Christophe Salomon - A Laser Cooled Primary Cesium Clock in Space

On April 21, 2025, the Atomic Clock Ensemble in Space (ACES) was launched towards the International Space Station by a SpaceX Falcon rocket. Within one week, the ACES payload was installed on an external platform of the European module Columbus and turned on by the ACES control center located in CNES Toulouse (FR) via the Columbus and ISS control centers.

ACES is an ESA-CNES fundamental physics mission carrying two atomic clocks, a cold atom clock, PHARAO, designed for operating in microgravity, and a Space Hydrogen Maser (SHM). These clocks are connected to a worldwide network of microwave or optical clocks in national metrology institutes with advanced time transfer systems, operating either in the microwave domain or in the optical domain with short laser pulses. The ACES mission is currently in the commissioning phase, where the onboard clocks and time transfer links are turned on and characterized.

In this talk, we will start by recalling the scientific objectives of the mission and give the current status of the main instruments. We will present the first signals of the PHARAO clock in microgravity and preliminary frequency comparisons with cesium and rubidium fountain clocks located at Paris Observatory. The commissioning phase will end with the full deployment and operation of 7 ground terminals located on three continents, four in Europe (PTB, NPL, Paris Observatory, Wetzell Observatory), two in the USA (JPL and NIST) and one in Japan (NICT).

Intercontinental time and frequency comparisons of microwave and optical clocks with increased accuracy as compared to GPS time comparison systems will enable new tests in fundamental physics and provide additional support for a future redefinition of the second based on optical clocks.