# Tag Info

## Hot answers tagged epr-experiment

18

Even for things that seem very clear from the theory, you will want to check them. You asked I mean, what were they expecting ----- were they expecting the states of the particles to be not in co-relation? How would they explain for that? Well, of course they were expecting the entanglement. But finding that this is NOT there, would have been a huge ...

14

What you seem to not get from the paper is that the EPR-thought experiment actually made a prediction: It predicted that there are correlations within quantum systems that are stronger than in any possible classical system or any local hidden variable theory. The "spooky action at a distance" is just failing classical intuition. Don't read too much into it ...

14

The original goal of the EPR paper was to show that quantum mechanics is incomplete. Hence, that extra variables have to be added to complete it, contrary to what Cedric claims. The goal of EPR is to show that either nature is non-local (and thus in conflict with SR) either quantum mechanics is incomplete. Since Einstein was not ready to abandon locality and ...

12

The standard test for whether two things are really entangled with one another in the spooky-action-at-a-distance sense of the EPR picture is to see whether measurements of the states of the two particles violate one of the Bell inequalities, meaning that the correlation between the states is stronger than can be explained by any local hidden variable ...

9

The particle does not know anything. The information is what we (physicists, experimentalists) can communicate to someone else. Ask yourself: what can $A$ do, sharing an EPR pair with $B$, to tell something (let's say $0$ or $1$) to his friend $B$ after they have left each other (but still sharing at a distance the EPR pair), in a superluminal way? You will ...

8

The problem with this sort of scheme is that Alice has no control over the results of her measurements, since those are random. This means that she can control which basis Bob's spin is projected on, but she cannot control which of the basis states gets chosen. Bob will then see a random mix of results which turns out to contain no trace of what Alice was ...

8

In quantum mechanics, two observables that cannot be simultaneously determined are said to be non-commuting. This means that if you write down the commutation relation for them, it turns out to be non-zero. A commutation relation for any two operators $A$ and $B$ is just the following $$[A, B] = AB - BA$$ If they commute, it's equal to zero. For ...

6

The Heisenberg's relation is not tied to quantum mechanics. It is a relation between the width of a function and the width of its fourier transform. The only way to get rid of it is to say that x and p are not a pair of fourier transform: ie to get rid of QM.

5

In precise terms, the Heisenberg uncertainty relation states that the product of the expected uncertainties in position and in momentum of the same object is bounded away from zero. Your entanglement example at the end of your edit does not fit this, as you measure only once, hence have no means to evaluate expectations. You may claim to know something but ...

5

Consider the situation where you have two electrons which are entangled with one another, you know one has spin up and the other has spin down, but not which is which. All you know is that for a given electron the chance of it having either spin is 50/50. Now suppose you go on to measure the spin of one of these. Consider the following two theories for what ...

5

I have to think David will agree, on reflection, that his answer has failed to capture the essence of entanglement. Any stream of particles, if not specially prepared, will measure +h/2 or -h/2 at detector A; they will do with respect to the x axis, or the y axis, or any axis. Exactly the same is true at detector B. How can this very ordinary circumstance ...

4

The answer to the question depends a bit on what is meant by "mediated". A composite quantum system composed of two or more quantum subsystems can be in a quantum state in which the subsystems are entangled from the beginning, i.e., from the initial state. If the composite system evolves without any interaction among the subsystems, then the form and degree ...

4

In the original EPR gedanken experiment, they assumed two particles that have perfect correlations in position, i.e., they are described by a delta function. That does not pose a problem for a thought experiment but cannot be performed in a lab because such a state cannot be normalized and is therefore unphysical. However, in quantum optics, many ...

4

Jherico, I see that you are keen in finding answers to your questions, or putting your views across for a debate, and this is really good. This is what science is all about. I think your questions deserve attention and proper debate. Here is an effort from my side to help dilute some of the misunderstanding through the comments section of this forum. ...

