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

Abstract

We argue that the modification proposed by Li et al. [Chin. Phys. Lett. 32,\n050303 (2015)] to the experiment of Danan et al. [Phys. Rev. Lett. 111, 240402\n(2013)] does not test the past of the photon as characterised by local weak\ntraces. Instead of answering the questions: (i) Were the photons in A? (ii)\nWere the photons in B? (iii) Were the photons in C? the proposed experiment\nmeasures a degenerate operator answering the questions: (i) Were the photons in\nA? (ii) Were the photons in B and C together? A negative answer to the last\nquestion does not tell us if photons were present in B or C. A simple variation\nof the modified experiment does provide good evidence for the past of the\nphoton in agreement with the results Danan et al. obtained.\n