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Quantum information differs from classical information in several respects, among which we note the following:
However, despite this, the amount of information that can be both stored and retrieved in a single qubit is equal to one bit. It is in the ''processing'' of information (quantum computation) that a difference occurs. The ability to manipulate quantum information enables us to perform tasks that would be unachievable in a classical context, such as unconditionally secure transmission of information. Quantum Information Processing is the most general field that is concerned with quantum information. There are certain tasks which classical Computer s cannot perform "efficiently" (that is, in Polynomial Time ), at least not with any known Algorithm . However, a quantum computer can compute the answer to some of these problems in polynomial time; one well-known example of this is Shor's Factoring Algorithm . Other algorithms can speed up a task less dramatically - for example, Grover's Search Algorithm which gives a polynomial speed-up over the best possible classical algorithm. Quantum information, and changes in quantum information, can be quantitatively measured by using an analogue of Shannon Entropy . Given a Statistical Ensemble of quantum mechanical systems with the Density Matrix ''S'', it is given by : Many of the same entropy measures in classical information theory can also be generalized to the quantum case, such as the Conditional Quantum Entropy . SEE ALSO
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