4

"But in many worlds interpretation is in one world only one particle at the one place" For the awful pop-sci version of MWI yes, but for actual MWI no. MWI is formulated with the math of quantum mechanics, and that means the Schrodinger equation. Have you read Everett's thesis? Assuming you already know some of how QM works, it's actually a fairly easy read ...

4

Bell's theorem shows that standard QM is inconsistent with local realism. Local realism is a very general principle that was not originally thought to make any testable physical predictions. A major part of Bell's achievement was showing that Bell's inequality is implied by local realism, while standard QM predictions violate it. Experiments like Aspect's ...

4

And you cannot communicate information through entanglement anyway. So my question again : why the experiments? I think you will find that the technology is important in communication. Quantum cryptography is a way of sending secure messages with, for example, entangled photons used to send a key. If photons are intercepted between the sender and the ...

4

Because they have less information that you imagine. They do not have separate bits of information describing them that gets somehow "transported" between them. From an information POV they are only one particle until measured and then they acquire separate but related information describing each separately.

3

Since you're an electronics student, I'll speak your language. Think of momentum and position as parameters in time and frequency domain of a signal rather than classical observable that are well defined. If you do so, you can easily realize that your frequency isn't well defined if you don't do an infinitely long measurement. This is simply due to the wave ...

3

please don't blame simplicity Sending information means : 1) identify the information you want to send, ie up or down 2) send it But , in EPR context , the best you can do is in reverse order 2) send an ( unknow yet ) information by doing a measure ( with a detector ) 1) identify the measure sent ( by reading the detector ) You cannot send what ...

3

There are a few cases. First case, you measure your particle then you write a letter to your friend and tell your result to your friend before your friend measures their stuff. They can be amazed to know the result of their measurements before they do them. Or they can be unamazed since by then your measurement has had time to affect them without violating ...

3

What is wrong with this logic is that you are supposing a particle has simultaneously well-defined position and momentum. This is not true - a state localized in real space is delocalized in momentum space, and vice versa. The classical conservation laws hold on the quantum level as operator laws, not as laws on the states.

3

The wave formulation has in its seed the uncertainty relation. Let me be precise what is meant by the wave formulation: the amplitude over space points will give information about localization on space, while amplitude over momenta will give information about localization in momentum space. But for a function, the amplitude over momenta is nothing else but ...

3

The majority opinion is that Einstein was wrong. However, I see some problems with the standard quantum mechanics (SQM) approach that you outlined. SQM contains two major parts: unitary evolution (described, e.g., by the Dirac equation) and the measurement theory (e.g., collapse, or the projection postulate, which, loosely speaking, states that, after ...

3

I think you answered your question. "According to EPR experiments measurements of the entangled states are at odds with SR": if you mean that we cannot consider that the result of a measurement made on one entangled particle "propagates" to another one because this propagation would violate SR principles, you have to rules out a interaction in the sense of ...

2

Pretty sure that EPR does not state entanglement is at odds with SR or if it does it is incorrect. The point of the EPR paper was that the consequences of entanglement were so strange they could not be real. Experimental evidence however supports entanglement and has never shown any violation of SR.

2

There are no problems with second detector measurements. They occur as they would without first detector. The "problem" is that if you check them with the knowledge of the results of first detector, you might notice that measurements on both ends are correlated. If you measured x components in both, you definitely got opposite results. While if you check x ...

2

An experiment very close to the proposed experiment has been done by Zeilingers grad-student Dopfler in 1998. She used a down-conversion crystal to produce pairs of entangled photons in a quantum-eraser type of experiment involving a dual-slit. One of the members went through the dual-slit and was detected by a detector A which is scanning the space behind ...

2

The process used in this kind of source is the spontaneous parametric down conversion (SPDC, see, e.g. Wikipedia for details). It is a nonlinear optical process in which from a photon with angular frequency $\omega_0$ you get two photons with frequencies $\omega_1$, $\omega_2 = \omega_0-\omega_1$. These photons are then phase matched and have correlated ...

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