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The Appearance of Sunspot 981 Marked the Beginning of Solar Cycle 24.....ssn# 981 appeared on 04 Jan 2008

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"The quality of a clock depends on its stability and accuracy—whether the clock provides a constant, unchanging output frequency, and how close the measured frequency is to the fundamental atomic resonance that provides the clock’s “tick.”  NIST

The Official Time Keeper for the U.S. 

U. S.  Atomic Clocks

This official Time service is cooperatively provided by the two time agencies of United States: a Department of Commerce agency, the National Institute of Standards and Technology (NIST), and its military counterpart, the U. S. Naval Observatory (USNO). Readings from the clocks of these agencies contribute to world time, called Coordinated Universal Time (UTC). The time maintained by both agencies should never differ by more than 0.000 0001 seconds from UTC

Go to Atomic Clock Part 1    NASA's Contribution

Go to Atomic Clock Part 2    NASA's Contribution

"New Optical Clock Promises More Accuracy than Cesium. NIST researchers have demonstrated a new kind of atomic clock that has the potential to be up to 1,000 times more accurate than today’s best clock. The new clock is based on an energy transition in a single trapped mercury ion (a mercury atom that is missing one electron). Building a clock based on such a high-frequency transition was previously impractical because it requires both “capturing” the ion and holding it very still to get accurate readings, and having a mechanism that can “count” the ticks accurately at such a high frequency."

"The quality of a clock depends on its stability and accuracy—whether the clock provides a constant, unchanging output frequency, and how close the measured frequency is to the fundamental atomic resonance that provides the clock’s “tick.” One advantage of the new clock is that it ticks much faster. Today’s international time and frequency standards, such as NIST-F1, measure an atomic resonance of about 9 billion cycles per second. By contrast, the new NIST device monitors an optical frequency more than 100,000 times higher or about 1 quadrillion (US) cycles per second." NIST

In December of 1999, NIST introduced the NIST-F1, cesium atomic clock. At the time, cesium was used in several clock configurations. When introduced, the NIST-F1, Fountain Clock, became the most accurate clock in the world. The "Optical Clock" mentioned above takes accuracy to a new level.

"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 ultra precise 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."

"NIST-F1 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." NIST

New clocks on the drawing board will make the accuracy of the above clocks seem like a wind-up watch.

	NIST-F1 Cesium Fountain Atomic Clock
	Primary Atomic Reference Clock in Space
	NIST Time Scale Data

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