The gold standard of timing accuracy is the cesium fountain clock, a type of atomic clock. There are a handful of these clocks around the world. The most accurate of these elite timepieces, according to a new study by Krzysztof Szymaniec of the National Physical Laboratory, United Kingdom and by Kurt Gibble and Ruoxin Li from Pennsylvania State University, is to be found at Teddington, within the United Kingdom.
Atomic clocks operate by measuring the oscillation frequency of electrons as they change energy levels. By international agreement, a second is defined as the transition frequency between the two ground-state sublevels of a cesium 133 atom. The most accurate atomic clocks use cesium atoms that are cooled to nearly absolute zero, and tossed upward (like in a fountain) through a microwave detector. Measurement takes place while the atoms are in the weightless portion of that arc.
Thanks to some upgrades, the U.K. clock, known as NPL-CsF2, has an inaccuracy rate of only 2.3 x 10-16. In contrast, the most accurate U.S. clock, the NIST-F1 located in Boulder Colorado, has uncertainty rating of 3 x 10-16. Okay, that’s not a big difference, considering that the U.S. clock is not expected to gain or lose a second within the next 100 million years. However, as only one clock can reign supreme, the U.K. clock will now be used to calibrate and synchronize all other clocks around the world, which is critical for global communications.
Caption: A caesium fountain clock that keeps the United Kingdom's atomic time is now the most accurate long-term timekeeper in the world, according to a new evaluation of the clock that will be published in the October 2011 issue of the international scientific journal Metrologia by a team of physicists at the National Physical Laboratory (NPL) in the United Kingdom and Penn State University in the United States. This image shows the clock, NPL-CsF2, which is located at the National Physical Laboratory in Teddington, U.K. The whole device is approximately 8.2 feet (2.5 m) high. Atoms are tossed up 3.2 feet (1 m), approximately 12 inches (30 cm) above the cavity that is contained inside a vacuum vessel. The large external cylinder screens the atoms inside the clock from the relatively large and unstable external magnetic field.
Credit: National Physical Laboratory, United Kingdom.