Keeping Time to the Nanosecond: Atomic Clocks

A New Standard for a New Age

Since the first creation of the sundial, thousands of years ago, man has struggled to keep accurate track of the time. Until the 20^th century, all developments with clocks still depended ultimately on one thing, the rotation of the earth in the creation of a full day. With origin of the atomic age in the first half of the 20^th century, a new standard was created to keep track of the time: the atomic clock.

The first atomic clock was created by the National Institute of Standards and Technology (NIST). Unlike modern atomic clocks that use the cesium ion cesium-133, this original atomic clock used an ammonia molecule standard. In 1952, NIST made an accurate measurement of a cesium resonance, which would become the base of the NBS-1 clock. However, the first cesium beam clock was built in 1955 at the National Physics Laboratory in England by physicist Louis Essen.

As these clocks continued to develop, they became increasingly accurate with telling time. In 1967, the 13^th General Conference on Weights and Measures redefined the second as being 9,192,631,770 vibrations of the cesium atom. This was the first time in the history of mankind that time was not defined on an astronomic basis.

This new method led to the creation of two international time standards: International Atomic Time (TAI) and the Coordinated Universal Time (UTC). International Atomic Time is based purely on the atomic clock, while UTC, used to disseminate civic time, is based on the TAI readings but also adjusted to synchronize more precisely with the natural transitions between day and night.

The current time standard in the United States is the NIST-F1 atomic clock located in Boulder, Colorado. This is the official clock of the United States, that also plays a role in the calculation of both TAI and UTC. It was first built in 1999, and is the continuation of a long line of atomic clocks built by the National Institute of Standards and Technology, including a long line of NBS clocks and the NIST-7.