Opt Lett. 2018 Dec 15;43(24):6073-6076. doi: 10.1364/OL.43.006073.
A phase-sensitive (PS) heterodyne detector is intrinsically resistant to classical noises and useful in measurement of low-frequency signals below the shot noise. Despite the existence of image band vacuum, we show that the quantum-noise power level of this heterodyne detector sensing a coherent signal is exactly one light quantum per measurement time, i.e., twice the vacuum fluctuation power, which can be further reduced by use of squeezed light. We then report on an experiment on a PS heterodyne detector with a 10 Hz 1.0×10 W optical signal (1064 nm wavelength) at its input. The noise floor of the unmodulated coherent light is 2.2(±0.1)×10 W/Hz from 2 Hz to 20 Hz, and the signal-to-noise ratio is about 6.6 dB for the measured signal when the resolution bandwidth is 1 Hz. The quantum noise floor is reduced by 1.6(±0.3)dB when squeezed light is used, and the sub-shot-noise power spectral density is 1.6(±0.1)×10 W/Hz between 2 Hz and 20 Hz. This work should be an important advance towards squeezing-improved precision measurements of low-frequency signals with heterodyne detectors, including audio-band gravitational-wave detection.
相敏（PS）外差探测器本质上能抵抗经典噪声，且可用于测量低于散粒噪声的低频信号。尽管存在虚频带真空，但我们证明了这种外差探测器感应相干信号的量子噪声功率水平恰好是每次测量一个光量子，即真空涨落功率的两倍，而使用压缩光可以进一步降低该噪声功率水平。随后，我们报告了一个实验，该实验采用 PS 外差探测器，输入的是 10 Hz、1.0×10 W 的光信号（1064nm 波长）。未调制相干光的噪声底为 2.2(±0.1)×10 W/Hz，从 2 Hz 到 20 Hz，当分辨率带宽为 1 Hz 时，测量信号的信噪比约为 6.6 dB。当使用压缩光时，量子噪声底降低了 1.6(±0.3)dB，在 2 Hz 到 20 Hz 之间，亚散粒噪声功率谱密度为 1.6(±0.1)×10 W/Hz。这项工作应是朝着利用外差探测器进行低频信号的压缩增强精度测量迈出的重要一步，包括音频波段的引力波探测。
