The space cold atom interferometer for testing the equivalence principle in the China Space Station

Einstein's equivalence principle (EEP), a cornerstone of general relativity (GR), comprises three parts: the weak equivalence principle (WEP), local Lorentz invariance (LLI), and local position invariance (LPI). The WEP posits that objects with different compositions experience the same acceleration in a given gravitational field. The incompatibility between GR and quantum mechanics (QM) has led to theories (like loop quantum gravity and noncommutative geometry) that predict WEP violations, highlighting the need for high-precision WEP tests. Historically, WEP tests have employed various methods, including Newtonian pendulums, torsion balances, free-fall experiments, Lunar Laser Ranging, and astronomical observations, achieving ground-based precisions up to 10⁻¹³. Atom interferometers (AIs), using microscopic atoms, offer a path to higher precision. Early AI-based WEP tests achieved 10⁻⁷ precision, later improved to 10⁻¹² using ultracold Rb atoms. The microgravity environment offers advantages for both macroscopic and microscopic WEP tests; the MICROSCOPE satellite achieved 10⁻¹⁵ precision using macroscopic objects. Space-based AI experiments like QUANTUS, MAIUS, ICE, and STE-QUEST are aimed at further improving WEP test precision. The China Space Station (CSS), particularly its MSLC offering 10⁻⁷g microgravity, provides a platform for this research, but with constraints on payload size and power consumption.

The paper reviews existing literature on WEP tests, highlighting various methodologies and their achieved precision. It emphasizes the limitations of ground-based experiments and the advantages of microgravity environments for enhancing the accuracy of these tests. It then surveys existing and proposed space-based atom interferometer missions, noting their goals, technologies, and progress. The literature review establishes the context for the current research by showcasing the state-of-the-art in WEP testing and the motivations behind the development of a new, compact space-based interferometer.

The research methodology focuses on designing and implementing a compact dual-species rubidium atom interferometer payload for the CSS's MSLC. To overcome power consumption limitations within the MSLC, a 3D velocity selection method is used instead of evaporative cooling to achieve an effective low atomic ensemble temperature. This method selects a desired 3D atom velocity distribution by manipulating Raman pulses and the detection area. The paper provides a mathematical description of this selection process, calculating the selection ratio and effective temperature. The interferometer utilizes a double-diffraction Raman interference scheme with the phase shear method for fringe extraction, which is robust against noise and dephasing. To compensate for the residual rotation of the magnetic suspension bench (MSB), a closed-loop controlled piezoelectric deflector and IMU are employed. The design also incorporates an optimization of Raman laser ratios to eliminate AC-Stark shifts, considering the simultaneous operation of dual-species interferometers and the use of fiber electro-optical modulators (FEOMs). The payload comprises three systems: a physical system (vacuum chamber, atom source, laser cooling and interference components), an optical system (lasers, fibers, modulators, and detectors), and an electronic system (power supply, control unit, and functional circuits). The paper provides detailed descriptions and diagrams of each system, emphasizing miniaturization and thermostability. The experimental process in space involves laser cooling (2D-MOT, 3D-MOT, PGC), atom interference (double-diffraction Raman pulses with rotation compensation), and fluorescence detection using a SCMOS camera. The paper mathematically models the sources of error, including quantum projection noise, detection noise, rotation, gravity gradient, magnetic field, AC-Stark shift, and wavefront aberration, providing quantitative estimations of their contributions to the overall uncertainty.

The study presents a compact (460 mm × 330 mm × 260 mm) dual-species rubidium atom interferometer payload designed for the China Space Station's MSLC. The 3D velocity selection method successfully reduces atomic temperature without excessive power consumption. The double-diffraction Raman interference with phase shear and rotation compensation allows for accurate measurements. Analysis of error sources shows that quantum projection noise, detection noise, rotation, gravity gradient, and magnetic field are the most significant contributors to uncertainty. The estimated overall uncertainty for the WEP test is 4.0 × 10⁻¹⁰ for 100 shots. Ground-based experiments demonstrated successful atom cooling and interference, though with lower contrast due to limitations in free-fall time and interference time. The ground tests verified the functionality of the payload's subsystems and its robustness to environmental stresses. The expected test precision in space is estimated to be in the order of 10⁻¹⁰, with potential improvement to 10⁻¹¹-10⁻¹² with increased data acquisition time and further refinement of techniques.

The results demonstrate the successful design and ground testing of a compact and robust atom interferometer for space-based WEP tests. The achieved precision in ground tests, while lower than the expected space-based precision, validates the core functionalities of the instrument. The identification and quantification of major error sources provide a roadmap for future improvements, focusing on averaging noise with more data, reducing magnetic field effects, and refining rotation and gravity gradient compensation. The significant error contribution from rotation and gravity gradient suggests that future iterations might benefit from employing advanced compensation techniques or selecting a more stable platform. The successful launch and deployment of the payload to the China Space Station mark a significant step towards achieving higher-precision WEP tests in a microgravity environment.

This research successfully designed, constructed, and ground-tested a compact dual-species atom interferometer payload for high-precision WEP tests aboard the China Space Station. The payload's compact design addresses the constraints imposed by the MSLC, while its functionality was validated through rigorous ground testing. The expected space-based WEP test precision is on the order of 10⁻¹⁰, with potential for improvement. Future work could focus on advanced error reduction techniques and data analysis to achieve even greater precision. This project provides a valuable foundation for future space-based atom interferometry experiments.

The size and power constraints of the MSLC limit the interferometer's performance compared to ground-based experiments with less restrictive conditions. The limited interference time and the impact of the MSB's residual rotation contribute to the uncertainties. While the ground-based experiments validated the system's components, they did not fully replicate the space environment and the resulting data has lower contrast than expected for space-based experiments. The precision of the WEP test is primarily limited by the rotation of the MSB, gravity gradient, and the magnetic field. Further improvements in the control of these factors are needed to achieve higher precision.