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'''Entanglement''', also called the quantum non-local connection, is a property of a [[quantum]] [[mechanical]] [[state]] of a [[system]] of two or more objects in which the quantum states of the constituting objects are [[Connected|linked]] together so that one object can no longer be adequately described without full mention of its counterpart—even if the [[individual]] objects are spatially separated in a [[Space|spacial]] [[manner]]. The property of entanglement was understood in the early days of [[quantum theory]], although not by that name. Quantum entanglement is at the heart of the EPR [[paradox]] developed by [[Albert Einstein]], Boris Podolsky, and Nathan Rosen in 1935. This interconnection leads to non-classical correlations between observable physical properties of remote systems, often referred to as nonlocal correlations.

'''Entanglement''', also called the quantum non-local connection, is a property of a [[quantum]] [[mechanical]] [[state]] of a [[system]] of two or more objects in which the quantum states of the constituting objects are [[Connected|linked]] together so that one object can no longer be adequately described without full mention of its counterpart—even if the [[individual]] objects are spatially separated in a [[Space|spacial]] [[manner]]. The property of entanglement was understood in the early days of [[quantum theory]], although not by that name. Quantum entanglement is at the heart of the EPR [[paradox]] developed by [[Albert Einstein]], Boris Podolsky, and Nathan Rosen in 1935. This interconnection leads to non-classical correlations between observable physical properties of remote systems, often referred to as nonlocal correlations.

Theories involving hidden variables have been proposed in order to explain this result. These hidden variables would account for the spin of each particle, and would be determined when the entangled pair is [[created]]. It may [[appear]] then that the hidden variables must be in [[communication]] no matter how far apart the particles are, that the hidden variable describing one particle must be able to [[change]] instantly when the other is measured. If the hidden variables stop interacting when they are far apart, the [[statistics]] of multiple measurements must obey an inequality (called Bell's inequality), which is, however, violated, both by quantum mechanical theory and in experiments.

Theories involving hidden variables have been proposed in order to explain this result. These hidden variables would account for the spin of each particle, and would be determined when the entangled pair is [[created]]. It may [[appear]] then that the hidden variables must be in [[communication]] no matter how far apart the particles are, that the hidden variable describing one particle must be able to [[change]] instantly when the other is measured. If the hidden variables stop interacting when they are far apart, the [[statistics]] of multiple measurements must obey an inequality (called Bell's inequality), which is, however, violated, both by quantum mechanical theory and in experiments.

When pairs of particles are generated by the decay of other particles, naturally or through induced collision, these pairs may be termed "entangled", in that such pairs often necessarily have linked and opposite [[qualities]] such as spin or charge. The [[assumption]] that measurement in effect "[[creates]]" the [[state]] of the measured quality goes back to the [[arguments]] of [[Einstein]], Podolsky, and Rosen[citation needed] and [http://en.wikipedia.org/wiki/Erwin_Schrodinger Erwin Schrödinger] concerning Heisenberg's [http://en.wikipedia.org/wiki/Uncertainty Principle uncertainty principle] and its [[relation]] to [[observation]] (see also the [http://en.wikipedia.org/wiki/Copenhagen interpretation Copenhagen interpretation]).

When pairs of particles are generated by the decay of other particles, naturally or through induced collision, these pairs may be termed "entangled", in that such pairs often necessarily have linked and opposite [[qualities]] such as spin or charge. The [[assumption]] that measurement in effect "[[creates]]" the [[state]] of the measured quality goes back to the [[arguments]] of [[Einstein]], Podolsky, and Rosen and [http://en.wikipedia.org/wiki/Erwin_Schrodinger Erwin Schrödinger] concerning Heisenberg's [http://en.wikipedia.org/wiki/Uncertainty_principle uncertainty principle] and its [[relation]] to [[observation]] (see also the [http://en.wikipedia.org/wiki/Copenhagen_interpretation Copenhagen interpretation]).

The [[analysis]] of entangled particles by means of [http://en.wikipedia.org/wiki/Bell's theorem Bell's theorem] can lead to an impression of [[Simultaneity non-locality], i.e. that there [[exists]] a [[connection]] between the members of such a pair that defies both classical and [[Relativity|relativistic]] [[concepts]] of [[space and time]]. This is reasonable if it is assumed that each particle departs the location of the pair's [[creation]] in an [[Ambiguity|ambiguous]] state (thus yet unobserved, as per a possible [[interpretation]] of Heisenberg's principle). In such a case, for a given observable [[quality]] of the particle, all outcomes remain a [[possibility]] and only measurement itself would precipitate a distinct [[value]]. As soon as just one of the particles is observed, its entangled pair collapses into the very same state. If each particle departs the scene of its "entangled creation" with properties that would unambiguously [[determine]] the [[value]] of the [[quality]] to be subsequently measured, then the postulated [[Immediate|instantaneous]] transmission of [[information]] across [[space and time]] would not be required to account for the result of both particles having the same value for that quality. The [http://en.wikipedia.org/wiki/De Broglie–Bohm theory Bohm interpretation] postulates that a guide [[wave]] exists connecting what are perceived as individual particles such that the supposed hidden variables are actually the particles themselves existing as [[functions]] of that wave.

The [[analysis]] of entangled particles by means of [http://en.wikipedia.org/wiki/Bell's_theorem Bell's theorem] can lead to an impression of [[Simultaneity non-locality], i.e. that there [[exists]] a [[connection]] between the members of such a pair that defies both classical and [[Relativity|relativistic]] [[concepts]] of [[space and time]]. This is reasonable if it is assumed that each particle departs the location of the pair's [[creation]] in an [[Ambiguity|ambiguous]] state (thus yet unobserved, as per a possible [[interpretation]] of Heisenberg's principle). In such a case, for a given observable [[quality]] of the particle, all outcomes remain a [[possibility]] and only measurement itself would precipitate a distinct [[value]]. As soon as just one of the particles is observed, its entangled pair collapses into the very same state. If each particle departs the scene of its "entangled creation" with properties that would unambiguously [[determine]] the [[value]] of the [[quality]] to be subsequently measured, then the postulated [[Immediate|instantaneous]] transmission of [[information]] across [[space and time]] would not be required to account for the result of both particles having the same value for that quality. The [http://en.wikipedia.org/wiki/Bohm_interpretation Bohm interpretation] postulates that a guide [[wave]] exists connecting what are perceived as individual particles such that the supposed hidden variables are actually the particles themselves existing as [[functions]] of that wave.

[[Observation]] of wavefunction collapse can lead to the impression that measurements [[performed]] on one [[system]] instantaneously [[influence]] other systems entangled with the measured system, even when far apart. Yet another [[interpretation]] of this [[phenomenon]] is that quantum entanglement does not [[necessarily]] enable the transmission of classical information faster than the speed of [[light]] because a classical information [[channel]] is required to complete the [[process]].[http://en.wikipedia.org/wiki/Quantum_entanglement]

[[Observation]] of wavefunction collapse can lead to the impression that measurements [[performed]] on one [[system]] instantaneously [[influence]] other systems entangled with the measured system, even when far apart. Yet another [[interpretation]] of this [[phenomenon]] is that quantum entanglement does not [[necessarily]] enable the transmission of classical information faster than the speed of [[light]] because a classical information [[channel]] is required to complete the [[process]].[http://en.wikipedia.org/wiki/Quantum_entanglement]

==Background==

==Background==