Phys. Rev. Lett. 97, 020801 (2006). PRL 14 July 2006| Abs |

ticking for life

Atomic clock's timekeeping is significantly more constant than the rotation of the Earth. Its history can be traced to the first caesium (Cs) atomic clock designed by Louis Essen and built in Teddington in 1955, which clock would drift 1 second in three hundred years. In 1967, the 13th CGPM (Conférence Générale des Poids et Mesures, 1967/68, Resolution 1; CR, 103 and Metrologia, 1968, 4, 43) defined the second based on the frequency of the cesium ground-state hyperfine transition--The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. It follows that the hyperfine splitting in the ground state of the caesium 133 atom is exactly 9 192 631 770 Hz. This definition has been modified by the CIPM (Comité International des Poids et Mesures) in 1997 with a statement: this definition refers to a caesium atom at rest at a temperature of 0 K. 

Now, J. C. Bergquist and his colleagues from NIST (National Institute of Standards and Technology in Colorado, USA) reported an optical frequency standard based on an ultraviolet transition in a single, laser-cooled mercury ion (¹⁹⁹Hg⁺). The absolute frequency of the transition was measured versus cesium to be 1 064 721 609 899 144.94 (97) Hz. This mercury clock hits the timekeeping record and is expected to drift by only 1 second in 400 million years compared with 1 in 60 million in the caesium clock of their lab's NIST-F1in 2005 (developed by Steve Jefferts and Dawn Meekhof).  

Whether or not the CGPM will redefine the second, this measurement is too precise beyond endurance. I can't help to remembering simple rhythm of a pendulum.

* Lin Pu is in the Physics Department of Nanjing University, Nanjing 210093, CHINA.

References

Scidea Sketch calls for review of this article. We are especially interesting in why we need to measure time at high level of accuracy and its importance for modern navigation systems, mobile telephones, digital television and computer…   

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