I read up on wikipedia.com about entanglement and teleportation but it left me with a few questions. If you go to This Link. You'll see that they give the analogy "Bob has created two atoms called I and II which are maximally entangled". Now obviously, bob can't create two atoms at will so how do two particles become entangled? Other texts in the article suggest that due to the fact that they're identicle particles in the same instant of time, they're basically one particle so what happens to one will happen to the other. But then it says the transmition of information can not go faster than the speed of light. If this is true then I would assume that the communication between the particles is transmitted through some sort of EM wave. There's a lot of confusion right now, could someone clear this up for me? I just realized that my questions might not be obvious from the text so I will list them. 1) What defines which identicle particles in the same time period are entangled and which are not? Is there an "entanglement process" or are two identical particles that exist in the same point in time automatically entangled? 2) Does the transfer of particle state information happen instataneously no matter what the distance? 3) If not, is the information transmitted through some sort of EM wave?
An entangeled pair can be created by a technique called parametric down conversion. i suggest you google for that. Besides, two entangeled particles can indeed be seen as one 'bigger' particle with twice as much energy (suppose that each particle has the same amount of energy) or twice as much shorter wavelength. This is how entangled pairs can beat the diffraction limit. i refer to my journal for more info. Just look at the faster then light communication entry and the for qubit-lovers entry marlon ps this is nice : http://marcus.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf
Marlon, I read your faster than light article. Just to verify, you said that the communication between the two particles themselves is instataneous but the ability to read these particles has to be enabled through a classical communication platform? Do physicsists know how the particles communicate faster than the speed of light or is there not an explanation behind that?
The faster then light aspect directly comes from the entaglement. If you have an entangled pair of two atoms (one has spin up along the x-axis and the other has spin down, for example) and you measure the spin of one atom along the x-asis, then you automatically know the spin value of the other atom because it has to be the opposite direction. First of all, communication like that is impossible because of the necessary classical phonecall that is required between the two observers. Secondly, both observers need to measure along the very same axis but who say they will do that ? regards marlon
I must have my concept of entanglement wrong then. I thought entanglement meant that what was done to one entangled partical was automatically done to the other. But this is just a matter of "If this particle is moving this way, the other particle has to be moving that way". No communication is done between the particals themselves. right?
Lets say; two observers, separated a light year from eachother, have come to the agreement that spin up refers to 1 and spin down to 0 (assuming they live quite long...). (They have come to this agreement trough the conventional ways of communication, in this example taking multiple light years, discerning the side effects such as signal-loss and such). When they agreed this, they also agreed to prepare two isolated photons (one here and one there) and to bring them into a 'long-lasting' state of entanglement. Lets assume they have the possibility (by chance or agreed before) to each measure the photon the exact same time or in a time frame allowing FTL. Then, Observer A puts the photon into a forced 'spin up' state, which will, due to entanglement, be instantaneously sent to the other photon, thus forcing that one at observer B into an immediate spin down state. Via this way, observer B will be able to read 0 from his photon. Apart from the fact that measuring the spin state requires classical communication, the whole procedure will highly exceed light speed.
In fact it is neither ! If it were "what is done to one, is also done to the other", you obviously would have a faster-than-light communication channel. Some fools even thought that you could build a rocket motor that way, by having entangled atoms, one in the rocket, and one on earth, and then accelerate those on earth, so that those in the rocket would also accelerate :-) On the other hand, it is not just learning about an unknown parameter of the other atom either. Bell's theorem tells us that that is not the case. Let us separate two issues: one is discussions on the *mechanism* that is responsible for entanglement: there are many discussions about it, people have different views on what is actually going on (I have my own view which I don't stop defending over here :-). The other issue is about what is actually observed: here, most people agree (there's still a "local realist" crowd who denies all experimental results and claims it are all tricked, or badly analysed, or oversold results, but they are, by most others, seen as kind of cranky). I won't go into the mechanism explanations. I will just try to state what is actually predicted by quantum theory, no matter what interpretational flavor. It is about 2 observers, Alice and Bob, who receive each one of the two entangled particles (photons, atoms, whatever). Now, they can do only one measurement on the particle, but they have a choice of WHICH experiment they can do, which is parametrised by a variable, theta-Alice, and theta-Bob. (usually a polarisation angle). So Alice makes a choice of theta-Alice, and then gets a result (up or down) for the measurement at hand. Bob on his side makes a choice of theta-Bob, and then gets a result (up or down) for the measurement at hand. Alice has a certain probability of getting "up", P(a_up, theta_Alice), which is only a function of theta_Alice. Bob has a certain probability of getting "up", P(b_up, theta_Bob), which is only a function of theta_Bob. So far, so good: this is what people mean by 'there is no information transfer': Bob, with his measurement, cannot learn anything about Alice's choice of theta-Alice and vice versa. BUT, but... If Alice and Bob COME TOGETHER, AND COMPARE NOTES, then they observe something strange: there is a correlation: the probability P(a_up, b_up, {theta_Alice, theta_Bob} ) is such that it does not satisfy a property which is called Bell locality. In order to explain