  We demonstrate an optical receiver that achieves the quantum Chernoff bound
for discriminating coherent states from thermal states in the multi-copy
scenario. In contrast, we find that repeated use of the receiver approaching
the Helstrom bound for single-copy measurement is sub-optimal in this
multi-copy case. Furthermore, for a large class of multi-copy discrimination
tasks between a pure and a mixed state, we prove that any Helstrom-bound
achieving single-copy receiver is suboptimal by a factor of at least two in
error-probability exponent compared to the multi-copy quantum Chernoff bound.
This behavior has a classical analog in the performance gap between
soft-decision and hard-decision receivers for detecting a multi-copy signal
embedded in white Gaussian noise.
