A Matter of Time the Arrival of a New Type of Timekeeper Heralds the End of the Second As We Know It, As Helen Margolis Explains

measure for measure A matter of time The arrival of a new type of timekeeper heralds the end of the second as we know it, as Helen Margolis explains. ifty years ago, the General Conference However, the on Weights and Measures decided that Earth’s period of the time had come for a new definition rotation is not F 1 of the second , and declared that “The constant. Although second is the duration of 9,192,631,770 the RGO applied periods of the radiation corresponding to corrections for the transition between the two hyperfine seasonal fluctuations levels of the ground state of the caesium 133 and movement of atom.” This decision marked a fundamental the rotation axis, shift in philosophy — a move away from it was known that previous astronomical definitions to one such astronomical based on a quantum phenomenon. timescales also The underlying principle — that a suffered from long- OF NPL COURTESY transition between discrete atomic energy term drift. Seeking levels could serve as a natural unit of a more stable unit of time, astronomers accuracy of caesium clocks has steadily frequency — had been understood for had chosen to move to ephemeris time, improved, and today’s best caesium fountain decades. But it was only twelve years based on the period of the orbital motion primary frequency standards are now previously that Louis Essen (pictured, right) of the Earth around the Sun. In 1956, the accurate to one part in 1016. and Jack Parry (left) had built the first International Committee for Weights and Recently, however, a new generation caesium atomic clock at the UK’s National Measures followed suit, selecting the second of optical atomic clocks has surpassed the Physical Laboratory (NPL). They first of ephemeris time to be the base unit of time performance of caesium atomic clocks in observed the atomic resonance line in May in the International System of Units. both stability and estimated uncertainty6,7, 1955 and Essen later wrote2: “We invited Although ephemeris time was more raising the prospect of a future redefinition the Director to come and witness the death stable, the period of the orbital motion of of the second. These new clocks derive their of the astronomical second and the birth of the Earth about the Sun was impractically improved precision from their much higher atomic time.” long for most measurement purposes. operating frequencies — about five orders By carefully studying the dependence of Nevertheless, Essen now needed to link the of magnitude higher than the microwave the resonance frequency on environmental caesium frequency to the ephemeris second. absorption frequency in caesium. Although conditions, Essen and Parry definitively He achieved this by collaborating with significant work remains to validate the showed that their new clock was much more William Markowitz from the United States uncertainty budgets of the optical clocks via stable than astronomical timescales based Naval Observatory (USNO), who used a international comparisons, to incorporate on the motion of celestial bodies. But this novel type of moon camera to observe the them into international timescales and to was not enough. A basic tenet of metrology Moon’s position over a three-year period, establish the leading contender for a new is that a new definition of any measurement thus obtaining a precise realization of definition, the days of the caesium-based unit should be consistent with the old one to the ephemeris second. Long-range radio definition are almost certainly numbered. ❐ within the measurement uncertainty. time signals were used to compare the It was therefore necessary to measure the timescales at NPL and USNO, enabling HELEN MARGOLIS is at the National frequency of the caesium clock against the the frequency of the NPL caesium clock Physical Laboratory, Teddington, London astronomical second. For Essen and Parry, to be measured relative to the ephemeris TW11 0LW, UK. this meant the timescale disseminated second5. The measurement uncertainty of e-mail: [email protected] by the Royal Greenwich Observatory 20 Hz was dominated by the uncertainties References (RGO), which was based on observations of the astronomical observations and the 1. Terrien, J. Metrologia 4, 41–45 (1968). of the Earth’s rotation about its axis. Their timescale comparisons rather than by the 2. Essen, L. The Memoirs of Louis Essen (ed. Essen, R.) first report on the operation of their uncertainty of the caesium clock itself, and (National Physical Laboratory, 2015). 3 3. Essen, L. & Parry, J. V. L. Nature 176, 280–282 (1955). caesium clock , including a preliminary the numerical value obtained is the one that 4. Essen, L. & Parry, J. V. L. Phil. Trans. R. Soc. London A frequency measurement, appeared in appears in the 1967 definition of the second. 250, 45–69 (1957). August 1955, and was later followed by The choice of the caesium transition 5. Markowitz, W., Hall, R. G., Essen, L. & Parry, J. V. L. Phys. Rev. Lett. 1, 105–107 (1958). a more detailed paper and an improved as the basis for the definition has proved 6. Margolis, H. Nat. Phys. 10, 82–83 (2014). frequency measurement4. in hindsight to be a very good one. The 7. Katori, H. Nat. Phys. 13, 414 (2017). 1234 NATURE PHYSICS | VOL 13 | DECEMBER 2017 | www.nature.com/naturephysics ©2017 Mac millan Publishers Li mited, part of Spri nger Nature. 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