Determine the error-exponent prefactor

Evaluate the prefactor governing the error probability in quantum illumination with an unknown constant return phase when receivers are restricted to independent and identical measurements.

Background

The paper derives a leading-order upper bound on the worst-case error exponent for independent and identical measurements in the low-reflectivity regime. It establishes that unentangled coherent light with heterodyne detection saturates this bound, so entanglement provides no leading-order advantage when the return phase is unknown.

The analysis focuses on the exponential decay rate of the error probability and does not determine the multiplicative prefactor. A more complete characterization of receiver performance would require evaluating this prefactor.