Quantum entanglement

Quantum entanglement is the phenomenon where the <a href="/facts/Quantum_state/7jbxd7Dx">quantum state</a> of each <a href="/facts/Subatomic_particle/tMEzcTWQ">particle</a> in a group cannot be described independently of the state of the others, even when the particles are separated by a large distance. The topic of quantum entanglement is at the heart of the disparity between <a href="/facts/Classical_physics/gGnINVPE">classical physics</a> and <a href="/facts/Quantum_physics/BRc3Mzgr">quantum physics</a>: entanglement is a primary feature of quantum mechanics not present in classical mechanics.: 867 
<a href="/facts/Measurement/rsLLbn1s">Measurements</a> of <a href="/facts/Physical_properties/ujcMUceW">physical properties</a> such as <a href="/facts/Position_(vector)/Tpk5d9f2">position</a>, <a href="/facts/Momentum/TTkGv5eY">momentum</a>, <a href="/facts/Spin_(physics)/4Jvp7e8P">spin</a>, and <a href="/facts/Polarization_(waves)/fP33m0eD">polarization</a> performed on entangled particles can, in some cases, be found to be perfectly <a href="/facts/Correlated/egkluAEm">correlated</a>. For example, if a pair of entangled particles is generated such that their total spin is known to be zero, and one particle is found to have clockwise spin on a first axis, then the spin of the other particle, measured on the same axis, is found to be anticlockwise. However, this behavior gives rise to seemingly <a href="/facts/Paradox/3VLGjuWf">paradoxical</a> effects: any measurement of a particle's properties results in an apparent and irreversible <a href="/facts/Wave_function_collapse/f7KvbBHK">wave function collapse</a> of that particle and changes the original quantum state. With entangled particles, such measurements affect the entangled system as a whole.
Such phenomena were the subject of a 1935 paper by <a href="/facts/Albert_Einstein/CNTet8qu">Albert Einstein</a>, <a href="/facts/Boris_Podolsky/aHp6zv1f">Boris Podolsky</a>, and <a href="/facts/Nathan_Rosen/0ks08uEe">Nathan Rosen</a>, and several papers by <a href="/facts/Erwin_Schr%25C3%25B6dinger/wCprepF2">Erwin Schrödinger</a> shortly thereafter, describing what came to be known as the <a href="/facts/EPR_paradox/rulT1oGp">EPR paradox</a>. Einstein and others considered such behavior impossible, as it violated the <a href="/facts/Local_realism/rulT1oGp">local realism</a> view of <a href="/facts/Causality/Swh496X7">causality</a> (Einstein referring to it as "spooky <a href="/facts/Action_at_a_distance/RcnpWomY">action at a distance</a>") and argued that the accepted formulation of <a href="/facts/Quantum_mechanics/BRc3Mzgr">quantum mechanics</a> must therefore be incomplete.
Later, however, the counterintuitive predictions of quantum mechanics were verified in tests where polarization or spin of entangled particles were measured at separate locations, statistically violating <a href="/facts/Bell%2527s_theorem/lg1V6kpk">Bell's inequality</a>. This established that the correlations produced from quantum entanglement cannot be explained in terms of <a href="/facts/Local_hidden_variable_theory/wXo2mmvp">local hidden variables</a>, i.e., properties contained within the individual particles themselves.
However, despite the fact that entanglement can produce statistical <a href="/facts/Correlation/egkluAEm">correlations</a> between events in widely separated places, it cannot be used for <a href="/facts/Faster-than-light_communication/HTCP4ht5">faster-than-light communication</a>.: 453 
Quantum entanglement has been demonstrated experimentally with <a href="/facts/Photon/mFNhAEzA">photons</a>, <a href="/facts/Electron/L0KBo5cW">electrons</a>, <a href="/facts/Top_quark/82qd7NPI">top quarks</a>, molecules and even small diamonds. The use of quantum entanglement in <a href="/facts/Quantum_communication/JDuxpHp8">communication</a> and <a href="/facts/Quantum_computing/nbbcgmvq">computation</a> is an active area of research and development.

Quantum entanglement open-in-new

Quantum entanglement