  A model of quantum measurement, illustrated using the spin--boson model, is
formulated in terms of a cascading pair of quantum phase transitions. The first
produces the desired superposition of macroscopic responses to the microscopic
state under investigation, while the second provides a mechanism for subsequent
"wavefunction collapse," suppressing superpositions of distinct macroscopic
states, producing instead a density matrix that implements the expected
classical observation outcome via the Born probability rule. Motivated by
numerous carefully designed measurements that may occur during the course of a
quantum computation, effects of entanglement are investigated when the
measurement is performed on only a subset of the microscopic degrees of
freedom.
