  In a quantum measurement, a coupling $g$ between the system S and the
apparatus A triggers the establishment of correlations, which provide
statistical information about S. Robust registration requires A to be
macroscopic, and a dynamical symmetry breaking of A governed by S allows the
absence of any bias. Phase transitions are thus a paradigm for quantum
measurement apparatuses, with the order parameter as pointer variable. The
coupling $g$ behaves as the source of symmetry breaking. The exact solution of
a model where S is a single spin and A a magnetic dot (consisting of $N$
interacting spins and a phonon thermal bath) exhibits the reduction of the
state as a relaxation process of the off-diagonal elements of S+A, rapid due to
the large size of $N$. The registration of the diagonal elements involves a
slower relaxation from the initial paramagnetic state of A to either one of its
ferromagnetic states. If $g$ is too weak, the measurement fails due to a
``Buridan's ass'' effect. The probability distribution for the magnetization
then develops not one but two narrow peaks at the ferromagnetic values. During
its evolution it goes through wide shapes extending between these values.
