The detection of gravitational waves has ushered in a new era of observing the universe. Quantum resource advantages offer significant enhancements to the sensitivity of gravitational wave observatories. While squeezed states for ground-based gravitational wave detection have received marked attention, the generation of squeezed states suitable for mid-to-low-frequency detection has remained unexplored. To address the gap in squeezed state optical fields at ultra-low frequencies, we report on the first direct observation of a squeezed vacuum field until Fourier frequency of 4 millihertz with the quantum noise reduction of up to 8.0 dB, by the employment of a multiple noise suppression scheme. Our work provides quantum resources for future gravitational wave observatories, facilitating the development of quantum precision measurement.

引力波的探测开创了观测宇宙的新时代。量子资源优势显著提高了引力波天文台的灵敏度。虽然用于地面引力波探测的压缩态受到了极大的关注，但适用于中低频探测的压缩态的产生仍未得到探索。为了解决超低频压缩态光场的差距，我们报告了通过采用多重噪声抑制方案，首次直接观测到傅里叶频率为 4 毫赫兹的压缩真空场，量子噪声降低高达 8.0 dB。我们的工作为未来的引力波天文台提供了量子资源，促进了量子精密测量的发展。