Key Takeaways: Chapter 10 — Superdense Coding, Quantum Key Distribution, and the Communication Applications of Entanglement
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Superdense coding doubles classical capacity. With pre-shared entanglement, 1 qubit can carry 2 classical bits. This saturates the entanglement-assisted classical capacity.
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QKD provides information-theoretic security. Unlike classical cryptography, QKD's security rests on the laws of quantum mechanics, not computational assumptions. An adversary with unlimited computing power cannot break it.
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BB84 is the workhorse of QKD. Its simplicity (single photons, two bases) has enabled commercial deployment. The ~11% QBER threshold provides a clear go/no-go criterion.
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E91 uses Bell's inequality for security. Entanglement-based QKD can certify security through CHSH violation — a fundamentally different approach from prepare-and-measure protocols.
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Implementation matters. Quantum hacking exploits real-world imperfections: multi-photon pulses, detector loopholes, and side channels. Protocol security does not guarantee system security.
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The quantum internet is the long game. QKD, teleportation, and superdense coding are the primitive operations. Building a scalable, fault-tolerant quantum internet will take decades.
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Noise is the enemy. Every protocol in this chapter degrades under noise. Error correction, entanglement purification, and privacy amplification are the tools we use to fight back — but they all consume resources.
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We're at the beginning. Current QKD systems are limited to ~100 km over fiber and ~1000 km via satellite. Quantum repeaters, when they become available, will extend this range dramatically.