Termed NIST F1, 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 F1 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 F1 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 F1 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 F1 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 F1'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.