is believed to be three times more accurate than anything previously achieved. Photographed at the National Physical Laboratory in Teddington, UK.