How do entangled states contribute to the security of quantum key distribution (QKD) protocols like BB84 and E91?

Entangled states are crucial for the security of QKD protocols by enabling the detection of eavesdropping. In the BB84 protocol, the entanglement-like correlations ensure that any attempt to measure the quantum states by an eavesdropper will introduce detectable anomalies. In the E91 protocol, entangled particles shared between two parties ensure that any eavesdropping attempt will disrupt the entanglement, revealing the presence of an interceptor and thus preserving the security of the key distribution.