  Sudden changes are ubiquitous in nature. Identifying them is of crucial
importance for a number of applications in medicine, biology, geophysics, and
social sciences. Here we investigate the problem in the quantum domain,
considering a source that emits particles in a default state, until a point
where it switches to another state. Given a sequence of particles emitted by
the source, the problem is to find out where the change occurred. For large
sequences, we obtain an analytical expression for the maximum probability of
correctly identifying the change point when joint measurements on the whole
sequence are allowed. We also construct strategies that measure the particles
individually and provide an online answer as soon as a new particle is emitted
by the source. We show that these strategies substantially underperform the
optimal strategy, indicating that quantum sudden changes, although happening
locally, are better detected globally.