this in detail, you should study a bit Bell's theorem. In short, it comes down to the following point. Bell worked out what would be the requirement on the joint probability P(a_up,b_up,{theta_Alice,theta_Bob}) when we assume that the two particles share some common "hidden variables", and then have to generate the probability of "up" or "down" at Bob and Alice, INDEPENDENTLY. So Bell assumed that there is a common variable lambda, and that P(a_up, theta_Alice) is in fact given by P(a_up,lambda,theta_Alice), and that at Bob's the probability is given by P(b_up,lambda,theta_Bob) ; and that these probabilities are independently generated, once we know lambda. This means then that the joint probability is a product: P(a_up,b_up,lambda,{theta_Alice,theta_Bob}) = P(a_up,lambda,theta_Alice) x P(b_up,lambda,theta_Bob). But we don't know anything about lambda, is just has an unknown probability distribution, P(lambda), so our observed correlation is then, according to Bell: P(a_up,b_up,{theta_Alice,theta_Bob}) = Integral P(lambda) P(a_up,b_up,lambda,{theta_Alice,theta_Bob}) d lambda Of course, there is still a lot of freedom, because of the choice of P(lambda), and P(a_up,lambda,theta_alice) and so, but Bell succeeded nevertheless in writing down some INEGALITIES which the joint probability needs to satisfy. Well, it turns out that the joint probabilities for entangled particles in quantum theory DO NOT ALWAYS SATISFY those Bell inequalities. What does this mean, statistically ? Well, it just means that one of Bell's hypotheses are not satisfied. And Bell's hypotheses are that the probabilities of Alice and Bob observing "up" for their chosen angles are generated INDEPENDENTLY as a function of a COMMON SET OF (HIDDEN) VARIABLES. This is a very reasonable hypothesis when "statistical" things happen and when a correlation is observed. If somehow you arrange that there cannot be any DIRECT influence (because there's a big distance, a concrete wall etc.. between Alice and Bob), then if you observe a correlation, you normally assume a COMMON CAUSE (the hidden variable). So this is somehow not true in quantum theory: you can have correlations without having a "common cause". But it is also true that Bob cannot learn anything from Alice's CHOICE from his local measurement, nor can Alice learn anything from Bob's choice. So this thing doesn't allow you to send information from Alice to Bob. cheers, Patrick.
Well your definition is correct but the communication part is just the fact that if you measure one spin, you automatically know what the other observer will have as outcome when he measures the other atom of the entangled pair regards marlon
Their behavior wrt some detection scheme or other is correlated. For example, different, separated parts of the (same) television signal (wave) are entangled. "Entanglement processes" produce the entangled phenomena observed experimentally. The entangled phenomena have a common cause (including, but not necessarily requiring, that they've interacted prior to detection). Marlon mentioned PDC. There are also other experimental processes that produce entanglement. "Particle state information" is something that *we* generate via theory and observation. Are the separated, entangled physical phenomena *causing* each other (instantaneously or superluminally)? There's no direct evidence of that. But some interpretations have it that that's what's happening. My personal opinion is that that sort of *causation-at-a-distance* probably isn't what's happening. The correlations are a function of analyzing (even via spacelike separated events) motional properties that the entangled phenomena have in common due to their having interacted in the past, or being created at the same time and place (eg., a wave moving omnidirectionally away from its source and rotating parallel to some plane-- separate, individual points on the wave are entangled wrt the rotation). Separated objects in any *system* of objects moving together as a group are entangled wrt the movement of the system as a whole. Information, in the sense of something being communicated from one place to another, is transmitted electromagnetically. There might be other waves in nature moving faster than EM waves, but nobody has detected that yet. So, as far as anybody knows, the speed of electromagnetic radiation in a vacuum is the upper limit. Nothing *needs* to be being transferred instantaneously or superluminally to understand why the correlations of entangled phenomena are what they are. For example, in the case of photons entangled in polarization, light waves emitted (presumably by the same atom) during the same interval are analyzed by crossed linear polarizers. No nonlocal causation needs to be happening -- the polarizers are simply, in effect, analyzing the same light at the same time, and a cos^2 theta correlation for coincidental detection emerges (which is what would be expected if the same light is being analyzed by crossed linear polarizers). Now, I'm aware of analyses of this that conclude that the light incident on the polarizers can't have been made the same by the emission process, that it must happen at the instant the detection that initiates a coincidence interval occurs. But, these analyses are flawed, imho. One way to approach it is by considering where the qm projection along the plane of transmission (by the polarizer at the initially detecting end) comes from. There's, imo, a sound physical basis for it. Anyway, what results is a probability of 1 for the initiating detection, and a cos^2 theta probability at the other end for the same interval. So, the joint probability of detection (the probability of coincidental detection) wrt any interval is 1(cos^2 theta). And, experiments support this prediction. The assumption of the causal independence of spacelike separated individual results holds as long as one is careful to modify the probabilistic picture following the initiating detection. Maybe current 'pictures' of spin and polarization are inadequate to describe exactly what is happening. But, the plane of polarization, and the intensity, of the light transmitted by the first polarizer (associated with the start of the coincidence interval) is a subset of the emitted light incident on each polarizer for the common interval. This light produced a photon, which represents maximal intensity for that coincidence interval, at the first detector. So, it follows from standard optics that the probability that it will produce a photon at the second detector (via analyzing the light from the same emission, or set of emissions) is cos^2 theta, where theta is the angular difference between the settings of the two polarizers.
