  We investigate performances of pure continuous variable states in
discriminating thermal and identity channels by comparing their M-copy error
probability bounds. This offers us a simplified mathematical analysis for
quantum target detection with slightly modified features: the object -- if it
is present -- perfectly reflects the signal beam irradiating it, while thermal
noise photons are returned to the receiver in its absence. This model
facilitates us to obtain analytic results on error-probability bounds i.e., the
quantum Chernoff bound and the lower bound constructed from the Bhattacharya
bound on M-copy discrimination error-probabilities of some important quantum
states, like photon number states, N00N states, coherent states and the
entangled photons obtained from spontaneous parametric down conversion (SPDC).
Comparing the $M$-copy error-bounds, we identify that N00N states indeed offer
enhanced sensitivity than the photon number state system, when average signal
photon number is small compared to the thermal noise level. However, in the
high signal-to-noise scenario, N00N states fail to be advantageous than the
photon number states. Entangled SPDC photon pairs too outperform conventional
coherent state system in the low signal-to-noise case. On the other hand,
conventional coherent state system surpasses the performance sensitivity
offered by entangled photon pair, when the signal intensity is much above that
of thermal noise. We find an analogous performance regime in the lossy target
detection (where the target is modeled as a weakly reflecting object) in a high
signal-to-noise scenario.
