# Perfect eavesdropping on a quantum cryptography system

Quantum cryptography involves encoding messages using a key that is rendered secret by a quantum-mechanical principle – that the act of measuring affects the system being measured. In one popular scheme, the sender “Alice” sends a key in the form of a series of polarized single photons to the receiver “Bob”. Alice polarizes each photon at random using either a horizontal–vertical polarizer or a polarizer with two diagonal axes. Bob detects each photon by also randomly selecting one of the two different polarizers.

If Bob happens to pick the same polarizer as Alice, then he will definitely measure the correct polarization of a given photon. Otherwise, as the uncertainty principle dictates, there is a 50% chance he will get it wrong. Once he has made all the measurements, Bob asks Alice over an open channel which polarizers she used for each photon and he only keeps the results for those measurements where he happened to pick the correct polarizer, and this series of results becomes the secret key.

While in principle unbreakable, quantum cryptography is known to have weaknesses in practice.

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