  Heterodyne detectors as phase-insensitive (PI) devices have found important
applications in precision measurements such as space-based gravitational-wave
(GW) observation. However, the output signal of a PI heterodyne detector is
supposed to suffer from signal-to-noise ratio (SNR) degradation due to image
band vacuum and imperfect quantum efficiency. Here we show that the SNR
degradation can be overcome when the image band vacuum is quantum correlated
with the input signal. We calculate the noise figure of the detector and prove
the feasibility of heterodyne detection with enhanced noise performance through
quantum correlation. This work should be of great interest to ongoing
space-borne GW signal searching experiments.
