![]() Not one but two FOCS are currently being tested, one by METAS and one by the University of Neuchâtel’s Institute of Physics. This apparently simple system is, in reality, highly complex. The atoms, slowed by laser beams, are vertically launched then fall back through a hole where they are measured. Rather than launch atoms in batches, the Swiss opted for a continuous stream… not unlike the water jet on Lake Geneva. A cloud of caesium 133 atoms is vertically launched into a vacuum. Traditional primary frequency standards are known as pulsed fountains. Thomann is currently testing FOCS-2 © Fabrice Eschman “Of course we could have copied others, but we thought it would be far more interesting to use a different concept then compare results.” Right to left: Pierre Thomann, Director of the Time-Frequency Laboratory at Neuchâtel University, and father of the FOCS, with two members of his team, Laurent Devenoges and Gianni Di Domenico. Appointed by METAS in 1992, the professor and his team spent ten years developing and building a standard based on a different technology. Director of the Time-Frequency Laboratory at Neuchâtel University, he also leads the FOCS project. “Because we began working on our primary standard after everyone else, we chose a different route,” explains Pierre Thomann, his eyes sparkling at the thought. Switzerland, however, decided to do things a little differently. Each laboratory regularly adjusts the rate of its commercial atomic clocks to that of the primary frequency standard. By way of comparison, a mechanical watch movement beats at between three and five Hz, a quartz movement at 32,768 Hz. Their “heart” beats at 9.2 Gigahertz (9.2 billion Hz). These are not clocks, as they don’t count seconds, but provide the absolute reference for the duration of the second. Never content, scientists have developed primary frequency standards to check these clocks’ rate. The average of these averages gives Coordinated Universal Time (UTC). Every five days, the Time and Frequency Laboratory of the Federal Office of Metrology sends the mean measurement of these clocks to the International Bureau of Weights and Measures (BIPM) which gathers data from 350 clocks worldwide. Their conventional starting point is 00:00:00 on January 1st in year 1. At METAS, five “commercial” atomic clocks count time by numbering each second. Time is now the physical quantity that can be most precisely measured. “When defining something more precise than before, in this case the new second versus the old one, the final solution has to be an arbitrary value.” “Hence the second was defined as equal to 9,192,631,770 periods of oscillation of this frequency.” Why not one more period, or one less? “It is a convention,” admits Dr Stefanov. “When a caesium atom electron moves from one orbit to another, its frequency remains the same,” André Stefanov explains. The first consequence of this was to redefine the second. In 1967, twenty years after its debut, the atomic clock was adopted as the instrument of time measurement. Hence they were replaced by an atomic phenomenon: the oscillation of the caesium 133 atom. Twenty-four leap seconds have been added to time since the system was adopted in 1972.Īnd so the scientific community had to face a disconcerting fact: astronomical phenomena were no longer a sufficiently reliable basis for defining units of time. The last leap second was added on December 31st, 2008. The impact of this is such that, for clock time and the Earth’s rotation to remain synchronised, the International Earth Rotation and Reference System Service (IERS) must sporadically add an intercalated or leap second to universal time. “The days are now two milliseconds longer than a century ago,” comments André Stefanov with a smile. The reason: the tides and the varying distance between the Earth and the Moon. ![]() What’s more, the speed of the Earth’s rotation on its axis is slowing. With atomic clocks, the first of which appeared in the late 1940s, came evidence of the unthinkable: the duration of the Earth’s revolution around the Sun isn’t regular.
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