Are you familiar with how atomic clocks operate and their applications?

The purpose of an atomic clock is to define the second, the basic measure of time, with high-level precision. These clocks calibrate to the number of a radiation’s oscillations, corresponding to two levels of energy of a specific atom, hence its name, atomic clock. Examples of the atoms used are caesium-133, rubidium, and hydrogen.

Created in the 1950s, atomic clocks were developed through the work of several Nobel laureates in physics, including: I. I. Rabi (1944); A. Kastler (1966); N. Ramsey (1988); S. Chu with C. Cohen-Tannoudji and W. Philips (1997); J. Hall with T. Haensch, and R. Glauber (2005); and S. Haroche with D. Wineland (2012).

In practice, a caesium-133 atom is irradiated by electromagnetic waves with a precise frequency. This irradiation, if at the correct frequency, will allow an atom to pass from one energy level to another. At this radiation, one second divided by 9,192,631,770 constitutes one oscillation. In other words, 9,192,631,770 oscillations of radiation make up a single second.

The high number of oscillations is significant, as the higher the number of oscillations, the greater the clock’s precision and stability. The result is a level of precision to the nanosecond, or an error of one second every tens of millions of years for an atomic clock. But what use is all of this to mere mortals ?

How we define a metre is determined by the measurement of a second. One metre is the distance travelled by light in a vacuum for 1/299,792,658 of a second. Without a sufficiently precise measure of time and distance, systems such as GPS would not be possible. Because in general terms, for the purposes of satellite navigation, internal clocks must be accurate to the nanosecond (ns), so that positioning errors are no greater than one metre. **In practical terms, an error of one second would cause a positioning error of 300,000 kilometres !** This is why GPS satellites are fitted with atomic clocks. Quartz cannot guarantee precision to the nanosecond.

Other applications include telecommunications and power distribution networks. To minimise information losses in the former and overloads in the latter, each network hub must be precisely synchronised, this time to the microsecond. Although less exacting than GPS, precision remains crucial. **The dating of financial transactions and synchronisation of banking data (big data) may also rely upon atomic clocks in the near future.**

These commercial and industrial applications are made possible notably through the work conducted at the Neuchâtel Time Frequency Laboratory, which is often in the news for good reason. In March 2018, the recent publication about the production of microwave cavity with 3D printers, was integrated in the editor's pick of Applied Physics Letters journal.

The Laboratory also leads projects in atomic clocks in collaboration with other members of Microcity, such as CSEM and the EPFL’s Neuchâtel campus. **Recent developments – in miniaturisation, the use of lasers to improve the accuracy of caesium clocks tenfold, and 3D-printed components** – all demonstrate that despite being 70 years old, the atomic clock has moved with the times.

**Radio show (in French) CQFD, 23 février 2018, meeting with Gaetano Mileti, deputy director of the Time Frequency Laboratory at Neuchâtel University**