Quantum measurements constrained by symmetry
The Wigner-Araki-Yanase (WAY) theorem dictates that quantum measurements are constrained in the presence of conserved quantities. I'll review this theorem and sketch some generalisations which strengthen the existing results. The observation that the WAY constraint vanishes if the observable-to-be-measured is "symmetrised" points to connections with the subjects of reference frames and superselection rules. It also provokes analysis of the general problem of determining expectation values of subsystem observables given a symmetry constraint on a larger system. I'll give an overview of some recent progress on this problem.