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Quantum entanglement
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Quantum mechanics
Introduction
Glossary · History
Bra–ket notation
Classical mechanics
Hamiltonian
Interference
Old quantum theory
Complementarity
Decoherence
Entanglement
Energy level
Nonlocality
Quantum state
Superposition
Tunnelling
Uncertainty
Wave function
Wave function collapse
Symmetry
Measurement
Afshar
Bell's inequality
Davisson–Germer
Delayed choice quantum eraser
Double-slit
Elitzur-Vaidman
Franck-Hertz
Mach-Zehnder inter.
Popper
Quantum eraser
Schrödinger's cat
Stern–Gerlach
Wheeler's delayed choice
Formulations
Heisenberg
Interaction
Matrix mechanics
Schrödinger
Sum over histories
Phase space
Dirac
Klein–Gordon
Pauli
Rydberg
Schrödinger
Interpretations (overview)
Bayesian
Consistent histories
Copenhagen
de Broglie–Bohm
Ensemble
Hidden variables
Many-worlds
Objective collapse
Quantum logic
Relational
Stochastic
Transactional
Quantum chaos
Quantum field theory
Density matrix
Quantum statistical mechanics
Quantum information science
Scattering theory
Fractional quantum mechanics
Relativistic quantum mechanics
Bell
Blackett
Bohm
Bohr
Born
Bose
de Broglie
Candlin
Compton
Dirac
Davisson
Debye
Ehrenfest
Einstein
Everett
Fock
Fermi
Feynman
Heisenberg
Hilbert
Jordan
Kramers
von Neumann
Pauli
Lamb
Landau
Laue
Moseley
Millikan
Onnes
Planck
Raman
Rydberg
Schrödinger
Sommerfeld
von Neumann
Weyl
Wien
Wigner
Zeeman
Zeilinger
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Quantum entanglement is a physical phenomenon that occurs when pairs (or groups) of particles are generated or interact in ways such that the quantum state of each member must subsequently be described relative to the other.Quantum entanglement is a product of quantum superposition. However, the state of each member is indefinite in terms of physical properties such as position, momentum, spin, polarization, etc. in a manner distinct from the intrinsic uncertainty of quantum superposition. When a measurement is made on one member of an entangled pair and the outcome is thus known (e.g., clockwise spin), the other member of the pair is at any subsequent time always found (when measured) to have taken the appropriately correlated value (e.g., counterclockwise spin). There is thus a correlation between the results of measurements performed on entangled pairs, and this correlation is observed even though the entangled pair may be separated by arbitrarily large distances. Repeated experiments have verified that this works even when the measurements are performed more quickly than light could travel between the sites of measurement: there is no lightspeed or slower influence that can pass between the entangled particles. Recent experiments have measured entangled particles within less than one part in 10,000 of the light travel time between them; according to the formalism of quantum theory, the effect of measurement happens instantly.This behavior is consistent with quantum theory, and has been demonstrated experimentally with photons, electrons, molecules the size of buckyballs, and even small diamonds. It is an area of extremely active research by the physics community. However, there is some heated debate about whether a possible classical underlying mechanism could explain entanglement. The difference in opinion derives from espousal of various interpretations of quantum mechanics.Research into quantum entanglement was initiated by a 1935 paper by Albert Einstein, Boris Podolsky, and Nathan Rosen describing the EPR paradox and several papers by Erwin Schrödinger shortly thereafter. Although these first studies focused on the counterintuitive properties of entanglement, with the aim of criticizing quantum mechanics, eventually entanglement was verified experimentally, and recognized as a valid, fundamental feature of quantum mechanics. The focus of the research has now changed to its utilization as a resource for communication and computation.
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