  The Kramers-Kronig (KK) receiver provides an efficient method to reconstruct
the complex-valued optical field by means of intensity detection given a
minimum-phase signal. In this paper, we analytically show that for detecting
coherent states through measuring the minimum-phase signal, while keeping the
radial quantum fluctuation the same as the balanced heterodyne detection does,
the KK receiver can indirectly recover the tangential component with
fluctuation equivalently reduced to 1/3 times the radial one at the decision
time, by adopting the KK relations to utilize the information of the physically
measured radial component of other time of the symbol period. In consequence,
the KK receiver achieves 3/2 times the signal-to-noise ratio of balanced
heterodyne detection, while presenting an asymmetric quantum fluctuation
distribution depending on the time-varying phase. Therefore, the KK receiver
provides a feasible scheme to reduce the quantum fluctuation for obtaining the
selected component to 2/ 3 times that of physically measuring the same
component of the coherent state. This work provides a physical insight of the
KK receiver and should enrich the knowledge of electromagnetic noise in quantum
optical measurement.
