STE-QUEST

The STE-QUEST (Space-Time Explorer and Quantum Equivalence Principle Space Test) satellite will host an atomic clock and an atom interferometer operated with the same atomic species, Rubidium-87 (87Rb). A second atom interferometer with a different isotope of the same species, Rubidium-85 (85Rb), will be operated as reference. STE-QUEST will test the far reaching consequences of Einstein's postulate and one of the most fundamental predictions of Einstein's Theory of General Relativity with high precision. It thereby searches for hints of quantum effects in gravity, contributing to the exploration of one of the current frontiers in fundamental physics. The mission will measure space-time curvature via the precise determination of gravitational time dilation, i.e. the difference in the ticking rate of the satellite's atomic clock when it is compared with a ground-based clock. At the same time, the satellite will allow for the comparison of the free propagation of coherent matter waves of 85Rb and 87Rb under the influence of the Earth's gravity with precision matter wave interferometry, striving for an accuracy of few parts in 1015 and beyond. The use of ultra-cold matter at quantum degeneracy will permit to go far beyond the current accuracy of quantum tests.

Sketch of the STE-QUEST mission concept using a highly elliptical orbit.

A highly elliptic orbit of the satellite is foreseen for STE-QUEST, providing a large variation in the gravitational potential between perigee and apogee and maximizing the accuracy of the measurements of the redshift. A mission duration of up to 5 years is intended. During apogee, the STE-QUEST atomic clock is compared to one or more ground based clocks, during perigee, the local acceleration of the two Rb isotopes is measured and compared via atom interferometry.

 

The STE-QUEST dual atom interferometer payload, which is based on the strong European developments in this field, is measuring the differential acceleration of the two atomic Rubidium isotopes along one axis with respect to the spacecraft with a resolution of 10-12 m/s2. The high common mode rejection of the dual atom interferometer permits to achieve this high differential accelerational resolution despite the ambient vibrational noise of the spacecraft. For achieving an optimal suppression of common-mode-noise, which accords to basically all possible sources of disturbance, the two atomic species are simultaneously prepared, coherently manipulated and detected with optimally overlapped atomic clouds of the two species.

 

The instrument payload is the central part of the spacecraft and includes the atomic clock and the atom interferometer. The key parameters of the atom interferometer are the atom number of 106, a free evolution time between beam splitter pulses of 5 s, and a resulting repetition rate of 20 s. With these parameters the single shot precision at the shot noise limit will be 2.5*10-12 and the target sensitivity at the 1-15 level will be reached after integration over 635 days.

Sketch of the STE-QUEST atom interferometer.

The atom interferometer consists of a physics package, a laser system and the corresponding control electronics. The physics package includes the coherent matter wave source, an atom chip combined with a dipole trap, the beam splitter unit, the detection system, and the accelerometer. The laser system comprises the laser sources for laser cooling and trapping, state preparation and detection as well as the coherent manipulation of the atoms for forming the interferometer. In addition the system includes the laser for confining the atoms in a dipole trap. The laser system also includes the light distribution module for modifying and controlling the laser frequency, polarization and power. The control electronics consists of a data management unit, a low noise RF generator, and several driver modules to control the laser system and physics package.

 

In the current assessment study, a baseline concept and a preliminary design of the atom interferometer are worked out.

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