Entanglement distillation is a quantum information process in which a shared quantum state, typically a set of entangled pairs, is transformed into a smaller number of higher-quality entangled pairs. The initial state may contain mixed or noisy entanglement, which may not be sufficient for certain quantum information protocols, such as quantum cryptography or quantum computation.
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Entanglement distillation uses local operations and classical communication on many noisy entangled states to produce fewer high-fidelity Bell pairs. It is also called entanglement purification. The protocol may fail or abort; naming a purification step does not guarantee successful output for arbitrary inputs. If the output is an exact pure state of Bell pairs, every purification of a density operator factors between those pairs and the environment, so measuring each pair in the computational basis gives a shared uniform bit independent of an eavesdropper. Approximate privacy follows when the entire retained output is sufficiently close to that pure target. A separable quantum state cannot be distilled into a Bell pair by LOCC, because each local branch preserves separability.