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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, using 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. However, information cannot be transmitted through entanglement faster than the speed of light.Quantum entanglement is the basis for emerging technologies such as quantum computing, quantum cryptography and has been used for experiments in quantum teleportation. At the same time, it produces some of the more theoretically and philosophically disturbing
aspects of the theory, as one can show that the correlations predicted by quantum mechanics are inconsistent with the seemingly
obvious principle of local realism, which is that all objects have a well defined state and information about that state should not be transferred instantaneously. Different views of what is actually occurring in the process of quantum entanglement give rise to different interpretations of quantum mechanics.
1 Background
Entanglement is one of the properties of quantum mechanics which caused Einstein and others to dislike the theory. In 1935, Einstein, Podolsky, and Rosen formulated the EPR paradox, demonstrating that entanglement makes quantum mechanics a non-local theory. Einstein famously derided entanglement as "spooky action at a distance."
On the other hand, quantum mechanics has been highly successful in producing correct experimental predictions, and the strong correlations associated with the phenomenon of quantum entanglement have in fact been observed. One apparent way to explain quantum entanglement is an approach
known as " hidden variable theoryIn physics, a hidden variable theory is urged by a minority of physicists who argue that the statistical nature of quantum mechanics implies that quantum mechanics is incomplete; it is really applicable only to ensembles of particles; new physical phenome", in which unknown
deterministic microscopic parameters would cause the correlations. However, in 1964Events January January 1 Federation of Rhodesia and Nyasaland is dissolved. January 3 Senator Barry Goldwater announces that he will seek the Republican nomination for President. January 5 In the first meeting between leaders of the Roman Catholic and Ort BellBell ( June 28 1928 October 1 1990) was an Irish physicist. Bell was born in Belfast, Northern Ireland. In 1964 he derived an inequality that must be satisfied for there to be a local hidden-variable theory of quantum mechanics. He showed that von Neumann showed that such a theory could not be "local", the quantum entanglement predicted by quantum mechanics being experimentally distinguishable from a broad class of local hidden-variable theories. Subsequent experiments have been consistent with quantum mechanics. However, due to " loopholesIntroduction There are two main avenues of escape from the logic of " Bell's Theorem" in quantum mechanics with its implications of " quantum entanglement" or " nonlocality". One can either argue that Bell's logic was wrong and his inequality can be infri" in these
experiments, they have not completely ruled out the possibility of quantum mechanics being incorrect.
Entanglement produces some interesting interactions with the principle of relativityGalileo first developed the principle of relativity which was the postulate that claimed that the laws of physics be the same for all observers, and advocated a classical view that time was a universal constant. Einstein's theory of relativity consists of that states that information cannot be transferred faster than the speed of light. Although two entangled systems can interact across large spatial separations, no useful information can be transmitted in this way, so causalityThe philosophical concept of Causality or Causation refers to the set of all particular "causal" or "cause-and-effect" relations. The Differentia (distinguishing properties/characteristics) of Causality which all causal relations have in common: The relat cannot be violated through entanglement. This occurs for two subtle reasons: (i) quantum mechanical measurements yield probabilisticThe word probability derives from the Latin probare (to prove, or to test). Informally, probable is one of several words applied to uncertain events or knowledge, being more or less interchangeable with likely risky hazardous uncertain and doubtful depend results, and (ii) the no cloning theoremQuantum mechanics Theorems The no cloning theorem is a result of quantum mechanics which forbids the creation of identical copies of an arbitrary unknown quantum state. It was stated by Wootters, Zurek, and Dieks in 1982, and has profound implications in forbids the statistical inspection of entangled quantum states.
Although no information can be transmitted through entanglement alone, it is possible to transmit information using a set of entangled states used in conjunction with a classical information channel. This process is known as quantum teleportation. Despite its name, quantum teleportation can not be used to transmit information faster than light, because a classical information channel is involved.
