"Quantum information is the physics of knowledge. To be more
specific, the field of quantum information studies the implications
that quantum mechanics has on the fundamental nature of
information. By studying this relationship between quantum theory
and information, it is possible to design a new type of
computer—a quantum computer. A largescale, working quantum
computer—the kind of quantum computer some scientists think
we might see in 50 years—would be capable of performing some
tasks impossibly quickly.

"To date, the two most promising uses for such a device are
quantum search and quantum factoring. To understand the power of a
quantum search, consider classically searching a phonebook for the
name which matches a particular phone number. If the phonebook has
10,000 entries, on average you'll need to look through about half
of them—5,000 entries—before you get lucky. A quantum
search algorithm only needs to guess 100 times. With 5,000 guesses
a quantum computer could search through a phonebook with 25 million
names.

"Although quantum search is impressive, quantum factoring
algorithms pose a legitimate, considerable threat to security. This
is because the most common form of Internet security, public key
cryptography, relies on certain math problems (like factoring
numbers that are hundreds of digits long) being effectively
impossible to solve. Quantum algorithms can perform this task
exponentially faster than the best known classical strategies,
rendering some forms of modern cryptography powerless to stop a
quantum codebreaker."