 Ytterbium clocks hold the record for stability with ticks stable to within less than two parts in 1 quintillion ( 2 × 10 − 18 ) . The clocks developed at the National Institute of Standards and Technology ( NIST ) rely on about 10 @,@ 000 rare @-@ earth atoms cooled to 10 microkelvin ( 10 millionths of a degree above absolute zero ) and trapped in an optical lattice — a series of pancake @-@ shaped wells made of laser light . Another laser that " ticks " 518 trillion times per second provokes a transition between two energy levels in the atoms . The large number of atoms is key to the clocks ' high stability . 
