Quantum entanglement is a physical phenomenon that occurs when pairs or groups of
particles are generated, interact, or share spatial proximity in ways such that the
quantum stateof each particle cannot be described independently of the state of the others, even when the particles are separated by a large distance.
Measurements of
physical properties such as
position,
momentum,
spin, and
polarization, performed on entangled particles are found to be
correlated. For example, if a pair of particles is generated in such a way that their total spin is known to be zero, and one particle is found to have clockwise spin on a certain axis, the spin of the other particle, measured on the same axis, will be found to be counterclockwise, as is to be expected due to their entanglement. However, this behavior gives rise to seemingly
paradoxical effects: any measurement of a property of a particle performs an irreversible collapse on that particle and will change the original quantum state. In the case of entangled particles, such a measurement will be on the entangled system as a whole.
Such phenomena were the subject of a 1935 paper by
Albert Einstein,
Boris Podolsky, and
Nathan Rosen,
[1] and several papers by
Erwin Schrödinger shortly thereafter,
[2][3] describing what came to be known as the
EPR paradox. Einstein and others considered such behavior to be impossible, as it violated the
local realism view of causality (Einstein referring to it as "spooky
action at a distance")
[4] and argued that the accepted formulation of
quantum mechanics must therefore be incomplete.
Later, however, the counterintuitive predictions of quantum mechanics were verified experimentally
[5] in tests where the polarization or spin of entangled particles were measured at separate locations, statistically violating
Bell's inequality. In earlier tests it couldn't be absolutely ruled out that the test result at one point could have been
subtly transmitted to the remote point, affecting the outcome at the second location.
[6] However so-called "loophole-free" Bell tests have been performed in which the locations were separated such that communications at the speed of light would have taken longer—in one case 10,000 times longer—than the interval between the measurements.
[7][8]
According to
some interpretations of quantum mechanics, the effect of one measurement occurs instantly. Other interpretations which don't recognize
wavefunction collapse dispute that there is any "effect" at all. However, all interpretations agree that entanglement produces
correlation between the measurements and that the
mutual information between the entangled particles can be exploited, but that any
transmission of information at faster-than-light speeds is impossible.
[9][10]
Quantum entanglement has been demonstrated experimentally with photons,[11][12][13][14] neutrinos,[15] electrons,[16][
