  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.
