Quantum Cryptography: QKD, Its Limits, and What Ships Instead

Quantum cryptography and post-quantum cryptography share a name and almost nothing else: one uses physics and needs hardware, the other is classical maths and ships as a software update. This cursus builds QKD properly, from no-cloning through BB84 to the error threshold and Ekert's Bell-test version. Then the gap between an unconditional proof and a product: the authentication it cannot bootstrap, trusted nodes, attacks on real detectors, and why NSA and NCSC advise against it. It ends on what is actually being deployed.