An Improved Experiment to Determine the 'Past of a Particle' in the Nested Mach–Zehnder Interferometer

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.