Curriculum / Quantum Cryptography / BB84: Quantum Key Distribution Foundations
BB84: Quantum Key Distribution Foundations
Understand the principles that make BB84 secure against eavesdropping.
BB84: Quantum Key Distribution Foundations
BB84, named for its inventors Charles Bennett and Gilles Brassard and published in 1984, is the original quantum key distribution protocol and still the most widely deployed. It is the reason "quantum cryptography" exists as a field. BB84 solves the key-distribution problem that kills every classical perfect-secrecy scheme by using quantum mechanics itself as the security assumption. The security of the protocol does not rely on any computational hardness conjecture; it relies on facts about measurement, no-cloning, and mutually unbiased bases. That is what makes it information-theoretically secure even against a quantum adversary.
If you completed the Entanglement & Protocols track, you already met BB84 and even built it in the capstone. This track goes deeper: sifting and error rates were the warm-up, and the real subjects here are information reconciliation, privacy amplification, and why the security argument actually holds.
The Key Insight: Measurement Disturbs, Cloning Fails
Two facts from quantum mechanics drive BB84:
- 1.You cannot copy an unknown quantum state. The no-cloning theorem says no unitary operation can take |ψ⟩ ⊗ |0⟩ to |ψ⟩ ⊗ |ψ⟩ for arbitrary |ψ⟩. An eavesdropper cannot make a backup of a passing qubit and measure the copy at leisure.
- 2.Measurement in the wrong basis disturbs the state. Measuring |+⟩ in the computational basis yields 0 or 1 with 50/50 probability, but the state is projected onto |0⟩ or |1⟩, and the original |+⟩ is destroyed. If Eve then tries to re-send something to Bob, she cannot restore the original.
Put these together and you get the essential property: Eve cannot extract information from a quantum channel without leaving a detectable statistical fingerprint. Alice and Bob use that fingerprint to decide whether to trust the key.
The Protocol
Preparation (Alice):
- Z basis, 0 → |0⟩
- Z basis, 1 → |1⟩
- X basis, 0 → |+⟩ = (|0⟩+|1⟩)/√2
- X basis, 1 → |−⟩ = (|0⟩−|1⟩)/√2
- •For each qubit, Alice randomly picks a bit value (0 or 1) and a basis (+ or ×, i.e. Z or X).
- •She encodes the bit in the chosen basis:
- •She sends the qubit to Bob over a quantum channel.
This is the opening of the lesson. The full walkthrough, the interactive circuit, and the graded challenge continue inside myqubit.
How this lesson works
A guided reading lesson with interactive knowledge checks. Concepts are explained step by step with circuit diagrams and runnable examples, and you confirm understanding before moving on.
Part of: Quantum Cryptography
Master quantum key distribution, the threat quantum computers pose to classical cryptography, and post-quantum cryptographic standards.
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