# Why does quantum cryptography give us uncrackable codes?

Why does quantum cryptography give us uncrackable codes? What makes it 'uncrackable'? Articles in for example pop science magazines always claim QC produces uncrackable coded, however I highly doubt these claims.

p.s. - Just type in 'quantum cryptography uncrackable codes' and you'll find a ton of hits.

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Why do you find the claims questionable? –  leongz May 15 '13 at 21:40
@leongz Uncrackable codes. Uncrackable. That's sounds hard to believe right? Especially if you don't know why it is uncrackable, that's why I am asking why it is. –  Qubit May 15 '13 at 21:50
It is worth noting that one time pads are classical uncrackable cyphers (with the usual caveats about how inconvenient they can be and how easily a mistake in procedure or application can invalidate the guarantee). –  dmckee May 15 '13 at 22:24
You do know there's a whole cryptography SE site, right? crypto.stackexchange.com/questions/3699/… –  Deer Hunter May 16 '13 at 4:55
Daniel J. Bernstein, Johannes Buchmann, Erik Dahmen. Post-Quantum Cryptography. Springer, 2008. ISBN 978354088701 - and other workshop proceedings with the same name. –  Deer Hunter May 16 '13 at 4:58

Because the very act of viewing the code changes it. Maybe not uncrackable but any sniffing will be evident because it will be changed. From RSA Laboratories:

Quantum cryptography has a special defense against eavesdropping: If an enemy measures the photons during transmission, he will use the wrong basis half the time, and thus will change some of the polarizations.

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@Siva: Good point. You could also clone the state $n$ times and measure an average, but that still only measures the average with good precision, not the "actual" value, which, as you point out, is not defined. –  Ben Crowell May 16 '13 at 14:51