At the end of summer 2010, a first series of time transfer experiments was carried out between the Observatory of the Cote d'Azur and the Observatory of Paris. For that purpose, a transportable laser station was set up on the roof of the Observatory of Paris and linked to the best cooled atom clocks. The objective of this experiment was to measure the time even more accurately.
The time measurement is omnipresent. The clocks spread all around the world and compare themselves to the others. Look around yourself, on your wrist, on the train station pediment, on your phone and in the corner of your computer. If you do not see any, look at the sky. Those of the GPS constellation and soon the Galileo ones are orbiting the Earth in order to enable you to localise and navigate thanks to a very accurate synchronization of several "atomic" clocks. In fact, the time measurement is currently based on a periodic phenomenon at the atomic level. The official time reference is based on the caesium atom: one second is defined as 9,192,631,770 rotation periods corresponding to the transition between two hyperfine states of the caesium 133 atom fundamental state.
Other atoms are also used. The clocks have been improving their accuracy and stability for multiple uses: from the most fundamental ones with theoretical physics, rethought since Albert Einstein and its relativity theory, to the daily uses such as navigation and telecommunications.
The absolute time does not exist. It was replaced by many discordant individual times which have to be synchronized. Time and space are interrelated in the four-dimensional spacetime and gravity also plays its role by warping the spacetime. Everything is getting more complicated. Not only do we need good clocks, but comparison and signal ranging devices are also required.
At the beginning, the remote clock synchronization was based on radio time signals which transferred the time with an accuracy in the millisecond (at the 3rd top, it will be...). In 1968 with the Loran-C navigation system, the accuracy reached the 2 microseconds. It was thus possible to build an average time scale based on the comparison of an atomic clock set.
Nowadays, the remote clock comparison method is based on radio frequency signal exchanges from the GPS constellation or dedicated signals exchanged by a geostationary satellite (Two Way Time Transfer). Thanks to the GPS, the time transfer accuracy reached some nanoseconds.
To be compared, the new ultra-stable clocks, developed in the time metrology laboratories, such as the laser-cooled atom clocks, cryogenic oscillators and trapped-ion clocks need time transfer systems with enhanced performances of two orders of magnitude compared to the current methods.
This task has been entrusted to the T2L2 experiment (Time Transfer by Laser Link) on board the Jason-2 altimetry satellite as a passenger in June 2008. The objective is to carry out a time transfer between two clocks by using not a radio frequency signal but an optical one encoded on a light pulse coming from laser telemetry stations featured with ground-based clocks.
To compare the remote ground-based clock signals, the ground-based laser stations jointly shot at the satellite (common-view).
More than some twenty laser stations of the ILRS (International Laser Ranging Service) international network implemented all around the planet are currently sending their data.
Datation accumulation associated with the analysis of the system calibration experiments enabled a residual noise to be announced with a measurement of less than a dozen of picoseconds and an accuracy of less than 100 picoseconds. This achievement defined a new state of the art for time transfer.
Our ancestors were looking at the sky to read the time when crossing the oceans. Finally, we also turn to the sky to compare and synchronize the remote clocks. Our horizon spreads and the optical links will link us to probes featured with clocks going to the solar system. But that is another adventure, in 50 years maybe.
- CNES Fundamental physics programme scientist: Sylvie Léon