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Entanglement means a composite quantum state cannot be factored into independent states of its parts; the parts must be described together.
Quantum entanglement is a quantum phenomenon where the state of each particle in a group cannot be described independently of the others, even if the particles are far apart. Instead, the particles share a single joint quantum state, so measurements performed on one particle can be strongly correlated with measurements on the other. For example, in a spin-singlet pair, measuring one particle’s spin along a chosen axis determines (perfectly anti-correlated) what the other particle’s spin will be along the same axis. These correlations can appear paradoxical because standard quantum-mechanical measurement is described as a wave-function collapse that changes the state of the measured particle. With entangled particles, that “collapse” effectively affects the entire entangled system, not just the local particle. The EPR paradox highlighted this tension with local realism, and later experiments testing Bell’s inequality showed that the observed correlations cannot be explained by any theory based on local hidden variables. Importantly, entanglement enables correlations but does not allow faster-than-light communication.
Entanglement means a composite quantum state cannot be factored into independent states of its parts; the parts must be described together.
Measurements on entangled particles can show perfect or strong correlations, leading to apparent “spooky” effects compared with classical intuition.
Bell test experiments demonstrate that entanglement correlations cannot be reproduced by local hidden-variable theories, though they still cannot be used for faster-than-light signaling.
A phenomenon where the joint quantum state of multiple particles cannot be described as independent states of each particle, even when separated by large distances.
A composite quantum state that cannot be written as a product of states of its local constituents.
An entangled two-particle state with total spin zero that yields perfect anti-correlations when measuring spins along the same axis.
The quantum-mechanical update of a system’s state upon measurement, which for entangled systems affects the description of the whole entangled pair.
A thought experiment by Einstein, Podolsky, and Rosen arguing that quantum mechanics may be incomplete because entanglement seems to conflict with local realism.
A bound on correlations predicted by any local hidden-variable theory, which quantum mechanics can violate for entangled states.
A class of theories that assume measurement outcomes are predetermined by hidden variables carried by particles and that no faster-than-light influence is involved.
The view that physical properties have definite values prior to measurement (realism) and that influences propagate no faster than light (locality).
The process by which entanglement is degraded when an entangled system interacts with its environment, preventing the system from maintaining its entangled state.
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