  A quantum measuring instrument is constructed that utilises symmetry breaking
to enhance a microscopic signal. The entire quantum system consists of a
system-apparatus-environment triad that is composed of a small set of spin-1/2
particles. The apparatus is a ferromagnet that measures the $z$-component of a
single spin. A full quantum many-body calculation allows for a careful
examination of the loss of phase coherence, the formation and amplification of
system-apparatus correlations, the irreversibility of registration, the fault
tolerance, and the bias of the device.
