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.

Quantum cryptology is the first cryptology that safeguards against passive interception. Since we can't measure a photon without affecting its behavior, Heisenberg's Uncertainty Principle emerges when Eve makes her own eavesdrop measurements.

Here's an example. If Alice sends Bob a series of polarized photons, and Eve has set up a filter of her own to intercept the photons, Eve is in the same boat as Bob: Neither has any idea what the polarizations of the photons Alice sent are. Like Bob, Eve can only guess which filter orientation (for example an X filter or a + filter) she should use to measure the photons.

After Eve has measured the photons by randomly selecting filters to determine their spin, she will pass them down the line to Bob using her own LED with a filter set to the alignment she chose to measure the original photon. She does to cover up her presence and the fact that she intercepted the photon message. But due to the Heisenberg Uncertainty Principle, Eve's presence will be detected. By measuring the photons, Eve inevitably altered some of them.

Say Alice sent to Bob one photon polarized to a ( -- ) spin, and Eve intercepts the photon. But Eve has incorrectly chosen to use an X filter to measure the photon. If Bob randomly (and correctly) chooses to use a + filter to measure the original photon, he will find it's polarized in either a ( / ) or ( ) position. Bob will believe he chose incorrectly until he has his conversation with Alice about the filter choice.

After all of the photons are received by Bob, and he and Alice have their conversation about the filters used to determine the polarizations, discrepancies will emerge if Eve has intercepted the message. In the example of the ( -- ) photon that Alice sent, Bob will tell her that he used a + filter. Alice will tell him this is correct, but Bob will know that the photon he received didn't measure as ( -- ) or ( | ). Due to this discrepancy, Bob and Alice will know that their photon has been measured by a third party, who inadvertently altered it.

Alice and Bob can further protect their transmission by discussing some of the exact correct results after they've discarded the incorrect measurements. This is called a **parity check**. If the chosen examples of Bob's measurements are all correct -- meaning the pairs of Alice's transmitted photons and Bob's received photons all match up -- then their message is secure.

Bob and Alice can then discard these discussed measurements and use the remaining secret measurements as their key. If discrepancies are found, they should occur in 50 percent of the parity checks. Since Eve will have altered about 25 percent of the photons through her measurements, Bob and Alice can reduce the likelihood that Eve has the remaining correct information down to a one-in-a-million chance by conducting 20 parity checks [source: Vittorio].

In the next section, we'll look at some of the problems of quantum cryptology.

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