Experimental Study of the Past of a Photon

Asking photons where they have been in plain language

“…4). Crucially, this probability cannot be made exactly zero, and the evidence of photons entering and leaving the inner MZI is always present, contrary to the claims made in [1], [7], and [9].…”

“…Secondly, simple models, similar to the one used in [1], do provide nonvanishing evidence of the photons entering and leaving the inner loop. It is, therefore, highly likely that the picture describing the "surprising" behaviour of the photons [1], [7], and [9] is a yet another artefact [13] of resorting to exotic interpretations of quantum mechanics, without consulting first with the standard quantum theory.…”

“…Further criticism can be found in [6], where it was pointed out that the relevant experimental evidence disappears, should the experiment be modified slightly. The authors of [1] responded by noting that what matters is that the evidence can be present, not that it may be absent under certain conditions [7]. Finally, a critical analysis of a similar MZI setup in terms of the consistent histories approach can be found in a recent work by Griffiths [8].…”

Experimental observation of anomalous trajectories of single photons

“…In particular, it rules out the option of a continuous trajectory. The results have stirred a lively discussion in the literature [22][23][24][25][26][27][28][29][30][31][32][33] including criticism on the use of classical light and the sensitivity of the experiment. Unfortunately, the clever technique used for obtaining the "which path" information, could not be applied to single photons.…”