THE long-running battle between coders and code-breakers could soon be over, as a breakthrough in quantum cryptography has brought uncrackable codes a step closer.
To exchange a coded message, the sender and recipient must somehow share a secret sequence of 0s and 1s that is used as a key to encode and decode the message. The problem is finding a way to exchange the key without it being intercepted. To achieve this, cryptographers have developed the technique known as quantum key distribution, which uses the quantum properties of photons to encode the key. But the technique has an Achilles鈥� heel.
To generate a quantum key one person, conventionally known as Alice, sends a sequence of photons to another, Bob. The photons carry the string of 0s and 1s encoded in their quantum properties, and the rules of quantum physics ensure that if an eavesdropper, Eve, tries to intercept the key she will inevitably disturb the photons and so give herself away.
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鈥淭his is one of the major recent advances in quantum cryptography. It should be relatively easy to implement鈥�
The weak point lies with the lasers that generate the photons. For the exchange to be secure they should send only a single photon at a time, but in practice lasers often send out pulses made up of several photons. This gives Eve a way in: if she removes one photon from each of these pulses and allows the rest of the pulse to reach Bob, she can crack the code. Alice and Bob will be none the wiser even though the single photon pulses never get through.
Eve is able to keep her activities hidden because all communications links, including the direct link between Alice and Bob, are imperfect and lose a certain proportion of the photon pulses passing along them. But if Eve ensures that the link that runs from Alice to her and on to Bob loses fewer pulses than the direct link, she can use the greater efficiency to compensate for the loss of the pulses she intercepts. If she gets it right, Bob will detect the number of pulses he expects even though many others will have been removed along the way.
Now Hoi-Kwong Lo at the University of Toronto, working with colleagues Xiongfeng Ma and Kai Chen, has devised a way to counter this kind of attack. The idea is for Alice to send out decoy multi-photon pulses. Eve cannot tell the decoys apart from the other pulses, so she reads the pulses and sends them on to Bob.
This is her undoing. Alice and Bob can detect Eve by looking at the number of decoy pulses that make it through the channel. If this is unexpectedly high, they know an eavesdropper has been at work ().
Gr茅goire Ribordy, a physicist at id Quantique, a Swiss company that offers a commercial quantum key distribution system, says the idea should be relatively easy to implement with the encryption equipment now being used. 鈥淭his is one of the major recent advances in quantum cryptography,鈥� he says.