Innovative concepts of compact cold atom clocks, derived from cold atom fountains but allowing to be miniaturized, are currently being industrialized. In a cold atom fountain, the different interactions (laser cooling, atomic preparation, atom interrogation, clock signal detection) take place in different locations within the clock, during the time when the atoms are flying freely. In order to reduce the clock size of, the device is designed so that all these interactions take place sequentially in the same place, inside the microwave cavity, thus merging all the interaction zones into one. Thus, instead of being carried out by collimated laser beams, the atoms radiative cooling is carried out by an isotropic laser light generated by reflection/diffusion on the microwave cavity walls. Moreover, the use of rubidium (secondary representation of the SI second) instead of cesium makes it possible to significantly improve the clock long-term stability by reducing the frequency shift due to collisions between cold atoms, but also to improve reliability by using technologies derived from optical telecommunications. These compact cold atom clocks, which were the subject of a transfer of know-how from the SYRTE to the µQuanS company, were tested in microgravity during parabolic flights aboard the Airbus Zero-G. Ambitious performance goal aims at a stability of some 10^-13 at 1 second and a stability floor below 10^-15. Accuracy in the 10-¹⁵ range is also expected.

Rubiclock clock, using cold Rb atoms, tested in microgravity (Source: CNES, Novespace, SYRTE, µQuanS)

Other concepts aim at using cold atoms trapped in magnetic fields generated on atomic chips. As for ion clocks, trapping has the advantage of allowing long interaction times in a small device, but it has the disadvantage of inducing frequency shifts due to interactions not only with the trapping magnetic field but also between cold atoms. The trapping technique on an atomic chip also has the advantage of being able to compare the clock operation with classical cold atoms or with ultra-cold atoms placed in a single giant quantum state associated with the Bose-Einstein condensation phenomenon. The studies carried out have allowed to demonstrate exceptional coherence times (several tens of seconds) making it possible to hope for ultrafine resonance lines.