The improved tuning of the microwave frequency leads to a better realization and control of the resonance frequency of cesium. The result is an observation time of about one second, which is limited only by the force of gravity pulling the atoms to the ground.Īs you might guess, the longer observation times make it easier to tune the microwave frequency. The laser cooled atoms are launched vertically and pass twice through a microwave cavity, once on the way up and once on the way down. Laser cooling drops the temperature of the atoms to a few millionths of a degree above absolute zero, and reduces their thermal velocity to a few centimeters per second. Since the atoms are moving so fast, the observation time is limited to a few milliseconds. Traditional cesium clocks measure room-temperature atoms moving at several hundred meters per second. The combination of laser cooling and the fountain design allows NIST-F1 to observe cesium atoms for longer periods, and thus achieve its unprecedented accuracy. This frequency is the natural resonance frequency of the cesium atom (9,192,631,770 Hz), or the frequency used to define the second. Eventually, a microwave frequency is found that alters the states of most of the cesium atoms and maximizes their fluorescence. This process is repeated many times while the microwave signal in the cavity is tuned to different frequencies. In the process of creating this ball, the lasers slow down the movement of the atoms and cool them to temperatures near absolute zero. The lasers gently push the cesium atoms together into a ball. Six infrared laser beams then are directed at right angles to each other at the center of the chamber. First, a gas of cesium atoms is introduced into the clock's vacuum chamber. NIST-F1 is referred to as a fountain clock because it uses a fountain-like movement of atoms to measure frequency and time interval. It is now approximately ten times more accurate than NIST-7, a cesium beam atomic clock that served as the United State's primary time and frequency standard from 1993-1999. In 2000 the uncertainty was about 1 x 10 -15, but as of January 2013, the uncertainty has been reduced to about 3 x 10 -16, which means it would neither gain nor lose a second in more than 100 million years! The graph below shows how NIST-F1 compares to previous atomic clocks built by NIST. The uncertainty of NIST-F1 is continually improving. Because NIST-F1 is among the most accurate clocks in the world, it makes UTC more accurate than ever before. NIST-F1 contributes to the international group of atomic clocks that define Coordinated Universal Time (UTC), the official world time. NIST-F1, the nation's primary time and frequency standard, is a cesium fountain atomic clock developed at the NIST laboratories in Boulder, Colorado. The Primary Time and Frequency Standard for the United States Two-Way Satellite Time and Frequency Transfer (TWSTFT).
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