  A model of quantum measurement is proposed, which aims to describe
statistical mechanical aspects of this phenomenon, starting from a purely
Hamiltonian formulation. The macroscopic measurement apparatus is modeled as an
ideal Bose gas, the order parameter of which, that is, the amplitude of the
condensate, is the pointer variable. It is shown that properties of
irreversibility and ergodicity breaking, which are inherent in the model
apparatus, ensure the appearance of definite results of the measurement, and
provide a dynamical realization of wave-function reduction or collapse. The
measurement process takes place in two steps: First, the reduction of the state
of the tested system occurs over a time of order $\hbar/(TN^{1/4})$, where $T$
is the temperature of the apparatus, and $N$ is the number of its degrees of
freedom. This decoherence process is governed by the apparatus-system
interaction. During the second step classical correlations are established
between the apparatus and the tested system over the much longer time-scale of
equilibration of the apparatus. The influence of the parameters of the model on
non-ideality of the measurement is discussed. Schr\"{o}dinger kittens, EPR
setups and information transfer are analyzed.
