France

Christophe Salomon

2025 Balzan Prize for Atoms and Ultra-Precise Measurement of Time

For his pioneering contribution in paving the way for the application of ultra-cold atoms to the creation of atomic clocks which have revolutionised the measurement of time.

Time is one of the fundamental quantities of Nature and its definition has always stimulated scientific and philosophical debates of the utmost importance. Primary to all human activity, the measurement of time has been based for millennia on the cyclical succession of day and night, which allowed the development of the sundial, useful for the then prevailing activity of agriculture. More precise measurements of time were then made possible by counting increasingly rapid periodic phenomena, starting with the oscillations of Galileo’s pendulum and ending with modern clocks. The challenge of measuring the passing of time with ever greater precision is strongly motivated by today’s prevalent technological activities, ranging from financial exchanges on the stock market at speeds of billionths of a second, to the transfer of information at the speed of light, to the measurement of distances travelled by measuring the time it takes for light to cross them, to navigation and exploration, which are becoming increasingly more accurate, moving from Earth to space.

Quantum mechanics – the centenary of which is celebrated by UNESCO this year – has understood the structure of atoms and the mechanisms by which electrons can ‘jump’ from one energy level to another when driven by electromagnetic radiation oscillating at a frequency determined with extreme precision by the energy difference between atomic levels.

About sixty years ago, the definition of time changed, moving from the cyclical repetition of day and night to the ticking of caesium atoms, nine billion times faster than Galileo’s pendulum. Atoms, which are the same at all times and in all places, are therefore the new natural clocks.

In a gas at normal temperatures, atoms move very quickly, which limits the possibility of controlling them for a precise measurement of their clock oscillations. Quantum mechanics, in addition to revolutionising scientific understanding, has led to important technological revolutions, such as the invention of the laser, which, among its countless applications, has also made it possible to slow down the motion of atoms by cooling them to almost absolute zero. While the importance of laser cooling in general terms was recognised by the awarding of the Nobel Prize in Physics 1997 to Chu, Cohen-Tannoudji, and Phillips, the 2025 Balzan Prize is a recognition of the tangible revolution produced by cold atoms in time metrology and the opening up of new scenarios.

The winner, Christophe Salomon, is credited with inventing pioneering methods which, starting with laser cooling, enabled him to fully develop atomic clocks. In 1991, he invented and first demonstrated the atomic fountain clock, in which caesium atoms, cooled by laser beams to a temperature of only a few microkelvins (millionths of a degree above absolute zero) are gently thrown upwards, like the jet of water in a fountain, and are measured throughout their upward and then downward motion. With this scheme, it is possible to significantly increase the duration of the measurement, and therefore its accuracy. Today, the atomic fountain scheme is used universally in metrology institutes around the world to create the network of interconnected primary atomic clocks that define International Atomic Time.

Christophe Salomon recognised the importance of defining time for science and, as a leading scientist also interested in fundamental physics, he also developed in collaboration with the Paris Observatory the first ‘transportable’ atomic fountain. He used it in the laboratories led by Theodor W. Hänsch (Nobel Prize in Physics 2005) for famous measurements of the hydrogen spectrum, which provided a very rigorous verification of quantum electrodynamics (QED), the physical theory that has been experimentally verified with the highest precision to date.

He was also the first to imagine the operation of these clocks with ‘ultracold’ atoms in the microgravity of space. Thirty years ago, with the support of CNES (Centre National d’Études Spatiales) and ESA (European Space Agency), he began developing the first caesium clock for the International Space Station (ISS), where the microgravity environment can allow for significant improvements in precision and, at the same time, the realization of highly accurate tests of the effects of general relativity. Only a few months ago, the space clock developed by Christophe Salomon and collaborators arrived on the ISS, and the first experimental signals already show an increase in interaction time compared to cesium fountain clocks on Earth.

The scientific community is working on the development of even more accurate atomic clocks, such as those operating at optical frequencies, 100 000 times faster than in microwaves. In this field too, Christophe Salomon’s cold atom clock operating in space could prove crucial to understanding the reproducibility of these new accurate atomic clocks operating in different locations on Earth.