Tracing the past of a quantum particle

Abstract

The question: "Where was a quantum particle between pre- and postselection measurements?" is analyzed in view of a recent proposal that it was in the overlap of the forward and backward evolving wave functions. It is argued that this proposal corresponds not only to the criterion of where the particle leaves a weak trace, but also where the local interactions can affect the probability of postselection and where finding the particle in a strong nondemolition measurement is possible. The concept of a "secondary presence" of a pre- and postselected particle where local interactions affect the weak trace in the overlap region is introduced.