Universality of local weak interactions and its application to interferometer alignment

  Shimshon Bar-Ad [1]  ,  Demitry Farfurnik [1],  ,  Nimrod Benshalom [1],  ,  Jonathan Efroni [1],  ,  Lev Vaidman [1]  ,  Jan Dziewior [2],   ,  Lukas Knips [2],  ,  Katharina Senkalla [2],  ,  Jasmin Meinecke [2],  ,  Harald Weinfurter [2]  
[1] Raymond and Beverly Sackler School of Physics and Astronomy, Tel-Aviv University, Tel-Aviv 69978, Israel
[2] Max-Planck-Institute for Quantum Optics, 85748 Garching, Germany, and Department of Physics, Ludwig-Maximilians-University, 80797 Munchen, Germany

The modification of the effect of interactions of a particle as a function of its preselected and postselected states is analyzed theoretically and experimentally, and the universality of this modification in the case of local interactions of a spatially preselected and postselected particle is demonstrated. It allows us to define an operational approach for the characterization of the presence of a quantum particle in a particular place, namely the way it modifies the effect of local weak interactions. The experimental implementation, which entangles the transverse degrees of freedom of photons with their path in a Mach-Zehnder interferometer, also allows us to demonstrate a novel and efficient method of interferometer alignment, where the trajectory of the postselected beam on a single quadrant detector, throughout a single phase scan, yields all the misalignment parameters.