  We study the discrimination of weak coherent states of light with significant
overlaps by nondestructive measurements on the light states through measuring
atomic states that are entangled to the coherent states via dipole coupling. In
this way, the problem of measuring and discriminating coherent light states is
shifted to finding the appropriate atom-light interaction and atomic
measurements. We show that this scheme allows us to attain a probability of
error extremely close to the Helstrom bound, the ultimate quantum limit for
discriminating binary quantum states, through the simple Jaynes-Cummings
interaction between the field and ancilla with optimized light-atom coupling
and projective measurements on the atomic states. Moreover, since the
measurement is nondestructive on the light state, information that is not
detected by one measurement can be extracted from the post-measurement light
states through subsequent measurements.
