Experimental study of a phase-sensitive heterodyne detector

It is believed that the quantum behaviors of homodyne detectors and traditional heterodyne detectors can be fully understood in the context of the quantum theory of optical detection. According to the theory, a 3 dB extra quantum noise has been predicted in a traditional heterodyne detector, as a phase-insensitive device, due to the existence of the image sideband vacuum. However, regarding the noise performance of a phase-sensitive heterodyne detector, a fundamental dilemma inevitably arises: On one hand, the detector should suffer the 3 dB noise penalty caused by the image sideband vacuum, on the other hand, it, as a phase-sensitive device, should be noise free at the quantum level. We report on an experiment on the quantum noise performance of a phase-sensitive heterodyne detector with a bichromatic local oscillator. The results show that the studied detector is noise free, i.e., the quantum noise of the image sideband vacuum is absent in the observation. Revealing the mechanism for the absence of the image vacuum noise will be important for a full understanding of the origin of the quantum noise in optical detection.