An Improved Experiment to Determine the 'Past of a Particle' in the Nested Mach–Zehnder Interferometer
A. Ben-Israel1, L. Knips2,3, J. Dziewior2,3, J. Meinecke2,3, A. Danan1, H. Weinfurter2,3, L. Vaidman1**
1Raymond and Beverly Sackler School of Physics and Astronomy, Tel-Aviv University, Tel-Aviv 69978, Israel 2Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Straße 1, Garching 85748, Germany 3Department für Physik, Ludwig-Maximilians-Universität, München 80797, Germany
Abstract：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.
. [J]. 中国物理快报, 2017, 34(2): 20301-020301.
A. Ben-Israel, L. Knips, J. Dziewior, J. Meinecke, A. Danan, H. Weinfurter, L. Vaidman. An Improved Experiment to Determine the 'Past of a Particle' in the Nested Mach–Zehnder Interferometer. Chin. Phys. Lett., 2017, 34(2): 20301-020301.
2017, Vol. 34 
