Question about Quantum Eraser

[bennyboy]

I'm not sure whether this absolute dichotomy particle/wave is really something that exists in quantum mechanics. From my understanding, even the "particle" mode is just a wave which is localized to a small space corresponding to the resolution of the measurement apparatus.

That certainly makes sense.  However I think we can say "particle" as a shorthand for whatever it is that ends up with a 2-band dispersal pattern.

But I guess that's true anyway-- if you are engaged in an experiment, and I walk into the room after it's done, I assume I will be able to see a 2-stripe pattern, and say, "Aha!  I know you were detecting activity at the slits!"  If certain apparatus could also do this, then the results themselves might actually be useful.

My problem is that if you could do this experiment at very long range, it seems you could actually send information at a speed faster than light.  You could, for example, send a burst of photons from the moon, and then use their entanglement with the transmitting apparatus to set their state just before they arrive at a receptor.  Isn't this "illegal"?

This should be illegal. Usually, in such cases, the (delayed) decision whether to measure or not does not change the possible outcomes in a way that would allow a conclusion about the decision. The typical example I can think of to illustrate what I mean is that of two entangled particles which are entangled to have opposite spin up or down, but the spin of each can be both. If they part until they are separated by a large distance, the receiver of one of them, Alice, can destroy the entanglement (at least from her point of view) by measuring its state. She will get up or down, and then knows for sure that Bob will see the opposite result. From Bob's point of view, there will always be fifty-fifty up or down, whether Alice looks or not. He can't tell *just from measuring his particle*, whether Alice has looked or not, whether the entanglement is still intact. The same kind of situation should generalize to all possible experiments, no matter how elaborate.

Here's a question.  If you could split entangled pairs such that one observer had information about the system via a local detector, and the other couldn't possibly have information about the system, is it possible that they'd get DIFFERENT results?  As in, that even though the particles are entangled, observer A gets a 2-band result and observer B gets an interference pattern?

If you want to trip me out, show me a 4-state superposition (essentially a uniform distribution, right?), that only resolves itself to me when I look up my random symbols in a book.

I'll gladly oblige, but what do you mean by a four-state-superposition?

It's something I heard in response to some of these videos: that the "backward in time" effect really isn't, because all possible outcomes are propagated, so that whatever you do with regards to observation only changes which outcome the state collapses to at the end.  In other words, no information actually passes back in time though it seems to.  So my question really is how far can you mess around with things like delayed observation?  Could you theoretically make a system in which observations made several seconds after a photon was fired (and possibly detected) or even minutes could affect what pattern you get at the end?

And my biggest and deepest question: would it ever under any scenario be possible for two observers of the same system to see different results because of the way each is interacting (or not interacting) with the apparatus? "No dummy, look! It's obviously 2 bands, do you even science bruh?"