Question about Quantum Eraser

[bennyboy]

There are several threads about this.  This is just one of them:





Here's my question, and I'm assuming it will be only for Alex: let's say you have some detectors positioned at or before your slits.  If you observe them, you should get a photon-as-particle effect, i.e. 2 bands.  If you do not observe them, you should get an interference pattern.

My question is this: what if your detectors are set to randomly observe or not to observe particles passing them, and to give a character representing what they've "decided" to do.  Each character will represent one of: (1) decided to detect, and detected a particle (i.e. particle passed by its slit); (2) decided to detect, and didn't detect a particle (i.e. particle passed other slit); (3) decided NOT to detect.  You will then have to look up the symbols in a book printed by the system without you being able to see it.

How will your lack of knowledge of the meanings of the symbols affect the pattern you observe after photons are fired?  If a detector detected something, will you see a 2-band result?  Or could it be that since you don't know what the symbols mean, no information is really collected, and not only will you see an interference pattern, but also the detector will always give a "decided NOT to detect" result? What if there's no booklet, and the detectors just pump out random characters that mean something only to them in response to their detection state? Does this count as information or not?

[Alex K]

bennyboy,

I'll first try to think about the following: what if the measurement machine just saves the result internally but remains completely isolated from you. What this then means is that a pretty macroscopic system like the measurement device becomes entangled with the photon state. I suspect that this alone will suppress the interference effects on the screen by a factor dependent on the # of particles in the device (in mathematical lingo one would say the off diagonal entries of the density matrix are suppressed) but I am not 100% sure. Only when I have understood this question, can I sensibly address yours. We then have to consider how the symbols corresponding to outcomes are chosen, and how they are communicated to the outside world.

[bennyboy]

Okay, if what you suspect is right, then that means you could, at least sometimes, infer whether a photon was acting as a particle or a wave from the resultant spread 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"?

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.

[Alex K]

Okay, if what you suspect is right, then that means you could, at least sometimes, infer whether a photon was acting as a particle or a wave from the resultant spread pattern.  

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.

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.

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?

[purplepurpose]

It hurts my mind to see that QM proves "supernatural".

[Alex K]

It hurts my mind to see that QM proves "supernatural".

Because it doesn't? What is the "supernatural" supposed to be anyway? Noone ever manages to give a useful and coherent definition. But even if, whatever Quantum Mechanics entails (scientific theories don't "prove" anything), it's the consequence of a theory in physics and therefore by definition not supernatural.

[purplepurpose]

It hurts my mind to see that QM proves "supernatural".

Because it doesn't? What is the "supernatural" supposed to be anyway? Noone ever manages to give a useful and coherent definition. But even if, whatever Quantum Mechanics entails (scientific theories don't "prove" anything), it's the consequence of a theory in physics and therefore by definition not supernatural.

"Observation beings - matter behaves differently". Thats magic.

[Alex K]

Because it doesn't? What is the "supernatural" supposed to be anyway? Noone ever manages to give a useful and coherent definition. But even if, whatever Quantum Mechanics entails (scientific theories don't "prove" anything), it's the consequence of a theory in physics and therefore by definition not supernatural.

"Observation beings - matter behaves differently". Thats magic.

Nope, because observation of matter by beings means that the matter necessarily needs to interact with the matter the beings are made of. That it then behaves differently is not magic. The extent to which it behaves differently may be surprising, but not magic.

[purplepurpose]

"Observation beings - matter behaves differently". Thats magic.

Nope, because observation of matter by beings means that the matter necessarily needs to interact with the matter the beings are made of. That it then behaves differently is not magic. The extent to which it behaves differently may be surprising, but not magic.

Such grand complexity of matter that came from "NOTHING" or is "eternal" pains the mind and begs for an easy answer of "goddidit".

[Alex K]

Nope, because observation of matter by beings means that the matter necessarily needs to interact with the matter the beings are made of. That it then behaves differently is not magic. The extent to which it behaves differently may be surprising, but not magic.

Such grand complexity of matter that came from "NOTHING" or is "eternal" pains the mind and begs for an easy answer of "goddidit".

You seem to be changing topics by the post. The origin of matter or whether it is eternal is not part of the theory of quantum mechanics.