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Ben-Israel, A.; Knips, L.; Dziewior, J.; Meinecke, J.; Danan, A.; Weinfurter, H.; Vaidman, L. (2017): An Improved Experiment to Determine the 'Past of a Particle' in the Nested Mach-Zehnder Interferometer. In: Chinese Physics Letters, Vol. 34, No. 2, 20301
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We argue that the modification proposed by Li et al. [Chin. Phys. Lett. 32 (2015) 050303] to the experiment of Danan et al. [Phys. Rev. Lett. 111 (2013) 240402] does not test the past of the photon as characterized by local weak traces. Instead of answering the questions: (i) were the photons in A? (ii) were the photons in.. ? and (iii) were the photons in C? the proposed experiment measures a degenerate operator answering the questions: (i) were the photons in A? and (ii) were the photons in B and C together? A negative answer to the last question does not tell us if photons were present in B or C On the other hand, a simple variation of the proposal by Li et al. does provide conceptually better evidence for the past of the pre-and post-selected photon, but this evidence will be in agreement with the results of Danan et al.