Fig. 1. Schematics for heterodyne detection of light. (a) The signal mode enters into the detector together with an unexcited mode (image band vacuum) that gives rise to 3 dB heterodyne noise^[4,5,7]. (b) Before being received by the detector, the signal light passes through a parametric amplifier where quantum correlation^[17] is generated between the signal mode and the image band vacuum for reduction of the 3 dB heterodyne noise. ω_s,i,l, angular frequency of the signal/image band/local oscillator mode; E^s,a(+)(t), quantum field of signal/amplified light beam; ε_l⁽⁺⁾(t), classical field of local oscillator light beam; 50-50, balanced beamsplitter; LA, linear amplifier; J₋(t) ≡ J₂(t) − J₁(t), average differential photocurrent signal from the detector. Inset: a typical parametric amplifier consisting of a type-I periodically-poled KTiOPO₄ (PPKTP) crystal inside an optical cavity and a laser pump^[20] may be used to realize the proposed heterodyne detection with ω_p = ω_s + ω_i, where ω_p is the pump angular frequency. M₁ & M₂, cavity mirrors; DM, dichromatic mirror.