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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 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 [https://en.wikipedia.org/wiki/Erwin_Schrodinger Erwin Schrödinger] concerning Heisenberg's [https://en.wikipedia.org/wiki/Uncertainty_principle uncertainty principle] and its [[relation]] to [[observation]] (see also the [https://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/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.

The [[analysis]] of entangled particles by means of [https://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 [https://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]].[https://en.wikipedia.org/wiki/Quantum_entanglement]

==Background==

==Background==

Entanglement is one of the properties of [[quantum mechanics]] that caused [[Einstein]] and others to dislike the [[theory]]. In 1935, Einstein, Podolsky, and Rosen formulated the [http://en.wikipedia.org/wiki/EPR paradox EPR paradox], a quantum-mechanical [[thought]] [[experiment]] with a highly counterintuitive and apparently nonlocal outcome, in [[response]] to Niels Bohr's advocacy of the [[belief]] that quantum mechanics as a theory was complete.[1] Einstein famously derided entanglement as "spukhafte Fernwirkung"[2] or "[[Mystery|spooky]] [[action]] at a distance". It was his belief that future [[mathematicians]] would discover that quantum entanglement entailed nothing more or less than an error in their calculations. As he once wrote: "I find the [[idea]] quite intolerable that an [[electron]] exposed to radiation should choose of its own [[free will]], not only its [[moment]] to jump off, but also its direction. In that case, I would rather be a cobbler, or even an employee in a gaming house, than a [[physicist]]".[3]

Entanglement is one of the properties of [[quantum mechanics]] that caused [[Einstein]] and others to dislike the [[theory]]. In 1935, Einstein, Podolsky, and Rosen formulated the [https://en.wikipedia.org/wiki/EPR paradox EPR paradox], a quantum-mechanical [[thought]] [[experiment]] with a highly counterintuitive and apparently nonlocal outcome, in [[response]] to Niels Bohr's advocacy of the [[belief]] that quantum mechanics as a theory was complete.[1] Einstein famously derided entanglement as "spukhafte Fernwirkung"[2] or "[[Mystery|spooky]] [[action]] at a distance". It was his belief that future [[mathematicians]] would discover that quantum entanglement entailed nothing more or less than an error in their calculations. As he once wrote: "I find the [[idea]] quite intolerable that an [[electron]] exposed to radiation should choose of its own [[free will]], not only its [[moment]] to jump off, but also its direction. In that case, I would rather be a cobbler, or even an employee in a gaming house, than a [[physicist]]".[3]

On the other hand, [[quantum mechanics]] has been highly successful in producing correct [[experiment]]al [[predictions]], and the strong correlations predicted by the theory of quantum entanglement have now in [[fact]] been [[observed]].  One apparent way to explain found correlations in line with the predictions of quantum entanglement is an approach known as "local hidden variable theory", in which unknown, shared, local parameters would cause the correlations. However, in 1964 John Stewart Bell derived an upper limit, known as [http://en.wikipedia.org/wiki/Bell's inequality Bell's inequality], on the strength of correlations for any [[theory]] obeying "local realism". Quantum entanglement can lead to stronger correlations that violate this [[limit]], so that quantum entanglement is experimentally distinguishable from a broad class of local hidden-variable theories. Results of subsequent [[experiments]] have overwhelmingly supported quantum mechanics. However, there may be experimental problems, known as "loopholes", that affect the validity of these experimental findings. High-efficiency and high-visibility experiments are now in [[progress]] that should confirm or invalidate the [[existence]] of those loopholes.  

On the other hand, [[quantum mechanics]] has been highly successful in producing correct [[experiment]]al [[predictions]], and the strong correlations predicted by the theory of quantum entanglement have now in [[fact]] been [[observed]].  One apparent way to explain found correlations in line with the predictions of quantum entanglement is an approach known as "local hidden variable theory", in which unknown, shared, local parameters would cause the correlations. However, in 1964 John Stewart Bell derived an upper limit, known as [https://en.wikipedia.org/wiki/Bell's inequality Bell's inequality], on the strength of correlations for any [[theory]] obeying "local realism". Quantum entanglement can lead to stronger correlations that violate this [[limit]], so that quantum entanglement is experimentally distinguishable from a broad class of local hidden-variable theories. Results of subsequent [[experiments]] have overwhelmingly supported quantum mechanics. However, there may be experimental problems, known as "loopholes", that affect the validity of these experimental findings. High-efficiency and high-visibility experiments are now in [[progress]] that should confirm or invalidate the [[existence]] of those loopholes.  

