# Tag Info

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in Quantum Mechanics interferences is when the wave function of one (or more) particle can take several paths and then later cross. The complex amplitude adds up and you have an interference. Negative interference if the probability gets smaller at one point or positive if the probability gets higher. The definition is mostly qualitative here. Entanglement ...

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They are different things. Interference Is an intrinsically quantum phenomenon that arises form the fact that transition probabilities between different pure states can be non-zero, as opposed to classical mechanics, where such transition probability is always zero. Entanglement Is a property of a state of a composite system, expressed as the tensor ...

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For $z=2$ there is a study here http://arxiv.org/abs/quant-ph/0404026 in the context of ferromagnetic spin chain. The result is basically $\log L$. Swingle and Senthil argued in http://arxiv.org/abs/1112.1069 that "generally" the violation of area law for EE is at most $L^{d-1}\log L$ where $d$ is the space dimension. However, http://arxiv.org/abs/1408.1657 ...

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No idea if applying Fock algebras to the entanglement description would lead to anything "fruitful", I haven't seen any of that. What I've seen is the emergence of Tensor Network Methods in order to describe entanglement of many-body systems. There is the possibility that TNM can be related with holographic descriptions of gravity. Here is an ...

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From my initial reading of the paper it looks like the the quantum state Bob has access to is changed by Alice's choice of measurement but in such a way that he can't tell until Alice communicates her choice of measurement to him. In particular they present a way for Alice to prove to Bob that she has influenced the part of the state he has access to even ...

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What "entangled" means You probably have put too much in your head for the meaning of the word "entangled." Let me fix that for you: Two systems are entangled in quantum mechanics if the results of separate experiments on those two systems display strange correlations when you bring them back together and compare them. Notice that not all correlations can ...

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A situation like this already happens in classical physics. Suppose that I write either "0" or "1" on two pieces of paper, the same thing on both pieces, and I put them into two separate envelopes. Then, if you don't know which thing I wrote, the contents of the two envelopes are "correlated": opening one tells you about the state of the other. But once ...

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Suppose that system $S_1$ is entangled with system $S_2$. An observation is just coupling the measured system to some other system that allows you to easily make records of experimental results. Everything that happens after that can explained by unitary evolution of the joint system of the measuring instrument, the measured system and the environment, see: ...

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The answer is simple: measurement causes the wave-functions to collapse. It can be said that one of the fundamental properties that makes Quantum mechanics so strange is the idea of superposition, which is the property that if you have two physically valid descriptions of a state, then it is physically just as valid for a system to be in any linear ...

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"Entanglement" is a term describing economically the quantum mechanical state of a system of particles. It is a short hand way of saying : these particles are described by the solution of the Schrodinger equation, with a wave function that can predict the probability of finding the individual particles in a specific (x,y,z) each with specific quantum ...

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I'm not sure I can answer your questions about quantum mechanics honestly without equations, but I can tell you something about the details of generating entangled photons with BBO. First, there are two things you need to know about laser light: it has a definite polarization (orientation of the electric field) and definite energy (or, equivalently, a ...

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In order to generate entanglement you need an interaction, by which I mean that the dynamics have a term that is a function of two different degrees of freedom that you intend to entangle$^1$. The type of nonlinearity in this case is what is known as spontaneous parametric downconversion or SPD, which is a nonlinear optical process. 1) How does this ...

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$\newcommand{\HH}{\mathcal{H}}$As Martin says, entanglement is correlation rather than anything like "determining" the state of the other particle. We don't necessarily even need to talk about correlations, though, although they are one of the primary interesting features of entangled states. More precisely, your quote Quantum entanglement is a physical ...

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The problem is that entanglement doesn't mean that a measurement on the first particle determines the outcome of the second. Although this is often perpetuated, it's not the gist of entanglement. Entanglement is (mostly?) nonclassical correlations. Bipartite states are correlated, when the outcome of the measurement on one particle tells us something about ...

