English: JILA's strontium optical atomic clock is now the world's most accurate clock based on neutral atoms. Shining a blue laser onto ultracold strontium atoms in an optical trap tests how efficiently a previous burst of light from a red laser has boosted the atoms to an excited state. Only those atoms that remain in the lower energy state respond to the blue laser, causing the fluorescence seen here. Credit: Sebastian Blatt, JILA, University of Colorado