Quantum dynamics of a small symmetry breaking measurement device

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.