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Entanglement isn't about interaction or information transfer betweeen entangled particles. Consider spin-entaglement of two spin-$\frac{1}{2}$ particles: Let them be in singulet-state relative to an arbitrary axis (say z-axis): $$|\Psi \rangle = \frac{1}{\sqrt{2}} (\ |\uparrow_z, \downarrow_z \rangle - |\downarrow_z,\uparrow_z\rangle \ )$$ The ...

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This problem has been pointed out historically in what is now universally abbreviated as the EPR paper, for which I'll simply refer you to an answer to a very similar question. This seemingly paradoxical effect has been observed experimentally. Some people insist the question of whether it is "real" is still unresolved. The main difficulty, however, is ...

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Entanglement is a real property that can be shown by the violation of the Bell inequalities. How this is commonly done is that a pair of particles are created with entangled spin states in a configuration called Bell states. If entanglement is real, then measuring the state of one particle will give me definite knowledge of the state of the other particle. ...

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It is real, and experimentally verified. If you measure the spin of entangled particle A, particle B when measured will always have the opposite spin. Some physicists believe it has to do with superluminal communication, but there are many other theories.

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This is not possible. Please look at the following links: http://physics.stackexchange.com/a/170798/75518 http://physics.stackexchange.com/a/154051/75518 http://physics.stackexchange.com/a/170884/75518 http://en.wikipedia.org/wiki/No-communication_theorem The first describes in a good way what entanglement is (statistical correlation). The second gives a ...

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entanglement is not about instantanious actions or information transfer. Please look at this link, where a concrete example of spin entanglement was given to justify that statement.

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As I don't belong anymore to this site, my answer will be very brief. I apologize for not being interested to give explicit details, only certain hints. 1) Nature is local. 2) FTL signals are not only self-contradictory, as explained in my protocol, the nature doesn't use them. I repeat, they are not the way the nature works. (By the way, this is why we ...

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Entanglement isn't about interaction or information transfer betweeen entangled particles. Consider spin-entaglement of two spin-$\frac{1}{2}$ particles: Let them be in singulet-state relative to an arbitrary axis (say z-axis): $$|\Psi \rangle = \frac{1}{\sqrt{2}} (\ |\uparrow_z, \downarrow_z \rangle - |\downarrow_z,\uparrow_z\rangle \ )$$ The ...

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Quantum entanglement of two particles is a way of stating that there exists a unique quantum mechanical solution ( a mathematical function) that describes the probability of finding the particles with specific attributes at specific spacetime points with specific energy and momentum. The probability is what is described/known/fitted. The shorthand of ...

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There is no action at a distance and the question makes no sense. A pair of particles must, at any moment occupy some quantum state. That quantum state dictates the probabilities of the outcomes of any measurement you can make. What would it mean for there to be a "delay" in this, and how would that delay differ from a "delay" in any other measurement of ...

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The "instantaneous" collapse of the quantum entanglement is a direct consequence of the axioms of quantum mechanics. Also, in experiment, it has been proved that the speed of "action at a distance" is larger than 10000c. And there is no way to test the absolute instantaneous in experiment.

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Let me first say that I do not work in quantum foundations, really, so I might have a few misconceptions myself. I beg anyone to correct me, where I err and I will try to provide more references upon request. After the question seems to have cleared up in chat, let me rewrite my answer: You basically seem to ask: What if entanglement would allow ...

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See the following examples: $\rho_1 = \frac{1}{2}(|00\rangle + |11\rangle)(\langle 00| + \langle 11|)$ is a maximally entangled state. $\rho_2 = \frac{1}{2} (|0\rangle \langle 0| + |1\rangle \langle 1|)$ is a maximally mixed state. The difference is not related with "maximally". Your question can be changed to : What's the difference ...

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Suppose we have two Hilbert spaces $\mathcal{H}_A$ and $\mathcal{H}_B$. A quantum state on $\mathcal{H}_A$ is a normalized, positive trace-class operator $\rho\in\mathcal{S}_1(\mathcal{H}_A)$. If $\mathcal{H}_A$ is finite dimensinal (i.e. $\mathbb{C}^n$), then a quantum state is just a positive semi-definite matrix with unit trace on this Hilbert space. ...

