Remarkable clocks and watches: Switzerland’s most accurate clock

The most accurate clock ever developed in Switzerland, the FOCS-1 device, started operating in 2004. It stands in a laboratory of the Swiss Federal Office of Metrology METAS in Bern. Were you to come back and look at it in 30 million years’ time, it would not have deviated by more than one second.

It is one of five atomic clocks in the Time and Frequency Laboratory, which cooperates with 45 similar institutions around the world and a total of around 250 atomic clocks to provide the data used to compute Coordinated Universal Time (UTC), against which the times in all time zones are calculated.

Where a mechanical clock uses the oscillation of a pendulum to divide time into equal intervals, an atomic clock uses the oscillations that take place inside atoms when they are shot into a magnetic field. The frequency of this atomic oscillation is always the same, which is why it is so valuable as a standard for defining time.

The first atomic clock was built in the US in 1949, and the accuracy of such clocks has improved considerably over the years. Since the atoms oscillate so fast, they can normally only be observed for a short time, which limited the accuracy with which they could be measured.

The latest generation of atomic clocks – including the new Swiss one – uses lasers to cool the atoms to a temperature close to absolute zero. This slows them down from 200 meters/second to six meters/second. It is these «slow» atoms which are then fired into the magnetic field which makes them oscillate.

What is unique about the Swiss clock is that it shoots the atoms in a continuous beam rather than a spasmodic one. As a result, the atoms collide less frequently with each other, making the measurement more precise.

Such extreme accuracy might at first sight seem superfluous. But it has a number of applications, apart from setting the official time around the world. Satellite navigation systems depend on atomic clocks, and the more closely synchronised they are, the more accurate the data the systems can deliver. Radio telescopes round the world can be synchronised as they observe the same point in the heavens, creating in effect a single telescope whose diameter is that of the earth.

Additional information


  • METAS Laboratory for Time and Frequency