ACES-PHARAO is a space mission aiming to realise a time scale on board the International Space Station, and carry out comparisons with clocks on ground, at several National Metrology Institutes. This will allow a number of experimental tests of General Relativity, among which a gravitational redshift test, at unprecedented levels.

The ACES platform is the name given to the whole payload, under ESA supervision. PHARAO is a cold-atom caesium clock, which drives the time scale inside this platform and gives it its unprecedented accuracy in space. This clock has been funded by CNES and developped with expertise from Laboratoire Kastler Brossel and SYRTE, especially by the "Microwave metrology group".

Data analysis

The «Theory» team is heavily involved in the project since its beginning, including the definition and study of the science objectives, hardware specification, and preparation of data analysis, with a focus on data expected from the Microwave Link (MWL). It will therefore be in the first line to extract science products and interpret them in terms of a possible violation of fundamental laws of physics.

Signaux MWL de transfert de temps

The method relies on 3 signals for time transfer : one is uplink (ground to space) Ku band, another one is downlink with another frequency in the same band, and a second downlink signal at a much lower frequency (S band) in order to measure and compensate the ionospheric delay. We apply a two-way time transfer method to cancel, at first order, the effect of the uncertainty on the distance between the two terminals.

Expected scientific results

By realizing such a stable and accurate time scale in orbit over various ground stations, ACES will allow clock comparisons at an unprecedented level between the most advanced clocks throughout the world. Optical clocks using different atom species will be compared, so that ACES will be able to put new limits on possible variations of fundamental constants (α, m_e/Λ_QCD, and m_q/Λ_QCD).

Other fundamental physics tests will also be possible : the gravitational redshift effect will be compared to predicted values from General Relativity, with an uncertainty as low as 10^-6 (35 times better than previous measurements of this kind).

Finally, we will use those measurements to test and put constraints on alternatives theories to the General Relativity.

Publications

• Cacciapuoti, L., and Ch. Salomon, ‘Space Clocks and Fundamental Tests: The ACES Experiment’, The European Physical Journal Special Topics, 172.1 (2009), 57–68 <https://doi.org/10.1140/epjst/e2009...>
• Cacciapuoti, Luigi, Michele Armano, Rudolf Much, Omar Sy, Achim Helm, Marc Peter Hess, and others, ‘Testing Gravity with Cold-Atom Clocks in Space’, The European Physical Journal D, 74.8 (2020), 164 <https://doi.org/10.1140/epjd/e2020-...>
• Laurent, Philippe, Didier Massonnet, Luigi Cacciapuoti, and Christophe Salomon, ‘The ACES/PHARAO Space Mission’, Comptes Rendus Physique, The measurement of time / La mesure du temps, 16.5 (2015), 540–52 <https://doi.org/10.1016/j.crhy.2015...>
• Lilley, M., E. Savalle, M. C. Angonin, P. Delva, C. Guerlin, C. Le Poncin-Lafitte, and others, ‘ACES/PHARAO: High-Performance Space-to-Ground and Ground-to-Ground Clock Comparison for Fundamental Physics’, GPS Solutions, 25.2 (2021), 34 <https://doi.org/10.1007/s10291-020-...>
• Meynadier, F., P. Delva, C. le Poncin-Lafitte, C. Guerlin, and P. Wolf, ‘Atomic Clock Ensemble in Space (ACES) Data Analysis’, Classical and Quantum Gravity, 35.3 (2018), 035018 <https://doi.org/10.1088/1361-6382/aaa279>
• Salomon, Ch., N. Dimarcq, M. Abgrall, A. Clairon, P. Laurent, P. Lemonde, and others, ‘Cold Atoms in Space and Atomic Clocks: ACES’, Comptes Rendus de l’Académie Des Sciences - Series IV - Physics, 2.9 (2001), 1313–30 <https://doi.org/10.1016/S1296-2147(...>
• Savalle, E., C. Guerlin, P. Delva, F. Meynadier, C. le Poncin-Lafitte, and P. Wolf, ‘Gravitational Redshift Test with the Future ACES Mission’, Classical and Quantum Gravity, 36.24 (2019), 245004 <https://doi.org/10.1088/1361-6382/ab4f25>