Quantum Key Distribution (BB84)
Also known as: BB84, quantum cryptography
Quantum Key Distribution (QKD) BB84 is a cryptographic protocol allowing two parties to establish a shared secret key using quantum mechanics. Proposed by Bennett and Brassard in 1984, BB84 provides information-theoretic security: an eavesdropper's presence is guaranteed to be detected, and the secret key is provably secure against unlimited computational power.
Key highlights
- Information-theoretic security: provably secure against unlimited computation.
- Eavesdropping is detectable with high probability; no silent eavesdropping possible.
- Simple to implement; does not require quantum memory or entanglement.
- Proven secure under realistic assumptions (bounded eavesdropper resources).
- Foundation for practical quantum networks and quantum internets.
Intuition
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How it works
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When to use it
BB84 is used when unconditional security against eavesdropping is required, with no computational assumptions. It is practical for point-to-point quantum communication networks and is being deployed in real-world quantum internet backbones.
Strengths & limitations
- Information-theoretic security: provably secure against unlimited computation.
- Eavesdropping is detectable with high probability; no silent eavesdropping possible.
- Simple to implement; does not require quantum memory or entanglement.
- Proven secure under realistic assumptions (bounded eavesdropper resources).
- Foundation for practical quantum networks and quantum internets.
- Quantum channel required; suitable only for long-distance fiber optics or free space.
- Sifting efficiency: only ~50% of transmitted qubits become usable key bits.
- Vulnerable to photon loss and detector inefficiencies in real implementations.
- Requires authenticated classical channel for basis comparison; subject to man-in-the-middle attack without authentication.
- Practical implementations suffer from losses, limiting key distribution rates.
Common pitfalls
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Applications
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Frequently asked
How does BB84 ensure that an eavesdropper is detected?
If an eavesdropper Eve measures the qubits in the wrong basis (probability 50%), her measurement collapses the state. Bob's subsequent measurement (in a different wrong basis) will then have a 50% chance of disagreeing with Alice's original bit. These errors cluster in a detectable pattern.
What is sifting and why is it necessary?
Sifting removes bits where Alice and Bob used different bases. Alice and Bob publicly compare bases (not bit values) and keep only matching bases. This reduces the key length but ensures both parties measured in the same basis.
Is BB84 vulnerable to quantum computers?
No. BB84 is secure against quantum computers because it does not rely on computational hardness; it relies on quantum mechanics. A quantum computer cannot break BB84, and a quantum computer cannot eavesdrop silently.
Can BB84 transmit long distances?
Pure BB84 has distance limitations due to photon loss. Quantum repeaters and entanglement purification can extend range, but practical systems today are limited to ~100 km. Satellite QKD can reach longer distances.
What attacks are BB84 vulnerable to?
Side-channel attacks (detector vulnerabilities, timing), environmental noise, and practical implementation flaws. Theoretically, BB84 is secure; in practice, device imperfections and attacks like the photon-number splitting attack require care.
Sources
- 1.Bennett, C. H., Brassard, G. (1984). Quantum cryptography: public key distribution and coin tossing. Proceedings of IEEE International Conference on Computers, Systems, and Signal Processing, 175–179.
- 2.Shor, P. W., Preskill, J. (2000). Simple proof of security of the BB84 quantum key distribution protocol. Physical Review Letters, 85, 441–444.
- 3.Renner, R. (2008). Security of quantum key distribution. arXiv preprint arXiv:0512258.
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Cite this page
ScholarGate. (2026, June 3). Quantum Key Distribution (BB84). ScholarGate. https://scholargate.app/quantum-computing/quantum-key-distribution