Sensing subhertz optical signals at the quantum noise limit with heterodyne detectors

To sense quantum-noise-limited subhertz optical signals is of great interest to both fundamental science and technological development in practice. However, due to the presence of low-frequency classical noises, the sensitivity of optical signal detection based on existing technologies has never reached the quantum noise limit near 1 Hz. In this work, we explore heterodyne detection, the most sensitive method to capture low-frequency weak optical signals in the context of classical optics, to sense subhertz optical signals at the quantum noise limit. Starting from the quantum theory of optical coherence, we establish a model for balanced heterodyne detection and obtain the theoretical value of the detector's absolute quantum noise level. Then we present an experiment on sensing a subhertz optical signal with a heterodyne detector, achieving a quantum-noise-limited sensitivity from 0.2 Hz to 10 Hz. The achieved sensitivity makes it possible to sense subhertz optical signals at the power level of one photon energy per second at room temperature. This work paves the way to ultra-sensitive technological development for subhertz optical signal sensing at the quantum noise limit.