ROBUSTNESS IN QUANTUM MEASUREMENTS

Conditions are formulated under which a representation of an intrinsic C-algebra of (often quasilocal) observables of an infinite system is appropriate to describe measurement-type processes: such a representa tion should allow for the description of robust experiments, it should be separable, and the pointer observable should be in its weak closur e. If the pointer values are discrete the existence of such a measurem ent representation can be proven. If the pointer can take continuously many values, then the existence can only be proven under the addition al assumptions of having an asymptotically Abelian system or dealing w ith type I representations. In the constructed measurement representat ions the pointer observable turns out to be classical. The structure o f the representation suggests that spontaneous symmetry breaking might be a physical explanation of the emergence of the classical pointer.