More Accurate Time: NIST's New Year Gift To The World

Termed NIST F-1, the new cesium atomic clock at NIST's Boulder, Colo., laboratories, began its role as the nation's primary frequency standard by contributing to an international pool of the world's atomic clocks that is used to define Coordinated Universal Time (known as UTC), the official world time. Because NIST F-1 shares the distinction of being the most accurate clock in the world (with a similar device in Paris), it is making UTC more accurate than ever before. NIST F-1 recently passed the evaluation tests that demonstrated it is approximately three times more accurate than the atomic clock it replaces, NIST-7, also located at the Boulder facility. NIST-7 has been the primary atomic time standard for the United States since 1993 and is among the best time standards in the world.

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 infrared 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 meter 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 maximizes their fluorescence. This frequency is the natural resonance frequency for the cesium atom—the characteristic that defines the second and, in turn, makes ultraprecise timekeeping possible.

The NIST F-1 clock's method of resolving time differs greatly from that of its predecessor, NIST-7. That device—and the versions before it—fired heated cesium atoms horizontally through a microwave cavity at high speed. NIST F-1's cooler and slower atoms allow more time for the microwaves to "interrogate" the atoms and determine their characteristic frequency, thus providing a more sharply defined signal.

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 U.S. Naval Observatory in Washington, D.C., NIST provides official time to the nation.

As a non-regulatory agency of the U.S. Department of Commerce's Technology Administration, NIST strengthens the U.S. economy and improves the quality of life by working with industry to develop and apply technology, measurements and standards through four partnerships: the Measurement and Standards Laboratories, the Advanced Technology Program, the Manufacturing Extension Partnership and the Baldrige National Quality Program.