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A quantum bit (qubit) is the smallest unit of quantum information, which is the quantum analog of the regular computer bit, used in the field of quantum computing. A quantum bit can exist in superposition, which means that it can exist in multiple states at once. Compared to a regular bit, which can exist in one of two states, 1 or 0, the quantum bit can exist as a 1, 0 or 1 and 0 at the same time. This allows for very fast computing and the ability to do multitudes of calculations at once, theoretically.
The ability of a qubit to exist in a superposition state means that a quantum computer is not limited to two states and is therefore able to hold more information, giving quantum computers the potential to be millions of times more powerful than today's supercomputers. A qubit may represent anything very small, of quantum level, such as atoms, photons and electrons, which, when made to work together, can act like processors and memory.
The inherent parallelism of a quantum computer is due to the superposition of qubits, and according to physicist David Deutsch, this parallelism will allow a quantum computer to process millions of calculations during the time it takes a standard desktop PC to do a single calculation. Therefore, a 30-qubit computer could theoretically equal the power of a modern supercomputer that runs at 10 teraflops, whereas a modern desktop PC runs at only a few gigaflops.