Example Concept: In Quantum Key Distribution (QKD), entangled qubits are distributed between two parties. When one party measures their qubit, the state of the other qubit is instantaneously determined, regardless of the distance. This correlation allows the parties to detect any eavesdropping because any interception attempt will alter the entangled state, revealing the presence of an intruder. Protocols like BB84 and E91 utilize this principle to ensure secure communication.