# Understanding on quantum entanglement [duplicate]

Understanding on quantum entanglement? I am very vague on this topic and would appreciate a detailed explanation on this phenomenon. Also what are the possible applied uses for quantum entanglement? What are the problems of putting this phenomenon in practice?

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## marked as duplicate by Brandon Enright, Emilio Pisanty, Qmechanic♦Jun 4 '13 at 13:01

See, e.g., physics.stackexchange.com/q/54975 –  UwF Mar 5 '13 at 20:09

Quantum entanglement is a quantum mechanical phenomenon in which the quantum states of two or more objects have to be described with reference to each other, even though the individual objects may be spatially separated.. This leads to correlations between observable physical properties of the systems.For example, it is possible to prepare two particles in a single quantum state such that when one is observed to be spin-up, the other one will always be observed to be spin-down and vice versa, this despite the fact that it is impossible to predict, according to quantum mechanics, which set of measurements will be observed... As a result, measurements performed on one system seem to be instantaneously influencing other systems entangled with it.But quantum entanglement does not enable the transmission of classical information faster than the speed of light. Quantum entanglement has applications in the emerging technologies of quantum computing and quantum cryptography, and has been used to realize quantum teleportation experimentally.At the same time, it prompts some of the more philosophically oriented discussions concerning quantum theory.The correlations predicted by quantum mechanics, and observed in experiment, reject the principle of local realism, which is that information about the state of a system should only be mediated by interactions in its immediate surroundings.Different views of what is actually occurring in the process of quantum entanglement can be related to different interpretations of quantum mechanics.....

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Quantum entanglement occurs in any situation in which a system has parts with measurement outcomes that are not independent of each other--measuring one gives you some information about what the measurement outcomes on the other would be (but not necessarily determine them).

In other words, it is simply the quantum-probabilistic version of correlation. Having a system with parts that are correlated with one another is exactly the same statement as saying they are entangled. That's all there is to it.

Since it's so completely general, nearly every non-trivial quantum-mechanical situation involves entanglement. There is no problem in putting it into practice. For example, the state $$\frac{\left|\uparrow\downarrow\right\rangle - \left|\downarrow\uparrow\right\rangle}{\sqrt{2}}$$ can represent two particles with total spin $0$ where after measuring one to be spin-up, the other would always be measured to be spin-down, and vice versa. This is the situation in the EPR pseudo-paradox, but it's also a quite common situation that describes, for example, the electron spins in a bond between two hydrogen atoms in a hydrogen molecule.

In this particular example, the other measurement is determined with probability $1$, but any degree of non-independence counts as entanglement.

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