The Measurement of Time: Time, Frequency and the Atomic Clock

Abstract

This book provides an in-depth and comprehensive survey of the current state-of-the-art of the international atomic timescale and its underpinning relationship to practical and legal timescales. The authors are two of the leading figures in French time metrology. Claude Audoin, formerly Director of the Laboratoire de l'Horloge Atomique at Orsay, has had a long and distinguished career in the development of atomic frequency standards and atomic clocks. Bernard Guinot, formerly Director of the Bureau International de l'Heure (now absorbed by the Bureau International des Poids et Mesures, BIPM), has a lifetime's experience in the development of timescales. The work reviewed here is a revised and updated translation of Les fondaments de la mesure du temps (Paris: Masson, 1998). It is written at a level appropriate for an experienced graduate student or postdoctoral scientist entering either the field of time and frequency metrology or that of precise astronomical observation, whilst having sufficient depth to be useful as a work of reference for the established researcher in these fields. The first third of the book discusses the physical foundations of time metrology and introduces key concepts in the use of physical oscillators as clocks. The historical account of the complex process by which mean solar time came to be replaced by Coordinated Universal Time (UTC) will be particularly informative for newcomers to the field. About half of the volume is devoted to atomic frequency standards, with an exposition of the physics of caesium beam clocks and caesium fountains and a review of other commonly used standards such as the hydrogen maser, rubidium cell and stored ion clock. Although thorough, this 127-page chapter does not replace Audoin's earlier work with Jacques Vanier, The Quantum Physics of Atomic Frequency Standards (Bristol: IOP Publishing, 1989, out of print), which will remain the Bible for physicists working on clock development. The present time is one of great excitement in the development of atomic frequency standards. At the time the French edition was published, the first caesium fountain frequency standard, at the BNM-LPTF in Paris, had recently demonstrated 1.5 × 10-15 relative frequency uncertainty. Two such standards, at PTB and NIST, are now contributing to International Atomic Time (TAI). Clocks based on optical transitions in ions or atoms are just beginning to become a reality, thanks to Ted Haensch's breakthrough with the femtosecond optical frequency comb generator, which arrived on the scene just in time to receive a mention in this English edition. In a rapidly evolving field, any general survey is bound to be overtaken by events. Primary caesium frequency standards are designed to realize the SI definition of the second. The relationships between the SI second, TAI and UTC, as well as the national approximations to UTC generated by national standards laboratories such as NPL, the timescale available from GPS and timescales used in astronomy, are carefully set out. Finally, the practical applications of these timescales and frequency standards are reviewed, ranging from ultraprecise determinations of fundamental physical constants to positioning and navigation. The problem of longitude is as pressing as in Harrison's day, but for the foreseeable future, clockmakers will continue to have the upper hand over astronomers! Despite betraying the occasional Gallicism, the English translation reads with the clarity of the French original. The reader is guided painlessly through the multiplicity of acronyms with which the topic abounds. Audoin and Guinot have produced a book that will stand the test of time. Stephen N Lea