Most accurate atomic clock

The world’s most accurate atomic watch as of July 2006, is a prototype mercury optical clock which, if operated continually would neither gain nor lose a second in approximately 400 million years. The clock has been built and developed by the National Institute of Standards and Technology (NIST) in Boulder, Colorado, USA and is at least ten times more accurate than the NIST-F1 national standard clock in the USA. This experimental clock measures the oscillations of a mercury ion (an electrically charged atom) held in an ultra-cold electromagnetic trap, produces “ticks” at optical frequencies. Optical frequencies are much higher than the microwave frequencies measured in cesium atoms in the national standard (and one of the world’s most accurate) clocks.