Ytterbium clocks hold the record for stability with ticks stable to within less than two parts in 1 quintillion (). 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.