New quantum experiments and its implications

StevieTNZ

Would someone be able to write out the evolution of the photons to get to that state (above), in a way similiar to what the authors have done for the bell-states they detect (pg 14 of the pre-print).

(I also ask here: http://www.physicsforums.com/showthread.php?t=600624. But maybe it might be best asked here? Who knows!)

San K

SPDC is stimulated by random vacuum fluctuations, and hence the photon pairs are created at random times. The conversion efficiency is very low, on the order of 1 pair per every 10^12 incoming photons.

zonde

I would like to find that out too. For me derivation in paper (pg 14) is too short to follow it so I won't try to offer derivation for HV and VH separable states.

al onestone

Hey dr Chinese, this guy believes in FTL or superluminal communication. Do you think he's onto something?
Isn't it grand how many snags a person might come across when trying to create a thought experiment which could do such things. Problems with understanding that induced coherence usually does not occur when dealing with spontaneous processes. I recently had a snag with understanding that complete entanglement in position does not allow well defined momentum for the individual components. You remember.
There is alot of literature out there which will indicate the possibility of something miraculous to physics, but the miraculous never really manifests does it?
I think that indicating the Austrian study on entanglement swapping was a little beyond this thread. You should be spending your time preparing for the real "instant communication protocol", but it won't be sent superluminally, you'll just have to wait.

Am I getting this thread straight? Are we really considering signals being sent "backwards" in time. Please, first attempt to master the sending of signals in a manner that is simply "faster" than light, then move onto the heavyweights like "backwards" in time signalling.

My thought experiments never were so bold, and I even proffessed to have instantaneous signalling.

StevieTNZ

I've emailed a few of the authors regarding this. Hopefully they get back to me soon.

I gather the two bell-state evolutions in the paper are in superposition of being both (so 1/2 |equations for first bell state> + 1/2 |equations for second bell state>) up until one bell-state is detected at the end?

lugita15

Can you elaborate on this? Why can't you have a quantum system whose creation operators only produce entangled particle pairs? Or is this just a practical limitation?

gespex

Nope, I don't think you're getting this thread straight. I don't believe in either instant communicating or sending messages into the past. In fact, my possible ignorance on science swings the other way: I am still convinced that there is a hidden variable theorem for all of quantum mechanics. In fact, I like the way you put it:
Because I couldn't agree with you more. That was kind of my point in this thread: the article I read *seems* to imply that it can be done, I definitely don't believe it can be done, so, as I quote from my first post:
[quote ]
[...] so there's obviously a logic flaw in my logic.
[/quote]

I was simply trying to understand my misunderstanding of the article. Because I am open to be proven wrong, but Bell's Theorem simply doesn't cut it for me.

San K

Even if Victor could entangle particle at will (which I gather is fundamentally impossible) FTL information is still impossible because:

Alice and Bob still need to compare to find out if Victor entangled the pair or not.

Is that correct?


Side note: Per technical/nerd speak the name should have been Charlie or Chimp or Csomething than Victor. Perhaps Ma et. al. choose Victor because its easier to relate by having one slant of V point towards Alice and other towards Bob.

zonde

It's wrong. You can place Alice and Bob side by side but Victor at some remote location. That way Alice and Bob can compare their results very quickly and receive information from Victor FTL (or backward in time).

San K

you are right. I missed that.

So essentially all three (A, B and V) need to compare their results? Because Victor does not have control over which photons he can entangle.

however there another thought experiment where information can be sent FTL with some decent level of accuracy but not 100% accuracy:


When Victor tries entangling photons lets say One a Billion, on average, get entangled. However Victor cannot do it at will.

Now lets say Victor has 10 sets of a Billion entangled photons each. Thus a total of 10 billion photons.

If Victor does not entangle any of the 10 sets means ---> 0.
i.e. Victor does nothing, he just sits there and does not touch any of the photons. If he does not touch any of the photons they cannot get entangled, correct?

If Victor is able to entangle even one of the 10 billion photons means ---> 1

Alice and Bob are sitting close to each other and compare their 10 billion photons. If they find none is entangled then it means Victor is saying 0.

If even a single one is entangled its means Victor is saying 1.

This experiment was thought really quickly. There must be a flaw/catch somewhere. Where is it?

BTW the above experiment can be carried out with just two photons, we don't necessarily need four. For example the DCQE can be used.

gespex

@San K:
From what I understand now, simply comparing the results of A and B is not sufficient. The results of A and B will be completely random: only when you know which pairs to look at (because the other two from the four photons become entangled or not), you can find the correlation.

Imagine another experiment, in which something similar happens: V has a switch. Then A and B will either see a light flash red or green. If V turns the switch into one position, A and B will flash the same color, if V turns the switch into the other position, A and B will flash a different color. Now imagine that the position of the switch is completely random.
Now A and B will see a 50/50 correlation (on average). However, when V discloses the position of the switch, they'll be able to predict with a 100% accuracy the other person's color.
(Note that the only similarity between the two experiments is that the information about the correlation is completely useless until V discloses his information)

I'm "disappointed" (or relieved) to see that this experiment, once again, fails to prove any of the promised magic of quantum physics.

al onestone

You're quote "I am still convinced that there is a hidden variable theorem for all of quantum mechanics." tells me you need a little work on interpretational QM. I know that Bell's theorem and the violation of the CHSH inequalities does not convince alot of people about quantum nonlocality, but I would advise you that seeking out a hidden variables type explaination is not the right direction. If I could make two suggestions;

1>Read Ballentine's chapter on Bell's theorem and that will get you past any hidden variables type theory

if you've already done that then move on to the real single world interpretation of QM

2>Read Zeilinger's 1999 paper "A foundational principle for QM"(J. Found. Phys.). Believe me, as a believer in physical realism, there is no better explaination of the laws of QM then the information interpretation written by Zeilinger (and von Weisacker, Wheeler, etc)

I don't know much about the article you posted but I know that some other threads have some info on the matter (www.sciforums.com "retrocausality in action")

My explaination is that there is nothing strange or retrocausal happening in this experiment. The results of Alice and Bob's measurements can be later considerred "entangled" regardless of the outcome of their measurements at the time. It only requires that when Victor makes his measurement ( Bell state measurement) that the system (which knows the outcome of Alice and Bob's measurements already because it is the system after all) simply projects onto the appropriate symmetry state to make it seem like the results of Alice and Bob's measurements were entangled already, which they were not.

This is interesting to weak measurement theorists though, because it seems that Alice and Bob are priorly post-selecting Bob's measurement.

San K

You are right Gespex, for a moment/day I forgot that A and B are random and always 50% correlated.

Interestingly even V cannot tell which photons are entangled till all three (V, A and C) compare plus they need a coincidence counter too?

DrChinese

Thanks! Wasn't really sure what the order of magnitude was.

gespex

Thanks for your suggestions. I haven't read those articles yet, but at least the wikipedia article of CHSH fails to convince me as well. More specifically, this quote in wikipedia:
I fail to see why one would make the assumption that ρ(λ) has a constant distribution. It seems to me that it leaves a massive gap where hidden variable theorems may still exist, or is that mere ignorance on my side?

DrChinese

We expect random results. If the distribution was unbalanced, we would notice that in experiments pretty quickly. I.e. there are more H> than V> when we measure at 30 degrees. But even if the source does have some asymmetry in the hidden variables themselves, we can accept that too as long as there is a constant expectation value.

StevieTNZ

Will post the scan when they're sent through.