A quantum phenomenon where two or more qubits become correlated such that the state of one instantly determines the state of the others, regardless of distance. Einstein famously called it 'spooky action at a distance'.
Entangled qubits cannot be described independently, and measuring one immediately collapses the others' states in correlated ways. Entanglement is created via two-qubit gates (CNOT, CZ) and is the resource that enables quantum algorithms to achieve speedups over classical alternatives. Without entanglement, n qubits would behave like n independent two-state systems with no quantum advantage. Quantum teleportation, dense coding, and quantum key distribution all rely on entanglement.
Creating a Bell-state pair of entangled qubits, where measuring qubit A as 0 instantly determines qubit B is also 0, with 100% correlation across any distance.
Entanglement is the second pillar (after superposition) of quantum computational advantage, and its proper use distinguishes useful quantum algorithms from naive ones.
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