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. 2018 Jul 19;20(7):538.
doi: 10.3390/e20070538.

When Photons Are Lying about Where They Have Been

Lev Vaidman  1 , Izumi Tsutsui  2
Affiliations

When Photons Are Lying about Where They Have Been

Lev Vaidman et al. Entropy (Basel). .

Abstract

The history of photons in a nested Mach-Zehnder interferometer with an inserted Dove prism is analyzed. It is argued that the Dove prism does not change the past of the photon. Alonso and Jordan correctly point out that an experiment by Danan et al. demonstrating the past of the photon in a nested interferometer will show different results when the Dove prism is inserted. The reason, however, is not that the past is changed, but that the experimental demonstration becomes incorrect. The explanation of a signal from the place in which the photon was (almost) not present is given. Bohmian trajectory of the photon is specified.

Keywords: Dove prism; Mach–Zehnder interferometer; past of the photon; photon trajectory.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
Nested Mach–Zehnder interferometer with inner interferometer tuned to destructive interference towards F. Although our ‘common sense’ suggests that the only possible path for the photon detected in D is path C, the trace was found also inside the inner interferometer supporting the TSVF proposal according to which the particle was present in the places where forward (red continuous line) and backward (green dashed line) evolving wavefunctions overlap. The latter is demonstrated by the results of the measurement by Danan et al. [1].
Figure 2
Figure 2
Nested Mach–Zehnder interferometer with a Dove prism inside the inner interferometer as suggested by Alonso and Jordan [2]. The region of the overlap of the forward and the backward evolving states remains the same, but predicted results of an experiment similar to [1] include a signal from mirror E where the photon was not supposed to be.
Figure 3
Figure 3
Nested Mach–Zehnder interferometer tuned to destructive interference towards F when a single photon is detected in D. The dashed line represents a common sense proposal by Wheeler, the thick gray line describes the past according to Vaidman’s proposal as places where the particle leaves a weak trace, the continuous line represents the Bohmian trajectory.
See this image and copyright information in PMC

References

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