Observations pertaining to entangled states appear to conflict with the property of [[relativity]] that [[information]] cannot be transferred faster than the speed of light. Although two entangled [[systems]] appear to interact across large [[Spacetime|spatial]] separations, the current [[state]] of [[belief]] is that no useful information can be [[transmitted]] in this way, [[meaning]] that causality cannot be violated through entanglement. This is the [[statement]] of the no-communication theorem.

Observations pertaining to entangled states appear to conflict with the property of [[relativity]] that [[information]] cannot be transferred faster than the speed of light. Although two entangled [[systems]] appear to interact across large [[Spacetime|spatial]] separations, the current [[state]] of [[belief]] is that no useful information can be [[transmitted]] in this way, [[meaning]] that causality cannot be violated through entanglement. This is the [[statement]] of the no-communication theorem.

Even if [[information]] cannot be transmitted through entanglement alone, it is believed  that it is possible to transmit information using a set of entangled states used in conjunction with a [[classic]]al information channel. This [[process]] is known as [http://en.wikipedia.org/wiki/Quantum teleportation quantum teleportation]. Despite its name, quantum teleportation may still not permit information to be transmitted faster than light, because a classical information channel is required to complete the process. In addition experiments are underway to see if entanglement is the result of retrocausality.

Even if [[information]] cannot be transmitted through entanglement alone, it is believed  that it is possible to transmit information using a set of entangled states used in conjunction with a [[classic]]al information channel. This [[process]] is known as [https://en.wikipedia.org/wiki/Quantum teleportation quantum teleportation]. Despite its name, quantum teleportation may still not permit information to be transmitted faster than light, because a classical information channel is required to complete the process. In addition experiments are underway to see if entanglement is the result of retrocausality.

==External links==

==External links==

*[http://plato.stanford.edu/entries/qt-entangle/ Quantum Entanglement at Stanford Encyclopedia of Philosophy]

*[https://plato.stanford.edu/entries/qt-entangle/ Quantum Entanglement at Stanford Encyclopedia of Philosophy]

*[http://www.didaktik.physik.uni-erlangen.de/quantumlab/english/index.html Entanglement experiment with photon pairs - interactive]

*[https://www.didaktik.physik.uni-erlangen.de/quantumlab/english/index.html Entanglement experiment with photon pairs - interactive]

*[http://www.physorg.com/news63037231.html Multiple entanglement and quantum repeating]

*[https://www.physorg.com/news63037231.html Multiple entanglement and quantum repeating]

*[http://physicsweb.org/articles/world/11/3/9/1/world%2D11%2D3%2D9%2D3 How to entangle photons experimentally]

*[https://physicsweb.org/articles/world/11/3/9/1/world%2D11%2D3%2D9%2D3 How to entangle photons experimentally]

*[http://www.mathpages.com/home/kmath521/kmath521.htm Quantum Entanglement and Bell's Theorem at MathPages]

*[https://www.mathpages.com/home/kmath521/kmath521.htm Quantum Entanglement and Bell's Theorem at MathPages]

*[http://www.imperial.ac.uk/quantuminformation Recorded research seminars at Imperial College relating to quantum entanglement]

*[https://www.imperial.ac.uk/quantuminformation Recorded research seminars at Imperial College relating to quantum entanglement]

*[http://davidjarvis.ca/entanglement/ How Quantum Entanglement Works]

*[https://davidjarvis.ca/entanglement/ How Quantum Entanglement Works]

*[http://www.osa.org/meetings/topicalmeetings/ICQI/default.aspx Quantum Entanglement and Decoherence: 3rd International Conference on Quantum Information (ICQI)]

*[https://www.osa.org/meetings/topicalmeetings/ICQI/default.aspx Quantum Entanglement and Decoherence: 3rd International Conference on Quantum Information (ICQI)]

*[http://prola.aps.org/abstract/PR/v47/i10/p777_1 The original EPR paper]

*[https://prola.aps.org/abstract/PR/v47/i10/p777_1 The original EPR paper]

*[http://www.npl.co.uk/server.php?show=ConWebDoc.433 Ion trapping quantum information processing]

*[https://www.npl.co.uk/server.php?show=ConWebDoc.433 Ion trapping quantum information processing]

*[http://www.spectrum.ieee.org/aug07/5378/1 IEEE Spectrum On-line: ''The trap technique'']

*[https://www.spectrum.ieee.org/aug07/5378/1 IEEE Spectrum On-line: ''The trap technique'']

*[http://www.sciam.com/article.cfm?id=was-einstein-wrong-about-relativity Was Einstein Wrong?: A Quantum Threat to Special Relativity]

*[https://www.sciam.com/article.cfm?id=was-einstein-wrong-about-relativity Was Einstein Wrong?: A Quantum Threat to Special Relativity]

[[Category: Physics]]

[[Category: Physics]]