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What is quantum entanglement?

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Quick answer

Quantum entanglement is a link between particles so that measuring one instantly fixes the matching property of the other, no matter how far apart they are. The particles share one quantum state — what Einstein famously called 'spooky action at a distance.

Key things to understand

  1. Two entangled particles act as a single system: measure one and you instantly know the other's correlated value.
  2. It cannot send usable information faster than light — you can't control which result you get.
  3. Confirmed by experiments testing Bell's inequalities, ruling out hidden pre-set values.
  4. It is the backbone of quantum computing, quantum cryptography, and quantum teleportation.

People also ask

Does entanglement allow faster-than-light communication?
No. The outcomes are random, so you can't encode a message in them; comparing results still needs an ordinary, light-speed-limited channel.
Why did Einstein dislike it?
He called it 'spooky action at a distance' and argued quantum theory was incomplete — but experiments have repeatedly confirmed entanglement is real.
How is entanglement used?
It powers quantum computers' speed, ultra-secure quantum key distribution, and lab demonstrations of quantum teleportation.