Discriminating between the von Neumann and L\"uders reduction rule
  Given an ensemble of systems in an unknown state, as well as an observable $\hat A$ and a physical apparatus which performs a measurement of $\hat A$ on the ensemble, whose detailed working is unknown ('black box'), how can one test whether the L\"uders or von Neumann reduction rule applies? 
G. C. Hegerfeldt and R. Sala Mayato
quant-ph