Mach-Zehnder Interferometer

The Mach-Zehnder interferometer[117] is a light beam splitting apparatus that’s frequently used to demonstrate quantum effects. It’s easy to set up and easy to explain the experimental thesis. But what actually gets observed doesn’t conform to classical analysis.

In this experiment, start by shining a laser beam toward a beam splitter as shown here:

images/quantum_mechanics_with_qubelets/Single_Beam_Splitter_Experiment.png

The beam splitter reflects part of the beam, or light wave, which heads toward the mirror on the bottom left of the figure, and transmits the other part, which is directed towards the mirror shown on the top right. (We’ve implicitly assumed that the beam splitter reflects as much as it transmits. But this assumption isn’t germane to the discussion that follows. Beam splitters that are biased more toward one or the other way of changing the path of the light wave work equally well for the experiment we’re about to outline.) The beam that’s reflected off the top mirror, the top path, lights up sensor images/_pragprog/svg-563.png. Similarly, the beam that bounces off the bottom mirror, the bottom path, lights up sensor images/_pragprog/svg-564.png.

Next, introduce a second beam splitter that recombines the beam from the top and bottom path. According to classical ...

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