# Asking Neutrons where they have been – Past of a quantum particle

May 10, 2018 5:11 pm‘‘*What was the past of a photon which went through an interferometer ?*’’, this question is the central point in the work from Lev Vaidman and his co-workers published in ^{1}. In this particular paper Danan *et al.* use use a Mach-Zehnder interferometer (MZI) tuned such that theoutputs interfere constructively, ensuring that all photons exit the second beam splitter in the same direction. The authors probe where the photons were in two situations: *i)* * interference measurement* and

*when there is a*

**ii)****The which-way marking is achieved by vibrating mirrors at unique frequencies. The applied setup, together with the obtained power spectrum, is illustrated below on the right side. The authors suggest in their paper**

*which-way measurement.**of the photons and argue that this can be intuitively understood: one should consider not just the possible paths of the photons traveling forwards through the system but also the paths that a photon traveling*

**discontinues trajectories***. However, their experiment was carried out with a*

**backwards from the detector***classical laser beam*and their results can be fully explained within the framework of

*Maxwell theory*. To test the statement of Danan

*et al.*an experiment of

*pure quantum nature*is called for, which we have done in our

*which is schematically illustrated below on the right side. Our new results are mow presented in the journal “*

**neutron interferometric experiment****”**

*Physical Review A*^{2}.

Our experiment clearly confirms that standard quantum mechanics does provide an *intuitive picture* as well as a *correct quantitative argument* of the observed phenomena in the three-beam interferometer. Refuting the claim of Danan *et al.* that by the use of the *two-state vector formulation* a more intuitive understanding can be gained. Furthermore, in studying the interference effect, particularly in a *destructive* case, the occurrence of zero intensity is explained. This situation is interpreted in a mistaken manner as *non-continuous trajectories* in the paper of Danan *et al. Our results *witness the multifold presence of the neutron’s wave function in the interferometer. See * here* for a more detailed explanation and experimental details of our latest work. A photography of the applied setup is depicted below

1. A. Danan, D. Farfurnik, S. Bar-Ad, and L. Vaidman, *Phys. Rev. Lett.* **111**, 240402 (2013). ↩

2. Hermann Geppert-Kleinrath, Tobias Denkmayr, Stephan Sponar, Hartmut Lemmel, Tobias Jenke, and Yuji Hasegawa, *Phys. Rev. A * **97**, 052111 (2018). ↩