The mission payload consists of two instruments: a highly precise cold-atom clock and a differential atom interferometer.

The clock is derived from the microwave frequency standard Projet d'Horloge Atomique par Refroidissement d'Atomes en Orbite (PHARAO), which is also the core instrument of the Atomic Clock Ensemble in Space (ACES) mission. The performance of the clock is improved compared to the current implementation for ACES by an optically derived ultra-pure microwave signal.

The figure below shows the working principle of the PHARAO clock. During the mission, the tick rate of the clock will be compared almost continuously with atomic clocks on Earth using precise microwave frequency transfer methods similar to those developed for the ACES mission, as well as by using a laser coherent link based on the Laser Communication Terminal (LCT) technology in use by ESA.

Caesium atoms, launched in free flight along the PHARAO tube, cross a resonant cavity where they interact twice with a microwave field tuned to the transition between the two hyperfine levels of the ground state of rubidium. In a freely falling laboratory, the velocity of the atoms along the ballistic trajectories is constant and can be changed continuously over almost two orders of magnitude (5-500 centimetres per second), allowing the detection of atomic signals with sub-Hertz linewidth.  Credit: CNES.

The differential atom interferometer will compare the free propagation of the coherent matter waves of the two rubidium isotopes (⁸⁵Rb and ⁸⁷Rb) under the influence of Earth's gravity. The use of ultra-cold matter close to, or down to, quantum degeneracy (coherent atomic sources) and the long interrogation times possible in a freely falling laboratory will allow experimenters to go far beyond the accuracy levels of current measurements.