The latest atomic clock developed by the National Institute of Standard and Technology and the University of Colorado at Boulder is accurate to within one second per 15 billion years, as of April 2015. This accuracy level is three times more accurate than the previous strontium clock developed in 2014.
The latest atomic clock developed by the NIST is a strontium lattice clock, which measures time by measuring the resonant frequency of strontium electrons oscillating between energy levels. However, a cesium atomic clock that is accurate to within one second every 200 million years provides the basis for the international time standard. Atomic clocks operate by detecting microwave frequencies emitted by cesium electrons as they oscillate between two energy levels. The latest strontium clock detects frequencies in the optical range of resonant frequencies emitted by strontium atoms.