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No-cloning theorem

As explained in Real-World Cryptography

The no-cloning theorem states that an unknown quantum state cannot be copied perfectly by a physical operation.

An eavesdropper cannot passively observe a quantum exchange and create a perfect copy of an unknown quantum state. This property supports quantum key distribution (QKD), but it does not authenticate the exchange: a man-in-the-middle attack can still undermine it. section 14.1.1

Entangled particles in transit

Two peers receive entangled particles and measure them to form a shared key. An observer cannot secretly make an exact copy of one particle, but QKD still needs an authenticated channel to resist interception and impersonation. section 14.1.1

How Real-World Cryptography covers it

How deep it goes
Defines perfect-copying limits through a QKD scenario and stops short of detailed quantum-mechanics treatment or formal proof.
Examples
Uses entangled particles distributed to two peers as the concrete example.
What it assumes
Assumes basic awareness of particles and key exchange while introducing entanglement and QKD contextually.
What you can do after
Evaluate why QKD can expose passive copying and why authentication is still required against active interception.
How it connects
Connects quantum entanglement, quantum key distribution, man-in-the-middle attacks, and data authentication.
Cover of Real-World Cryptography
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Related concepts

This explainer was generated by Manning's knowledge graph from the text of Real-World Cryptography. The section links point to the parts of the book it draws on.