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Once two particles are entangled, they remain so at any distance, so long as you do nothing which would push them to lose coherence with each other. Entanglement is independent of distance. However, as a practical matter, it takes time to move the particles apart. Increasing distance increases how long it took to get them there. This means there's a ...

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"I was wondering if this type of communication..." Jimmy, as everyone has explained, you can not use entanglement to communicate information. (This is really unfortunate, but that's how it is!) There are many long explanations of this around, that make a good read. Regarding the entangled pair collapsing, I think it's perfectly reasonable to describe ...

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Instant communication is still impossible because the transfer of information occurs when the sender measures the quantum state of their photon. That causes the receiver’s entangled photon to instantly change. However, in order to understand the information, the receiver has to know what the original measurement was, along with some other instructions. ...

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A very good question. First of all, topological order strictly speaking is only defined for gapped states. But to some extent it can coexist with gapless degrees of freedom. A rather trivial example is just adding something gapless decoupled from the topological order (i.e. phonons). The example of the Kitaev model is quite different though, since the ...

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When the state space for a system can be expressed as a tensor product of the state spaces of individual components of the system, an entangled state is one that can't be expressed as a tensor product of states of those individual components. Thus an entangled state is a particular type of (pure, i.e. non-mixed) state. A mixed state, by contrast, is a ...

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rob's answer already touches on your misconception that quantum entanglement by itself could be used to send information. From an engineering point of view, the problems are even more basic: In order for Alice and Bob to create an entangled state, we must first choose a quantum system. For convenience, let's use the polarization of photons, i.e. an ...

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Quantum entanglement doesn't transmit information. When you have two experimenters (usually called Alice and Bob, because real people aren't named A and B) doing widely-separated experiments on entangled particles, each of them does a separate experiment on each particle. For example if the entangled particles are photons with opposite polarizations, each ...

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Not really. It is indeed perfectly possible to entangle two particles A and B without using direct interactions between them, the easiest example being to entangle A with some ancilla system C, carry C over to B, and then swap the states of B and C, which will transfer the A-C entanglement onto the A-B pair. This is of course not magical at all, and the ...

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To entangle a system of two particles with two states each, not-yet-decayed and already-decayed, you need to put them into a superposition of these two states and have their states at least partially depend on each other. Basically, you can have states of the type that either both or none have decayed yet (but you do not know if either has), or of the type ...

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They are entirely unrelated concepts. Two entangled particles are not "clones" of each other which magically do everything the same way; they merely have been put into a state which displays a strange statistical correlation when you "bring both parts back together." So for example a free neutron and an electron both have the same spin-1/2 structure; you ...

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Entanglement can be thought of as a correlation between states. When you say that 2 uranium atoms are entangled, you need to say how. For example, two electrons could be entangled in such a way that one is spin up, and the other spin down. This occurs when we look at a 2-electron system as a whole and determine that the expectation value (mean value) of ...

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One of the problems that inflation solves is the so-called horizon problem. The problem is that parts of the sky that don't appear to have been in causal contact have the same temperature. Inflation solves this problem because before the period of rapid expansion all parts of the sky were in causal contact. Let us consider whether entanglement - which is ...

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Your logic is flawed in that all mass-energy did not come from the same source. All mass-energy was created in a fairly even distribution across the universe, however without inflation, the different regions of the universe were not in causal contact with each other.

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Entangled particles simply do not have the information you think they have. Consider the simplest case, a particle that could spin up, down, or be in a superposition of both (this truly is as simple as it gets). If you have two particles, one possibility is that they each have those properties. For instance the first one could be up, the second could be ...

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Entanglement means that the particles involved are in a quantum mechanical state where all the phases and values are known. Take a specific decay of one particle into two. In the case of the pi_0 two photons come from the decay and their spins are "entangled" because pi_0 has zero spin and angular momentum has to be conserved. Both photons go at the ...

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