Two particles can be linked in a way that defies classical intuition. Measure one and you instantly know something about the other, no matter how far apart they are. That is quantum entanglement. Einstein called it spooky action at a distance, and he was uncomfortable with it. But experiments have confirmed it again and again. Entangled particles share a quantum state, and measuring one collapses the state of both.

Entanglement does not allow faster-than-light communication. You cannot send a message by measuring one particle. The result is random, and you need to compare notes with the other observer to see the correlation. That comparison happens at light speed or slower. Entanglement is a resource for quantum computing and quantum cryptography. In a quantum computer, entangled qubits can perform calculations that classical bits cannot. In quantum cryptography, entangled particles can detect eavesdropping. Entanglement is fragile. Any interaction with the environment breaks it, a process called decoherence. Keeping entangled particles coherent is one of the biggest challenges in quantum technology.

Entanglement facts

• Definition. Quantum link between particles.
• Non-local. Correlations persist across distance.
• No signaling. Cannot send messages faster than light.
• Applications. Quantum computing, cryptography.
• Decoherence. Environmental interaction breaks entanglement.

Entanglement is not the same as correlation. Classical objects can be correlated. Entanglement is a quantum phenomenon with no classical analogue.