The core instruments required to conduct STE-QUEST science investigations are:
- Differential atom interferometer;
- Science link operating in the microwave;
- GNSS receiver.
The atom interferometer provides differential acceleration measurements between ultra-cold samples of 85Rb and 87Rb in free fall. The microwave link, driven by the frequency reference provided by the internal ultra-stable oscillator or eventually by the cold-atom clock, is used to perform clock comparisons.
In addition, the following optional payload elements have been identified:
- High-performance cold-atom clock;
- Optical link.
The availability of a high-stability and accuracy cold-atom clock on-board STE-QUEST opens the possibility to perform space-to-ground clock comparisons, thus allowing a precision measurement of the gravitational redshift effect in the field of the Earth. The optical link is a complementary technique ensuring outstanding stability and very short averaging times for the comparison of atomic clocks on the ground.
The dual species atom interferometer (ATI) compares the free evolution of matter waves of ultra-cold rubidium 85Rb and 87Rb atoms. The differential acceleration between the two samples is continuously measured, while the spacecraft orbits around perigee where the signal-to-noise ratio of an eventual Weak Equivalence Principle (WEP) violating signal is maximized. The ATI instrument consists of three main functional units based on a modular design: the physics package (PP), the laser system (LS), and the electronics package (EP). Surrounded by a four-layer magnetic shield, the PP hosts the atom chip, the optical trap, and a 12 cm baseline interferometer inside an ultra-high vacuum system (see Figure). Additional coils and optics for atom manipulation, and the CCD cameras for atom detection are attached to the chamber or the chamber mounts. The vacuum pumps are located outside the magnetic shield. The laser system features high power diode lasers at 780 nm for cooling, detection, preparation, and coherent manipulation of both species. A telecom laser at 1560 nm generates the light fields for the Optical Dipole Trap (ODT) and serves, after frequency doubling, as a frequency reference locked to the atomic transition. Both PP and LS rely on the current drivers and controllers of the EP. A data management unit (DMU) executes the experimental sequences in real time, stores the data, and is capable of fitting images, determining atom numbers, and running differential evolution algorithms for autonomous optimization of the atomic source parameters.
The microwave link (MWL) design is an evolution of the science link presently under development for the ACES mission. The end-to-end system is composed of a flight segment unit and a distributed network of ground terminals, respectively connected to the clocks on-board the STE-QUEST spacecraft and on the ground. The input clock signal is up-converted and used to coherently generate the microwave signals that are transmitted through the atmosphere and received by the remote terminal at the other end of the link. The space segment provides four independent receiving channels capable of performing up to four simultaneous comparisons of the space clock with clocks on ground. The comparison of two ground clocks in common-view can be obtained by evaluating the difference of the two simultaneous space-to-ground comparisons. As the noise of the space clock is in common-mode, common-view comparison can be carried out without the need for a high performance on-board clock. A commercial ultra-stable oscillator (USO) is indeed sufficient for this purpose.