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'''Quantum entanglement''' is when two (or more) [[quantum]] [[particles]] relate to each other, as in taking spins opposite to each other.  When they are separated they retain the same opposite spins, and the observation of the spin of one of the states determines the opposite spin for the other state, even though the particles may have been separated by many miles from each other.

'''Quantum entanglement''' is the phenomenon of two (or more) [[Subatomic particle|particles]] relating to each other in a certain manner under the special rules of [[Quantum mechanics]], as in taking spins opposite to each other.  When they are separated they retain the same opposite spins, and the observation of the spin of one of the particles always obtains the opposite spin for the other particle, even though they may have been separated from each other by many miles.  This phenomenon lies outside of normal intuition.  Physicists refer to the normal state of affairs as "local realism".  Strict quantum mechanics is sometimes at odds with "local realism", and entanglement is an example of that.

The implications of this phenomenon are enormous, and form the basis for the new field of [[quantum computing]].  For example, there seems to be an instantaneous communication between the particles at the moment of observation of one of them.  

Experiments show that when local realism conflicts with strict (Copenhagen) quantum mechanics, the latter is correct.

On the other hand, some [[physicist]]s resist that notion and claim that no information is actually communicated. This is compatible with the [[relativity|theory of relativity]], since it is not possible for someone to transmit [[information]] faster than the [[speed of light]] by somehow encoding it with entangled particles.

The implications of this phenomenon are enormous, and form the basis for the new field of [[quantum computing]].

 

When the two particles reveal their states at their different locations, and those states always correspond, it might seem that there must have been some kind of instantaneous communication between them, and that this would constitute supraluminal information transfer.  However, this "transfer" lies outside of local realism.  There is no actual transfer that anyone could observe or exploit.

 

Some [[physicist]]s claim that no information is actually communicated. This is compatible with the [[relativity|theory of relativity]], since it is not possible for someone to transmit [[information]] faster than the [[speed of light]] by somehow encoding it with entangled particles.

== Local hidden variable theories ==

== Local hidden variable theories ==

== See also ==

== See also ==

* [[Action at a distance]]

*[[Action at a distance]]

[[Category:Quantum Mechanics]]

[[Category:Quantum Mechanics]]