Quantum computing has long confused and challenged the best of us. You can read only so many technical details before your head explodes. Yet it's likely that usable quantum cryptography will be a fact of life within the decade -- and that has huge ramifications.
Most of today's popular cipher algorithms (especially public/private key exchanges) work because the math involved is very difficult for conventional (nonquantum) computers to solve. Take some really big prime numbers, add, subtract, multiply, and divide them a bit (like you do with the RSA algorithm), and you quickly get a mathematical problem that is very hard to solve even with hundreds of billions of guesses.
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But conventional cryptography would be rendered useless if someone either came up with a very, very speedy computer or learned a method to do the math exponentially faster than the methods we have today for solving crypto problems.
Quantum computers are that solution -- or problem, depending on how you see it.
Quantum is not proven science yet
Quantum physics (or mechanics) is a nearly proven field of physics that explains many natural phenomena that cannot be explained by traditional physics, which is often controlled by gravity. In quantum physics, a very tiny particle can be in two places at once, be a wave and a particle at the same time, and be the backbone behind time travel, string theory, and other seemingly far-out notions.
At the same time, only the existence of quantum mechanics can explain how transistors, MRIs, and electron microscopes work. If you look at the underside of a CD-ROM and see the rainbow columns emanating from it, only quantum physics can explain it. It even befuddled Albert Einstein. Even though quantum physics has not been 100 percent proven, every experiment ever created to support it has succeeded, and every experiment to disprove it has failed.