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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 B? 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.
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. (a) were the photons in A? (ii) were the photons in B ?, and (iii) were the photons in C? the proposed experiment measures a degenerate (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.