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arXiv:1901.05673 · Physical Review A

Reply to “Comment on ‘Past of a quantum particle revisited’ ”

Berthold‐Georg Englert, Kelvin Horia, Jibo Dai, Yink Loong Len, Hui Khoon Ng

Published Feb 27, 2019• 17 citations
arXivHTMLPDF

Abstract

We stand by our findings in Phys. Rev. A 96, 022126 (2017). In addition to refuting the invalid objections raised by Peleg and Vaidman, we report a retrocausation problem inherent in Vaidman's definition of the past of a quantum particle.
DOI
10.1103/physreva.99.026104

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We here reply to a recent comment by Vaidman [Phys. Rev. Res. 5, 048001 (2023)] on our paper [Phys. Rev. Res. 5, 023048 (2023)]. In his Comment, Vaidman first admits that he is just defining (assuming) the weak trace gives the presence of a particle---however, in this case, he should use a term other than presence, as this already has a separate, intuitive meaning other than ``where a weak trace is.'' Despite this admission, Vaidman then goes on to argue for this definition by appeal to ideas around an objectively existing idea of presence. We show these appeals rely on their own conclusion---that there is always a matter of fact about the location of a quantum particle.
The recent criticism of Vaidman's proposal 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. [B. G. 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 behavior. 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.