Entanglement

Entanglement is a quantum correlation in which two or more particles share a single joint state that cannot be factored into independent descriptions of each particle. Measuring one entangled Qubit instantly constrains the outcomes of measurements on its partners, however far apart they are, a feature that Bell inequality experiments have repeatedly confirmed.

#Role in quantum computing

Entanglement, together with Superposition, is what separates quantum computation from classical probabilistic computation. Algorithms build large entangled states so that operations on a few qubits affect the joint state of many, producing the interference patterns that Shor's algorithm exploits to factor integers. Entanglement is also the resource behind quantum teleportation and quantum key distribution.

#Fragility

Entangled states are delicate. Any uncontrolled interaction with the environment leaks information about the state and degrades the entanglement, a process called decoherence. Preserving entanglement across many qubits for many operations is a core requirement for a useful quantum computer.