An ultranarrow bandwidth Faraday atomic filter is realized based on cold Rb87 atoms. The atomic filter operates at 780 nm on the 52S1/2, F=2 to 52P3/2, F′=3 transition with a bandwidth of 7.1(8) MHz, which is approaching the natural linewidth of the transition. The peak transmission achieves 2.6(3)% by the multi-pass probe method. This atomic filter based on cold atoms may find potential applications in self-stabilizing lasers in the future.

Many efforts have been taken in recent years to push atomic gravimeters toward practical applications. We demonstrate an atomic gravimeter named NIM-AGRb2 that is transportable and suitable for high-precision gravity measurements. Constraint-structured active vibration isolation (CS-AVI) is used to reduce the ground vibration noise. The constraint structure in CS-AVI ensures that the isolation platform only has vertical translation, with all other degrees of freedom (DoFs) being constrained. Therefore, the stability of active vibration isolation is enhanced. With the implementation of CS-AVI, the sensitivity of NIM-AGRb2 reached as low as 20.5 μGal/Hz^1/2. The short-term sensitivity could be further reduced to 10.8 μGal/Hz^1/2 in a seismologic observation station. Moreover, we evaluated the system noise of the gravimeter, and the results were consistent with our observations.

The paper describes a Raman-laser system with high performance for an absolute gravimeter that was based on ⁸⁷Rb atom interferometer. As our gravimeter is a part of the standard acceleration of gravity of China, the Raman lasers’ characteristics should be considered. This laser system includes two diode lasers. The master laser is frequency locked through the frequency-modulation (FM) spectroscopy technology. Its maximum frequency drift is better than 50 kHz in 11 h, which is measured by home-made optical frequency comb. The slave laser is phase locked to the master laser with a frequency difference of 6.8346 GHz while using an optical phase lock loop (OPLL). The phase noise is lower than −105 dBc/Hz at the Fourier frequency from 200 Hz to 42 kHz. It is limited by the measurement sensitivity of the signal source analyzer in low Fourier frequency. Furthermore, the power fluctuation of Raman lasers’ pulses is also suppressed by a fast power servo system. While using this servo system, Raman lasers’ pulses could be fast re-locked while its fast turning on again in the pulse sequence. The peak value fluctuation of the laser power pulses is decreased from 25% to 0.7%, which is improved over 35 times. This Raman-laser system can stably operate over 500 h, which is suited for long-term highly precise and accurate gravity measurements.

Atomic optical filters such as Faraday anomalous dispersion optical filters (FADOFs) or similar technologies can achieve very narrow optical bandwidth close to the scale of atomic linewidth, which can be greatly reduced in cold atoms. However, limited by the number of cold atoms and the size of the cold atomic cloud, the number of atoms interacting with the laser is reduced, and the transmission remains as low as 2%. In this work, we introduce the optical pumping into the cold atomic optical filter to solve this problem. Circular polarized optical pumping can produce polarization of the atomic ensemble and induce dichromatic as well as the Faraday rotation. We demonstrate a cold-atom optical filter which operates on the ⁸⁷Rb 5²S_1/2 (F=2) to 5²P_3/2 (F′=2) transition at 780 nm. The filter achieves an ultranarrow bandwidth of 6.6(4) MHz, and its peak transmission is 15.6%, which is nearly 14 times higher than that of the cold-atom optical filter realized by Faraday magneto-optic effect. This scheme can be extended to almost all kinds of atomic optical filters and may find applications in self-stabilizing laser and active optical clock.

Measuring low-frequency and ultra-low-frequency vibration signals is of critical importance in fields such as structural mechanics, geological exploration, aerospace, precision machining, and biomedicine. Existing methods face limitations in achieving both ultra-low-frequency range and high precision. We present an ultra-low-frequency vibration measurement method based on the atom interferometer, capable of measuring vibration signals from 0.01 Hz to DC. The performance of measurement was experimentally demonstrated for vibrations between 0.007 Hz and 0.01 Hz, achieving a sensitivity of $1.1 \mathsf{\mu }\mathrm{m}/{\mathrm{s}}^2/\sqrt{\mathrm{Hz}}$. Incorporating active vibration isolation can further enhance the measurement range, increasing the optimal sensitivity to $0.2 \mathsf{\mu }\mathrm{m}/{\mathrm{s}}^2/\sqrt{\mathrm{Hz}}$.

Frequencies of frequency standards are shifted by the local static gravity red shifts and also modulated by the tidal relativistic red shifts. We compute the tidal relativistic red shifts using a time-domain method and present the numerical results for the National Institute of Metrology (NIM) in Beijing, Laboratoire National de Métrologie et Essais-Systéme de Références Temps-Espace (LNE-SYRTE) in Paris and Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig. The differences of the tidal relativistic red shift approach as large as 1.1×10⁻¹⁶ when frequency standards at NIM are compared with those at SYRTE and PTB. Moreover, the tidal relativistic red shifts of frequency standards in space stations are also computed.