Quantum Computing and Cryptography Concepts

Core concepts

Superposition: A qubit exists in multiple states simultaneously until measured — the foundation of quantum advantage over classical bits.
Entanglement: Two qubits whose states are correlated regardless of distance. Measuring one instantly determines the other.
Decoherence: Loss of quantum state from environmental interference. The central hardware limitation — distinct from entanglement.
Quantum gates: Pauli-X flips qubit state. Hadamard creates superposition. CNOT combined with H produces entanglement. These are not classical logic gates.
Shor's algorithm: Factors large integers efficiently. Poses a direct threat to RSA encryption.
Grover's algorithm: Searches unstructured databases with quadratic speedup. Different purpose than Shor's — don't conflate them.
BB84: QKD using polarized photons. Security relies on the Heisenberg Uncertainty Principle — eavesdropping disturbs the state and is detectable.
E91: QKD based on entanglement, not superposition. Know which property underlies each protocol.
Quantum error correction: Multiple physical qubits protect one logical qubit. Core trade-off: qubit overhead vs reliability.
QKD security basis: Security derives from quantum physics principles, not computational hardness — the key distinction from classical cryptography.

Watch out for

Entanglement vs decoherence — correlated states vs loss of quantum state. These are opposites.
E91 is entanglement-based; BB84 is not. Know which property each protocol relies on.
Quantum circuits use quantum gates — not classical logic gates.
Shor's vs Grover's — different problems, different speedups. Don't swap their use cases.

→ This page was created with help from Claude AI.