  State discrimination is a useful test problem with which to clarify the power
and limitations of different classes of measurement. We consider the problem of
discriminating between given states of a bi-partite quantum system via
sequential measurement of the subsystems, with classical feed-forward of
measurement results. Our aim is to understand when sequential measurements,
which are relatively easy to implement experimentally, perform as well, or
almost as well as optimal joint measurements, which are in general more
technologically challenging. We construct conditions that the optimal
sequential measurement must satisfy, analogous to the well-known Helstrom
conditions for minimum error discrimination in the unrestricted case. We give
several examples and compare the optimal probability of correctly identifying
the state via global versus sequential measurement strategies.
