# Tag Info

66

Entanglement is being presented as an "active link" only because most people - including authors of popular (and sometimes even unpopular, using the very words of Sidney Coleman) books and articles - don't understand quantum mechanics. And they don't understand quantum mechanics because they don't want to believe that it is fundamentally correct: they always ...

43

I wish to complete @Luboš Motl's answer, to which I agree. My point is on why people continue to make this mistake of an active link. This mistake is connected with one of the most interesting properties of quantum mechanics, Bell's theorem. One can argue that any physical theory is an hidden variable theory, the hidden variable being the description of the ...

39

I understand your confusion, but here's why people often feel that quantum entanglement is rather strange. Let's first consider the following statement you make: 2 things have some properties set in correlation to each other at the point of entanglement, they are separated, measured, and found to have these properties A classical (non-quantum) version ...

20

A short summary of the relation between MPS/PEPS and ground states local Hamiltonians: First the direction from MPS/PEPS to Hamiltonians: Every MPS/PEPS naturally appears as the exact ground state of a frustration free local Hamiltonian. ("parent Hamiltonian") For generic MPS/PEPS, this ground state will be unique. There is a number of cases beyond the ...

20

It's not possible to communicate faster than light using entangled states. All you get out of entanglement is a correlation between the values of two measurements.; the entanglement doesn't allow you to influence the value measured at another location in a non-causal way. In other words, the correlation only becomes evident after combining the results from ...

17

To answer this kind of question properly, it's important to clarify the foundational issues of why SR forbids superluminal speeds and what kind of superluminal speeds it forbids. There are several independent arguments of this kind that tell us several different things. Superluminal transmission of information would violate causality, since it would allow ...

14

First of all, it is very important to note that quantum entanglement is not a spooky action-at-a-distance as Einstein once called it! It is a strong correlation of measurements that is stronger than any classical correlation could ever be. This has been experimentally verified by the so-called violation of Bell's inequalities. Second, quantum entanglement ...

13

Electromagnetic waves travel at the speed of light, and nothing can carry energy or information faster than light. Quantum entanglement doesn't carry information from one particle to another: all you get on one end is a random value from some distribution that has a relationship to a random number somewhere else. They can't be used to transmit information ...

13

Even in non-relativistic quantum mechanics, entanglement is no symptom of any non-locality. The explanation of the entanglement as a non-local effect is a mystification spread by numerous popular books that ultimately boils down to a misinterpretations of quantum mechanics by Albert Einstein who misleadingly called entanglement "the spooky action at a ...

12

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

Collapsing an entangled pair occurs instantaneously but can never be used to transmit information faster than light. If you have an entangled pair of particles, A and B, making a measurement on some entangled property of A will give you a random result and B will have the complementary result. The key point is that you have no control over the state of A, ...

11

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 ...

11

From what I understand, entanglement isn't destroyed, it is only obscured by subsequent interactions with the environment. Depends on how you view it. There is an explanation of quantum measurement (called decoherence) in which this is true. I will not be using that explanation in this post because it's unnecessarily complicated. This will have some ...

11

Just a nice analogue Prof. Jürgen Audretsch told me once: Imagine at home you put one glove in your coat without looking (and noticing it's only one of the two). After exiting the train you notice it's cold and you pull out that single glove. At this very instant you know it's either the left or the right glove, and you therefore know which one is left ...

11

Rather than repeat some very good standard answers, I want to discuss this issue from the perspective as to why classical systems should be viewed as strange. If we accept quantum mechanics as being fundamental, then in some sense we shouldn't really find things like entanglement to be strange at all. As pointed out by the answer given by joshphysics, as ...

11

This is a heavy question, that contains many topics in it that are worthy of their own questions, so I'm not going to give a complete answer. I am relying mainly on this excellent review paper by Nayak, Simon, Stern, Freedman and Das Sarma. The first part can be skipped by anyone already familiar with anyons. Abelian and non-Abelian anyons Anyons are ...

11

In principle, any of the fundamental "forces" could be used to transmit information. In practice, humans are only able to use electromagnetism. And in any case, none of these "forces" travel faster than light. Gravitational waves basically travel as fast, but no faster. Any massive particles (including neutrinos) will travel strictly slower than light. ...

9

I'm assuming you're talking about plane-polarized photons, where a photon that passes a 0º analyzer is horizontally polarized, a photon that passes a 90º analyzer is vertically polarized, and there's an orthogonal polarization basis at ±45º. Here's the trouble: Now whoever is at A measures A first at 0°, then (if it goes through), at 40°. Like a ...

9

Entanglement is a quantum correlation between two (or many) objects - a correlation means that these two objects' properties are not independent of each other - which was created in the objects' common past when they were close to one another i.e. when they were two parts of the same physical system. Quantum mechanics changes the character of possible ...

9

No, there is no retrocausal causation in the delayed choice entanglement swapping experiment (or any other experiment or process in the Universe, for that matter), see http://motls.blogspot.com/2012/03/has-anton-zeilinger-created-time.html for a detailed explanation. Correlations between Alice, Bob, and Victor's outcomes may obviously be verified only ...

9

Aram's answer seems perfect, but since you are also asking about the case for higher dimensional systems, let me add that there is a simply way to get somewhat non-trivial upper and lower bounds on $C(j_S,j_L)$. As a lower bound, you can simply synthesize an arbitrary gate which implements communication between the quantum systems (for an explicit algorithm ...

9

Dear Gunther, a partial trace contains all the information about possible predictions made for the subsystem that you haven't traced over. It's trivial to see why. First, take the whole system A+B. They may be entangled but we're only interested in future measurements of A. If the description for A+B is a pure state $|\psi\rangle$, then define $$\rho = ... 8 The normal state of subsystem is entangled, as follows from the Schroedinger equation, which turns a separable state immediately into an entangled state if there is some interaction. The state of a subsystem is obtained from the state of the universe by tracing out all other degrees of freedom. This leaves a density matrix, which is a perfectly good state ... 8 In quantum mechanics, two observables that cannot be simultaneously determined are said to be non-commuting. This means that if you write down the commutation relation for them, it turns out to be non-zero. A commutation relation for any two operators A and B is just the following$$[A, B] = AB - BA If they commute, it's equal to zero. For ...

8

In fact your view is quite official one; entanglement occurs just because both particles are described with one weave-function; the magic is in our classical habit of thinking that separate objects are described with separate "coordinates".

8

Dear sb1, the assumptions implicitly contained in your question are upside down. Exactly the fact that the degrees of freedom of the Universe are becoming increasingly entangled means that we cannot observe the quantum phenomena in practice. The entanglement that emerges after a relatively short time is the entanglement among a huge number of degrees of ...

8

No. What you describe is not what is meant by quantum entanglement. What you describe would allow instantaneous communication across large distances which would allow violations of causality and would violate special relativity. Quantum entanglement occurs when you prepare two particles such that one is spin up and the other is spin down, but you don't ...

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