Atomic Clock Ensemble in Space

TL;DR

The ACES mission aims to develop and deploy high-precision atomic clocks in space to test fundamental physics, including Einstein's relativity, and to enable global clock comparisons and geophysical measurements.

Contribution

This paper details the development, testing, and scientific objectives of the ACES space-based atomic clock ensemble, advancing space-based timekeeping and fundamental physics tests.

Findings

Achieved fractional frequency stability of 1E-16 in space clocks

Successfully tested the performance of the PHARAO and SHM clocks

Validated the space-to-ground clock comparison links

Abstract

The Atomic Clock Ensemble in Space (ACES) mission is developing high performance clocks and links for space to test Einstein's theory of general relativity. From the International Space Station, the ACES payload will distribute a clock signal with fractional frequency stability and accuracy of 1E-16 establishing a worldwide network to compare clocks in space and on the ground. ACES will provide an absolute measurement of Einstein's gravitational redshift, it will search for time variations of fundamental constants, contribute to test topological dark matter models, and perform Standard Model Extension tests. Moreover, the ground clocks connected to the ACES network will be compared over different continents and used to measure geopotential differences at the clock locations. After solving some technical problems, the ACES flight model is now approaching its completion. System tests involving the laser-cooled Cs clock PHARAO, the active H-maser SHM and the on-board frequency comparator FCDP have measured the performance of the clock signal delivered by ACES. The ACES microwave link MWL is currently under test. The single-photon avalanche detector of the optical link ELT has been tested and will now be integrated in the ACES payload. The ACES mission concept, its scientific objectives, and the recent test results are discussed here together with the major milestones that will lead us to the ACES launch.