Delayed Choice Quantum Eraser

Today I'd like to write something about quantum physics.

Quantum physics has gained my interest since several years and although I've seen all the most important experiments by now, it still peaks my interest when something new is discovered (such as the FTL neutrinos recently). An interesting experiment I read about is the Delayed Choice Quantum Eraser. It is most interesting, because nobody really has a conclusive answer for the behavior of this experiment yet. Many experiments in quantum physics can be explained both mathematically and with good reason. Some experiments can be explained only mathematically and Niels Bohr would say "Don't bother with the reason, the math is all you need to know.", but I always need an answer to the "Why?". This experiment reveals something that is still a complete mystery to us and I would like to know why it behaves the way that it does...

The Delayed Choice Quantum Eraser is an extension of the Double Slit Experiment. If you don't know what that experiment is, go see this movie for the short version and/or this movie for the long and more accurate version. The reason why the interference pattern disappears when we measure through which slit the electron went, is because we interact with the electron, which forces it to make a decision. This is also called the 'collapse of the wave function' and the 'wave function' is the probabilistic nature which causes the particle to look like a wave. Also note that while the movie demonstrates the experiment with electrons, it works exactly the same for photons. In the Delayed Choice Quantum Eraser (see this image below) we use photons.

So here it is. On the left-top we see the Double Slit Experiment where the photon goes through one slit (or both, or none) after which the photon is directly split into two entangled photons. This is called quantum entanglement which allows us to know the state of one photon by measuring the other (they always have opposite state, see Pauli exclusion principle). One photon (which we call the "signal" photon) follows the path at the top to detector D0. This is just a normal Double Slit Experiment. The other photon (which we call the "idler" photon) takes a much longer path to any of the other detectors. With the mirrors in this path we can choose to measure from which slit the photon came (using detectors D3 and D4) or to destroy the information from which slit the photon came by combining both the red and blue paths (through BSc) and use detectors D1 and D2.

When we choose to destroy the which-path information (through BSc) we get an interference pattern. But when we choose to keep (and record) the which-path information, there will be no interference pattern!

This is particularly odd, because when the "signal" photon hits (and is recorded by) detector D0, the "idler" photon is still on its way through the experiment (before it reaches any of the mirrors with which we decide what to do with the path information) the "signal" photon already seems to know what we will be going to do with the "idler" photon and decides to show us interference or not.

First question I hear you ask is "Can we use this to look into the future?". And the answer is simple: No, we can't. Because the result of all detectors is needed to see the interference pattern (or not) so there is no causality violation here. But it is still odd (and a little scary, in a philosophical way) that the particle knows the future...

In a good book that I read, called Quantum Enigma, the author describes something that (I figure) could explain the behavior of the Delayed Choice Quantum Eraser. A quantum particle keeps all options open until it is forced to choose. (I already figured that out) But with that decision, the entire path in the history (yes, back in time) of that particle is also decided. I could set up an experiment as shown in this image:

Where a single photon goes through a half mirror with a 50% chance of reflecting off of that mirror and 50% chance of going straight through the mirror. It reaches one of the boxes which I constructed so that I can keep the particle captured until I am ready to look inside one of these boxes. Until I do, the photon has not made a decision yet and is both in Box 1 and 2 at the same time (this can be demonstrated through yet another experiment). This also means that it has taken both paths! And it means it both reflected off of the mirror and it went through the mirror. At the time I open one of these boxes to look where the photon actually is, the whole history of the particle is "rewritten" (or "collapsed") so that the the particle took only one path and either reflected off of the mirror or went through it, but not both.

This could explain the behavior of the Delayed Choice Quantum Eraser such that the result at D0 (all the particle actions and reactions that take place to record it) is not actually decided until the "idler" photon is recorded. Or even until we actually look at the results with our own eyes.

Another explanation that I came up with for the Delayed Choice Quantum Eraser is Special Relativity: As one approaches the speed of light, time slows down. At the speed of light, time has stopped. Thus, because both the "idler" and the "signal" photons travel with the speed of light, all events in their lifetime are instantaneous and they don't have to "look into the future" to make a decision. While from our point of view it takes time for the particle to travel from point A to B, everything from the particle's point of view is instantaneous no matter how far they have to travel to make a decision. Although I am not sure if this holds when the photon travels through a medium such as air. Then it would still travel at the speed of light, but not with the speed c (the speed of light in a perfect vacuum).

Now I am not quantum physics scientist and my math is not nearly good enough to deal with the mathematics of quantum physics, so please go easy on me when you prove me wrong. But I'd like to hear other theories and links to other interesting articles which can shed some light on these things.