This pretty much sums up my conception of the process. There just can't be any impossible or mysterious factors involved. We just haven't identified all the properties and restrictions on their motion. Unless I miss the point, communication at a distance is merely speculation, right? :shy:
Well, there can't be any *impossible* factors involved. :) But, there are mysterious factors involved, and they have, imo, as much (maybe more in the case of entanglement) to do with the way competing formulations are analysed as with the entangled phenomena themselves. The (speculative) inference of instantaneous or superluminal *causal* relationships between the separated phenomena is allowed, logically, given certain assumptions (or, more strictly, the experimental negation of certain *interpretations* of certain assumptions via the formulation of probability statements regarding joint detection, and the restriction of alternatives). I've outlined the reasons why I don't think that experimental violations of Bell inequalities are telling us what some people seem to think they're telling us. Was Bell wrong? No, he said his formulation regarding probability of joint detection is incompatible with qm. It is. It's also incompatible with experimental results, which support the qm formulation. The problem is that the usual lhv formulation, following Bell, doesn't take into account that the probabilities for individual detection have changed once a detection is registered and a coincidence interval is initiated. If you give the qm projection operator the correct, imo, physical interpretation in these experiments, then the qm formulation can be seen as a sort of lhv theory itself. It seems like a good bet that all the properties of light, electricity, etc. haven't been identified yet -- at least not precisely enough to give a clear picture of the physical details of what's happening in the entanglement experiments.
Oh, I see. So it's more saying that because the photons were produced at the same exact time, any reading of the particles will be probably the same depending on the point in time the photon was "read". (seeing the same light at the same time). I was thinking like the idiots that were going to use it for rocket propulsion. Haha. Thank you for clearing that up for me. I thought that the actions of Alice would produce an effect to Bob's photon.
I don't think so, but some pretty smart people do. The problems arise because of the way some lhv formulas are done. If you describe joint detection in terms of the product of the *initial* (prior to detection) individual probabilities, then you get some predictions that don't agree with qm (or experiment). But, the probability of individual detection changes upon a detection being registered at one end or the other. When that's taken into account, then the idea that the filters are analyzing a common property (or properties) imparted at emission is ok.
More like, because the photons were produced at the same exact time *and place* (like from the same atomic 'burp'), subsequent analysis of the photons by the same sort of device will produce results that are correlated. There's a lot of great stuff written about this sort of thing. If you're really interested, then you should read all of Bell's work on this (and check out all of the citations, including the EPR paper, the Aspect papers, etc). That should set you back at least a few months, but it will give you a much better understanding of the difficulties involved -- and the considerations that led to the belief by some that there are superluminal 'influences' (or whatever you want to call the nonlocal stuff) in nature.
Ok. I'll check out those articles. Thank you for helpin me out even through my confusion. I must go now.
I read, and re-read this several times, and I can't make up what you mean. I am reasonably well acquainted (or so I think) with Bell's reasoning. What do you mean by "the probabilities for individual detection have changed once a detection is registered" ?? cheers, Patrick. EDIT: btw, this has probably already been cited, but I just found a very very thorough reference on all things Bell: http://plato.stanford.edu/entries/bell-theorem/
Prior to detection the probability of individual detection at each end is .5. A detection at one end or the other starts the coincidence circuitry. A 'coincidence interval' is electronically defined and, for this interval, the probability of detection at the detecting end is no longer .5. It's 1. The probability of detection at the other end for this interval is no longer .5, but cos^2 theta (where theta is the angular difference of the polarizer settings). So, the probability of joint detection is 1(cos^2 theta). The transmission axis of the polarizer at the initially detecting end is taken or projected as the global emission parameter, because: (1) the intensity of the detected light is a subset of the emitted light. (2) the transmission axis of the polarizer at the initially detecting end represents the or 'a' plane of maximal transmission by the polarizer(s) wrt the light emitted during the interval (a photon *was* produced out of light that was filtered from the emitted light). (3) PMT response is directly proportional to the intensity of the light transmitted by the polarizer. (4) the intensities of the light between the polarizers and their respective PMT's are related by cos^2 theta, which therefore represents the probability of joint detection for any coincidence interval.
As far as anyone can tell, that is EXACTLY what happens. Of course it is just as likely that it is Bob's actions that affect Alice's results. These scenarios end up being indistinguishable, which is of course a bit puzzling.
That is certainly an unconventional description of the situation. Since the results change upon the "first" detection, as you also point out in other posts, and that "causes" the results at the other detector to immediately change, you are saying that the results ARE dependent on space-like separated observer settings. That is the opposite of a LHV interpretation.