(1) Introduction to the Gravimeter System

Gravitational acceleration g is a crucial physical quantity in geophysical research, and its precise measurement is vital in fields such as gravitational wave detection, geological surveying, disaster prediction, and national security. The ultimate measurement precision of an atomic interferometric gravimeter is closely related to the free evolution time of atoms in the gravitational field, with longer free evolution times resulting in higher measurement precision. In traditional free-fall or fountain-type atomic interferometers, large physical system volumes are necessary to achieve high-precision measurements, often with interference regions extending over several meters, which limits the potential for miniaturization.

To achieve high-precision measurements while enabling miniaturization, we have combined traditional atomic interferometry with the manipulation of moving optical lattices. This approach has led to the development of a Ramsey-Bordé ⁸⁷Rb atomic interferometric gravimeter. By utilizing the principle of Bloch oscillations in a moving optical lattice, we impart upward momentum to freely falling atoms, reversing their velocity. Multiple optical lattice pulses enable the atoms to undergo prolonged evolution within a smaller spatial region, thus achieving both high precision and compactness.

We were the first in China to achieve gravity measurement based on a moving optical lattice. The vertical displacement of the cold atoms is approximately 3 cm, which is two orders of magnitude smaller compared to the conventional Mach-Zehnder atomic gravimeters. The short-term sensitivity of the measurement can reach 4.52\times{10}^2\mu Gal/\sqrt{Hz}，a level approximately twice that of similar international instruments. The system's ultimate resolution can reach，a level comparable to that of traditional free-fall gravimeters.

Building on the cold atom interferometer, we further achieved Bose-Einstein condensation (BEC) of ⁸⁷Rb atoms in an all-optical trap. After evaporative cooling in a crossed optical dipole trap, we were able to prepare ultracold atoms with a temperature of approximately 52 nK and a particle number of around 1.6\times{10}^5. Based on the BEC system, we have conducted a series of scientific experiments.

(2) Current Work: Spatially Correlated Lattice Interferometer

Atomic interferometers, with their unprecedented precision, have become powerful tools for measuring fundamental physical constants and testing fundamental physical theories. Traditional atomic interferometry mainly focuses on the phase difference between two interference paths and relies on matter waves with well-defined coherence properties. In our recent work, we have implemented a Ramsey–Bordé interferometer of coherent matter waves loaded into a moving optical lattice along the direction of gravity. Furthermore, we have explored interference patterns arising from multi-path interference with tunable coherence. Through interferometric measurements, we have studied the spatial correlations of atoms within the optical lattice and between the two arms of the interferometer. Multiple interference peaks were observed, originating from the long-range coherence of the Bose–Einstein condensate. Interestingly, we observed that the positions of these interference peaks in both space and time deviate from conventional expectations. This led us to propose new insights into the question of when and where interference occurs in an atomic interferometer. Based on the dynamics of wavefunction expansion and collapse, we developed theoretical models that are in good agreement with experimental observations, providing a solid foundation for future high-precision measurements using ultracold atom interferometry. The details of this work can be found at https://doi.org/10.48550/arXiv.2406.16847

(3) Current Work: Investigation of Ramsey Fringes in a Microwave Atomic Clock Based on Optical Lattices.

The Ramsey interference technique is the core technology of most microwave atomic clocks. To be specific, extending the free evolution time during the process is key to narrowing the fringe linewidth and enhancing the performance of atomic clocks. At present, the space-for-time scheme is generally adopted and the height of the fountain clock is increased to meter level. In our research, a new scheme to narrow the fringe linewidth is proposed by using the principle of Bloch oscillation of ultracold atoms in the moving optical lattice. During the free evolution time of the Ramsey interference process, a Bloch pulse applies recoil momentum in the opposite direction of gravity to the ultracold atoms, allowing the atoms to perform free-fall motion within a centimeter-scale cavity space. The main content of this research is to modify the experimental time sequence to increase the free evolution time, narrow the linewidth of Ramsey fringe of microwave clock transition, and research the decoherence problems caused by optical lattice, on the basis of the optical lattice gravimeter that has been realized. At present, we have achieved Ramsey fringes with a linewidth of approximately 3.3 Hz using this scheme. In the future, by increasing the number of moving optical lattice pulses and incorporating optical cavity technology, we expect to further extend the interference time, ultimately achieving Ramsey fringes with a linewidth comparable to that of fountain clocks. This advancement will lay a solid foundation for the development of compact, high-precision microwave atomic clocks.