Quantum cryptography may be essentially solved, but getting the funky physics to work on disciplined computer networks is a whole new headache. Cryptography is an arms race, but the finish line may be fast approaching. Up to now, each time the codemakers made a better mousetrap, codebreakers breed a better mouse. But quantum cryptography theoretically could outpace the codebreakers and win the race. Forever.
Already the current state of the art in classical encryption, 128-bit RSA, can be cracked with enough raw, brute force computing power available to organisations like the US National Security Agency. And the advent of quantum computing will make it even simpler. The gold standard for secret communication will be truly dead.
Quantum cryptography solves the problem, and it will overcome the remaining stumbling block, the distribution of the code key to the right person, by using quantum key distribution (QKD).
Modern cryptography relies on the use of digital 'keys' to encrypt data before sending it over a network, and to decrypt it at the other end. The receiver must have a version of the key code used by the sender so as to be able to decrypt and access the data.
QKD offers a theoretically uncrackable code, one that is easily distributed and works in a transparent manner. Even better, the nature of quantum mechanics means that if any eavesdropper - called Eve in the argot of cryptographers - tries to snoop on a message the sender and receiver will both know.
That ability is due to the use of the Heisenberg Uncertainty Principle, which sits at the heart of quantum mechanics. The principle rests on the theory that the act of measuring a quantum state changes that state. It is like children with a guilty secret. As soon as you look at them their faces morph plausibly into 'Who, me?'
The practical upshot for cryptography is that the sender and receiver can verify the security of the transmission. They will know if the state of the quanta has changed, whether the key has been read en route. If so, they can abandon the key they are using and generate a new one.
QKD made its real-world debut in the canton of Geneva for use in the electronic voting system used in the Swiss general election last year. The system guaranteed that the poll was secure. But, more importantly perhaps, it also ensured that no vote was lost in transmission, because the uncertainly principle established there was no change to the transmitted data.