Why Quantum Computing is a Danger to Modern Cryptographic Algorithms
The Physician Richard Feynman was the first to introduce the concept of a computer which should exploit quantum effects. In the year 1982 he developed a theoretical model of a quantum computer.
A Quantum Computer uses the concept of superposition to store and process information. A traditional 8-Bit-Register such as used in conventional computers for example is representing always a well known state, namely a number between 0 and 255. A quantum computer in contrast produces a superposition of all 256 states and can therefore do calculations with all 256 possible states of the register at the same time.
As you might already know, an ordinary bit can only have two states, 0 or 1, so with the birth of the Quantum computers the notion of the bit also changed.
A qbit (Quantum Bit) can be in be in both states at the same time and each operation on a qbit takes influence on all states of the bit. The consequence of this concept is, that a quantum computer is much faster than ordinary computers because it can do computations in parallel. It is possible to represent 2^N bits with just N qbits in favour of superposition.
At the end of the computation of an algorithm, a quantum computer has stored all possible solutions of the algorithm as a result of the fact that it has stored all possible states as a superposition. The qbits are influencing each other like ripples of water. Each state has a different amplitude of possibility and these amplitudes interfere with each other in a certain way so that the result is, that one state has the possibility of 1 which means that it contains the result of the algorithm and can be read out.
When Feynman published his concept of a quantum computer, technology was far away from building such a machine, but the concept itself survived and people started thinking of how to use the parallelism of such computers.
For further information we recommend:
Feynman, R. P. (1982). "Simulating Physics with Computers". International Journal of Theoretical Physics, 21, 467–488.