Quantum Information Group, Secrecy of Quantum Keys
Quantum cryptography provides a test of the secrecy of each individual key. No other known method can provide such a test. If a third party gains any information about the key via hacking of the communication channel, or indeed in any other way, the laws of Quantum Physics dictate that errors will appear in the secret key. Alice and Bob compare a small portion of their key, and if they find a certain percentage of their bits disagree, they know their key may not be secret. Quantum cryptography cannot prevent hacking (no method can), but it can detect an eavesdropper on the communication channel. If a hacker is detected, the communication can be switched to a different route or simply delayed until a later time.
The diagram shows the effect of an attempt to read the key upon the encrypted picture. Errors in the key introduced by the eavesdropper mean that Bob can no longer recover the original picture.
Imperfections in real systems can also cause errors in the shared key. Close inspection of the decrypted photo in the unhacked demonstration on the previous page, reveals some 'freckles' due to these errors in the shared key. Usually we remove all the discordant bits from the key using an error correction routine. (Here we have not removed the errors purely for illustrative purposes.)
The errors in the key caused by imperfections in the system are indistinguishable from those caused by an eavesdropper. In order to be able to detect Eve, it is essential to engineer a low intrinsic error rate. The Toshiba system has the lowest quantum bit error rate ever recorded. In practice, this means the Toshiba system can work over the longest distances and with the highest bit rates
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