20000103

Most accurate

atomic clock made

ISLAMABAD: A new atomic clock has been introduced which will not gain or lose a single second in nearly 20 million years.

The US National Institute of Standards and Technology (Nist) has started using the new clock, more accurate than any clock ever made before.

Nist F-1 was developed by Steve Jefferts and Dawn Meekhof of the Time and Frequency Division of Nist's Physics Laboratory in Boulder, Colo. It was constructed and tested in less than four years.

This new standard is more accurate by a wide margin than any other clock in the United States and assures the nation's industry, science and business sectors continued access to the extremely accurate timekeeping necessary for modern technology-based operations.

Together with the US Naval Observatory in Washington, DC, Nist provides official time to the nation.

The clock runs on the movement of cesium atoms, stimulated by infra-red laser beams, in a microwave-filled cavity.

Nist F-1 is referred to as a fountain clock because it uses a fountain-like movement of atoms to obtain its improved reckoning of time. First, a gas of cesium atoms is introduced into the clock's vacuum chamber. Six infra-red laser beams then are directed at right angles to each other at the center of the chamber.

The lasers gently push the cesium atoms together into a ball. In the process of creating this ball, the lasers slow down the movement of the atoms and cool them to near absolute zero.

Two vertical lasers are used to gently toss the ball upward (the "fountain" action), and then all of the lasers are turned off. This little push is just enough to loft the ball about a metre high through a microwave-filled cavity. Under the influence of gravity, the ball then falls back down through the cavity.

As the atoms interact with the microwave signal-depending on the frequency of that signal-their atomic states might or might not be altered. The entire round trip for the ball of atoms takes about a second. At the finish point, another laser is directed at the cesium atoms. Only those whose atomic states are altered by the microwave cavity are induced to emit light (known as fluorescence).

The photons (tiny packets of light) emitted in fluorescence are measured by a detector.

This procedure is repeated many times while the microwave energy in the cavity is tuned to different frequencies. Eventually, a microwave frequency is achieved that alters the states of most of the cesium atoms and maximises their fluorescence. This frequency is the natural resonance frequency for the cesium atom-the characteristic that defines the second and, in turn, makes ultra precise timekeeping possible.ÑAPP