Let’s now see how we would actually go about determining the leg of the interferometer taken by a photon. Let’s insert a third polarizer between the output of beam splitter 2 and the ground glass screen, as shown in Figure 134b. This polarizer is the equivalent of the analyzing polarizer of Figure 121. At an angle of 0° (horizontal) we are only looking at photons that traveled one leg of the interferometer, and no interference fringes are seen. In the same way, setting the angle of the analyzing polarizer at 90° selects photons that have traveled down the other leg, and again no interference pattern is observed. As we have seen before, it isn’t even necessary to actually determine the path. The mere existence of which-way information suffices to make the interference fringes disappear.
Now you are in for a real treat. Turn the analyzer to 45°, as shown in Figure 134b. The interference pattern miraculously reappears! This happens because you have erased the which-way information so you can’t—even in principle—determine which path of the interferometer was taken by a photon.†
As shown in Figure 135, the interference pattern is also restored when a quantum eraser is placed before the screen in a traditional two-slit experiment with path labeling (Figure 133).
Figure 135 The interference pattern is restored when a quantum eraser is placed before the screen in a traditional two-slit experiment with path labeling (Figure 133b).
It is important to note that the path-labeling process in itself does not disrupt the photons and erase the interference pattern. If the path labeling were responsible for the destruction of the interference pattern, we wouldn’t be able to restore it with a quantum eraser. The fact is that the mere existence of the information about the path that the photon takes is what destroys the interference pattern.
Another mindboggling result from this experiment is that the interference pattern can be restored by destroying the which-way information after the photon has left the interferometer!
These experiments with quantum erasers produce results that are absolutely baffling. They demonstrate that information available to the observer is the critical factor that determines the collapse of a quantum system’s wavefunction. In quantum physics, unlike classical physics, the observer and the quantum system being observed become inescapably linked.
A word of caution is in order—the “observer” is not required in any way to be a conscious being. Interaction with a measurement apparatus suffices as an “observation.” As Richard Feynman put it: “Nature does not know what you are looking at, and she behaves the way she is going to behave whether you bother to take down the data or not.”
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