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by Gilles Brassard Département IRO, Université de Montréal.
The idea that a vote cast by a person remains the same after he submitted it is taken very seriously in any democracy. Voting is the right of the citizen, and it's how we choose the people who make important decisions on our behalf. When the security of the ballot is compromised, so, too, is the individual's right to choose his leaders.
Privacy is paramount when communicating sensitive information, and humans have invented some unusual ways to encode their conversations. In World War II , for example, the Nazis created a bulky machine called the Enigma that resembles a typewriter on steroids. This machine created one of the most difficult ciphers (encoded messages) of the pre-computer age. Even after Polish resistance fighters made knockoffs of the machines -- complete with instructions on how the Enigma worked -- decoding messages was still a constant struggle for the Allies [source: Cambridge University ]. As the codes were deciphered, however, the secrets yielded by the Enigma machine were so helpful that many historians have credited the code breaking as a important factor in the Allies' victory in the war.
Both the secret-key and public-key methods of cryptology have unique flaws. Oddly enough, quantum physics can be used to either solve or expand these flaws. The problem with public-key cryptology is that it's based on the staggering size of the numbers created by the combination of the key and the algorithm used to encode the message. These numbers can reach unbelievable proportions. What's more, they can be made so that in order to understand each bit of output data, you have to also understand every other bit as well.
Photons are some pretty amazing particles. They have no mass, they're the smallest measure of light , and they can exist in all of their possible states at once, called the wave function . This means that whatever direction a photon can spin in -- say, diagonally, vertically and horizontally -- it does all at once. Light in this state is called unpolarized .
Quantum cryptography uses photons to transmit a key. Once the key is transmitted, coding and encoding using the normal secret-key method can take place. But how does a photon become a key?
The goal of quantum cryptology is to thwart attempts by a third party to eavesdrop on the encrypted message. In cryptology, an eavesdropper is referred to as Eve . In modern cryptology, Eve (E) can passively intercept Alice and Bob's encrypted message -- she can get her hands on the encrypted message and work to decode it without Bob and Alice knowing she has their message. Eve can accomplish this in different ways, such as wiretapping Bob or Alice's phone or reading their secure e-mails .
Despite all of the security it offers, quantum cryptology also has a few fundamental flaws. Chief among these flaws is the length under which the system will work: It’s too short. The original quantum cryptography system, built in 1989 by Charles Bennett, Gilles Brassard and John Smolin, sent a key over a distance of 36 centimeters [source: Scientific American ]. Since then, newer models have reached a distance of 150 kilometers (about 93 miles). But this is still far short of the distance requirements needed to transmit information with modern computer and telecommunication systems.