量子鍵配分

A key that cannot be photocopied quietly

• A shop can send an encrypted file over an ordinary network, but first it must share a secret key. Making the key longer does not explain how the two sides obtain it safely.
• Quantum cryptography 量子密码学 includes quantum key distribution 量子密钥分发 (QKD): sharing key material using quantum states. The key exchange and the encrypted file are two separate jobs.

Photons carry the key material

• A sender encodes random bits in quantum states of photons 光子. The receiver measures the incoming photons. In a simple protocol, the two sides use different measurement choices and later keep results from compatible choices.
• Measuring an unknown quantum state can disturb it; an attacker cannot make a perfect copy of an arbitrary unknown state. Interception can therefore leave a statistical error signal in the retained results.

A sample test, not a detector on every photon

• The two sides compare a sample of retained bits over an authenticated channel: they must know who is speaking. They do not publish the whole secret key; revealed sample bits are discarded.
• Too many errors mean aborting the exchange and discarding the proposed key. Errors may come from interception or equipment noise, so an error does not identify an attacker. Accepted results still need error correction and privacy processing.

Worked example: reject a noisy exchange

The benefit follows from the mechanism

• In an ideal protocol, the key's secrecy is supported by quantum physics rather than only the difficulty of a mathematical problem. Attempts to measure the transmitted states can affect the test results.
• This helps with key distribution. It does not protect a stolen endpoint, authenticate the other party on its own, or make every practical implementation immune to attack.

Why ordinary networks still need ordinary encryption

• Practical QKD needs special equipment and a suitable quantum link, such as optical fibre or a free-space path. Loss, distance and key-generation rate constrain deployment; ordinary routers cannot simply copy and forward arbitrary quantum states.
• The agreed key is then used by an encryption scheme to protect the actual data on an ordinary channel. QKD supplies key material, not a new way of emailing a whole document as photons.

Marks that slip away

Choose the right job for each tool

Further reading: QKD in an authenticated key-exchange framework and practical deployment limitations. Sources checked 2 October 2026; this lesson is original classroom material.