Two photon interference under Einstein's locality condition

The quantum mechanics formalism tells us that a single particle can exhibit both, wave and particle nature. Which of this two properties is revealed depends on the measurement apparatus. To translate this into a single photon experiment, imagine that we have a Mach-Zehnder interferometer. If the observer chooses to measure the particle nature of the single photon wave-packet, he should remove the last beam-splitter and thus he acquires the which-path information by looking at the counts at the detectors Det1 and Det2 which are not sensitive to the relative phase change between the paths a and b. In 1984 J. Wheeler proposed his famous Gedankenexperiment in which “[…] the decision whether to put the final half-silvered mirror in place or to take it out at the very last picosecond, after the photon has already accomplished its travel.” [1] His proposal was realized in several experiments [2,3].