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A quantum computer is a machine, as-yet hypothetical, that performs calculations based on the behavior of particles at the sub-atomic level. Such a computer will be, if it is ever developed, capable of executing far more millions of instructions per second (MIPS) than any previous computer. Such an exponential advance in processing capability would be due to the fact that the data units in a quantum computer, unlike those in a binary computer, can exist in more than one state at a time. In a sense, the machine "thinks" several "thoughts" simultaneously, each "thought" being independent of the others even though they all arise from the same set of particles.

Engineers have coined the term qubit (pronounced KYEW-bit) to denote the fundamental data unit in a quantum computer. A qubit is essentially a bit (binary digit) that can take on several, or many, values simultaneously. The theory behind this is as bizarre as the theory of quantum mechanics, in which individual particles appear to exist in multiple locations. One way to think of how a qubit can exist in multiple states is to imagine it as having two or more aspects or dimensions, each of which can be high (logic 1) or low (logic 0). Thus if a qubit has two aspects, it can have four simultaneous, independent states (00, 01, 10, and 11); if it has three aspects, there are eight possible states, binary 000 through 111, and so on.

Quantum computers might prove especially useful in the following applications:

The main difficulty that the research-and-development engineers have encountered is the fact that it is extremely difficult to get particles to behave in the proper way for a significant length of time. The slightest disturbance will cause the machine to cease working in quantum fashion and revert to "single-thought" mode like a conventional computer. Stray electromagnetic fields, physical movement, or a tiny electrical discharge can disrupt the process.

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What is quantum computer? - Definition from WhatIs.com