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Entanglement
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Correlations between particles that cannot be explained classically.
Two or more particles share a joint state that can't be written as separate states; measuring one instantly fixes correlations in the other, however far apart.
Two photons with correlated polarisations.
Can't be used to send signals faster than light.
In more detail
Entanglement is a quantum correlation between particles, so that measuring one affects the results for the other, even at a distance. It cannot be explained by pre-set local values.
Real-world uses
- Quantum cryptography uses entanglement for secure key distribution.
- Quantum computers use entanglement to perform parallel computations.
- Quantum communication and cryptography use entangled photons, whose measurements are correlated even when far apart.
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Frequently asked questions
Does entanglement allow faster-than-light signals?
No. Measurement results appear correlated only when the outcomes are compared classically.
How is it created?
Through interactions, such as by splitting photons from a single source.
Can entanglement send messages faster than light?
No. The correlations are real, but they cannot be used to send information faster than light.
Related topics in Quantum Mechanics
Superposition · Measurement and collapse · Schrödinger's cat · Bell's inequalities · EPR paradox · Decoherence