While both protect data against future quantum computers, they approach the problem from completely opposite directions.
Quantum Key Distribution (QKD) is a secure communication method that uses principles of quantum mechanics to establish a shared, secret cryptographic key between two parties (traditionally called Alice and Bob). Key Concepts & Quantum Mechanics:
No-Cloning Theorem: It is impossible to create an identical copy of an unknown quantum state. An eavesdropper (Eve) cannot intercept a photon, copy it, and send the original along unnoticed.
Measurement Collapse: Measuring a quantum system inevitably alters its state. If Eve intercepts and measures the key-generating particles, she introduces detectable errors into the signal.
Photon Polarization: Most QKD systems encode bit values ($0$ and $1$) into quantum properties of individual photons, such as vertical/horizontal or diagonal polarization angles.
Unlike classical cryptography, which relies on computational complexity (e.g., the difficulty of factoring large numbers), QKD guarantees security through physical laws.
PQC does not use quantum hardware or physics at all. Instead, it refers to classical software algorithms redesigned to resist attack by future quantum computers running Shor's algorithm (which breaks RSA and ECC).
