  We consider online detection strategies for identifying a change point in a
stream of quantum particles allegedly prepared in identical states. We show
that the identification of the change point can be done without error via
sequential local measurements while attaining the optimal performance bound set
by quantum mechanics. In this way, we establish the task of exactly identifying
a quantum change point as an instance where local protocols are as powerful as
global ones. The optimal online detection strategy requires only one bit of
memory between subsequent measurements, and it is amenable to experimental
realization with current technology.
