Irreversibility and quantum measurement

It may not be necessary to invoke the classical nature of a measurement apparatus in order to justify the collapse postulate. A realistic example of a measurement is presented where system and "apparatus" are both treated quantum mechanically, and the collapse of the density matrix to diagonal form is justified by an assumption of irreversibility at the microscopic level (specifically, spontaneous emission), enforced by suitable boundary conditions. The result makes use of an epistemic interpretation of the quantum state, which nevertheless is compatible with the belief in the reality of definite physical properties, each in its appropriate context.