Is a quantum eraser instantaneous? I'm going through the basic and popular optical quantum-eraser experiment with laser and polarizers and I have a question.

After applying the diagonally aligned polarizer at a 45 degree angle, the B1 section of the beam carries an interference that can be observed by the fringes at the screen.
But let say we could, somehow, check the B0 section of the beam for the existence of interference without disturbing the beam... would there be interference there too?
According to quantum mechanics, once the eraser is applied to the beam, the which-path information is erased and the interference should exist throughout the apparatus, but I'm not sure what the math theory and the experimental experience tells us about a simple quantum eraser.
 A: The interference pattern only appears after the diagonal polarizer. I just performed the experiment with a Michelson interferometer. I place polarizers orthogonal to each other in each path of the interferometer. I also put a beam splitter in section b0. When the diagonal polarizer was position at B1 the interference pattern reappeared, but the beam splitter at B0 continued to project no interference pattern.
A: This is a pretty bad example of a quantum eraser experiment, because it can be explained entirely using classical electromagnetism. Specifically, the two concepts involved are: 


*

*Malus's Law, which describes the output intensity of light as it passes through a linear polarizer, and

*The Fresnel-Arago laws, which, among other things, state that coherent polarized light can only interfere with the component of another coherent light source which is parallel to its polarization. In particular, this means that two coherent light beams that are polarized perpendicular to each other cannot interfere. 


The $B_0$ section of the beam contains two orthogonally polarized light beams. Therefore, by the Fresnel-Arago laws, no interference can be present.
