Published online by Cambridge University Press: 01 April 2022
Usual derivations of Lüders's projection rule show that Lüders's rule is the rule required by quantum statistics to calculate the final state after an ideal (minimally disturbing) measurement. These derivations are at best inconclusive, however, when it comes to interpreting Lüders's rule as a description of individual state transformations. In this paper, I show a natural way of deriving Lüders's rule from well-motivated and explicit physical assumptions referring to individual systems. This requires, however, the introduction of a concept of individual state which is not standard.
I would like to thank Linda Wessels and Geoffrey Hellman who directed the research on which this paper is based. I also would like to thank an anonymous referee of this Journal for valuable comments.