Exploring Gravity with Optical Clocks



What is an optical clock?

Mission proposals


EU-FP7 project "SOC2"




Space-Time Explorer and Quantum Equivalence Principle Space Test

Recent news: ESA selects "STE-QUEST" as one of four medium-size candidate missions for a flight in 2022+

ESA Mission homepage

The Space-Time Explorer and Quantum Test of the Equivalence Mission (STE-QUEST) is devoted to a precise measurement of the effect of gravity on time and matter using an atomic clock and an atom interferometer. It tests a fundamental assumption and one of the most fundamental predictions of Einstein’s Theory of General Relativity with high precision and thereby searches for hints of quantum effects in gravity, contributing to the exploration of one of the current frontiers in fundamental physics.

The first primary goal of the mission will be to measure space-time curvature via the precise determination of gravitational time dilation. The ESA ACES mission on the ISS aims to improve on current knowledge by approximately a factor 10. STE-QUEST aims to achieve a further improvement compared to ACES, by a factor between 45 and 400 (in the advanced STE-QUEST scenario), making use of a highly elliptic orbit and advanced atomic clocks. In addition, STE-QUEST will also perform a precise measurement the redshift due to Sun’s gravitational field, thereby testing for the independence of the redshift on the nature of the mass producing the gravitational field., The improvement is a factor 1E6 compared to current knowledge and approx. 20 compared to ACES.

A second primary goal is a quantum test of one aspect of the Equivalence Principle, the universality of free-fall. STE-QUEST will test various aspects by interferometrically tracking the propagation of matter waves in the Earth field, striving for an accuracy better than one part in 1E15.

The satellite payload consists of two instruments: a cold atom-based clock of highest performance and an atom interferometer.

The clock is derived from the well-developed microwave clock PHARAO (ACES heritage). The performance of the clock is improved compared to the current implementation for ACES by an optically derived ultra-pure microwave signal and by using the more favourable atomic species Rubidium. During the mission, the space clock will be compared with atomic clocks on the Earth, using precise microwave frequency transfer methods similar to those developed for the ACES mission, as well as using a laser coherent link based on the successful LCT technology

The atom interferometer will compare the free propagation of coherent matter waves of two isotopes of rubidium (85Rb. 87Rb) under the influence of the Earth's gravity. The use of ultra-cold matter at quantum degeneracy will permit to go far beyond the current accuracy of tests.

The highly elliptic orbit of the satellite provides a large variation in the gravitational potential between perigee and apogee and maximizes the accuracy of the measurements of the redshift,. Clock and atom interferometer will operate alternately. A mission duration of up to 5 years is intended.

Besides the fundamental physics results, the mission provides a new tool for mapping the gravitational potential on the Earth surface with high spatial resolution and at a high level of accuracy (1 cm equivalent height). STE-QUEST will establish a global reference frame for the Earth’s gravitational potential.

The STE-QUEST mission concept is derived from the STE assessment study by ESA’s Concurrent Design Facility in June-July 2010, following the 2010 recommendations of ESA’s Fundamental Physics Working Group.

Many components and subsystems of the satellite payload have been developed up to engineering or flight models in the frame of the ESA ACES mission and other missions using lasers and optical techniques, and have technical readiness level of 6-8. In particular, the atomic clock PHARAO and the associated microwave link will have been validated in space on the ISS in 2014. The laser link is based on the currently operational LCT technology used by ESA. Some subsystems are currently under development and are expected to reach TRL 5 by 2014, as required by the call. This includes planned tests of hardware in the drop tower and on rocket flights.

Download presentation about the time dilation test of STE-QUEST (2011)

Download proposal text (2010)