Comment on "Past of a quantum particle revisited"

TL;DR

This paper defends Vaidman's approach to analyzing a quantum particle’s past in an interferometer, refuting recent criticisms and clarifying the applicability of the definition of the particle's past.

Contribution

It clarifies the validity of Vaidman's proposal and refutes the claim that all pre- and postselected particles follow a continuous trajectory.

Findings

Vaidman's definition applies to a specific subset of photons.

Criticism based on Englert et al.'s proof is invalid for the entire photon ensemble.

The behavior of pre- and postselected particles is more nuanced than previously claimed.

Abstract

The recent criticism of Vaidman's propsal for the analysis of the past of a particle in the nested interferometer is refuted. It is shown that the definition of the past of the particle adopted by Englert et al. [Phys. Rev. A 96, 022126 (2017)] is applicable only to a tiny fraction of photons in the interferometer which indeed exhibit different behaviour. Their proof that all pre- and postselected particles behave this way, i.e. follow a continuous trajectory, does not hold, because it relies on the assumption that it is intended to prove.