directly limited by the transmitted probe power, it is useful to consider the normalized mutual information, which is defined as the mutual information divided by the average number of transmitted probe photons. The expression of the normalized mutual information could serve as a theoretical measure for the correlation enhancement of the target detection performance, despite that its specific expression may not be directly used for practical performance characterization.

Intensity Correlation
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The intensity correlation that exists within the probe and reference light simply means that the number of generated probe photons and reference photons are always equal. In other words, the detection of a reference photon implies that one single probe photon has been sent to probe the target object. Intensity correlation has already been used in quantum imaging to reduce the noise below the standard quantum limit [@brida2010experimental]. In the implementations [@Lopaeva:2013; @england2019quantum; @He:2018] of intensity correlation enhanced target detection protocols (will be referred to as intensity correlation protocols in the rest of this paper), the correlated twin beams are generated through pulse pumped SPDC: a short wavelength pump light pulse with narrow temporal duration interacts with the nonlinear medium and generates a pair of short duration wave packets 