Reversibility
Although quantum gates work on qubits in a much different fashion from standard electronic circuits, they only differ in their basic effects in one sense: reversibility. Both types of gates take a bit, alter it, and give an output bit state. Quantum gates, however, have the additional property of
reversibility. The reason that quantum gates are reversible is that their mechanism of action on qubits is through Schrdinger evolution (which is reversible by virtue of being unitary).
This property of reversibility caused quantum gates to not be
aggressively developed until the late 1980Õs. The problem is that, according to
LandauerÕs principle (established in 1969), when you attempt to complete a reversible process, there is inevitably dissipation and some compression of phase space, requiring kTln2 units of work and creating heat [4]. However, Bennett discovered in 1973 that by using what are called Traffoli gates that store extra bits of information, we can create reversible gates without this dissipation. This revelation,
as well as the realization that non-reversable classical computations generate
heat that quantum computations do not, resulted in increased motivation for developing quantum gates: Quantum computers will be useful for longer than classical machines, which will eventually reach an upper limit as to how much heat the components can take [1].
Some classical gates, such as NOT, translate directly into quantum gates: a NOT gate is its own inverse. Others, such as the AND gate,
have been incorporated as parts of more advanced gates (most commonly the Toffili gate). Any non-reversable gate can be made reversible by recording the inputs in the output (as a Toffili gate essentially does in the case of imitating an AND or NAND gate).
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Created by Brian Patterson
Last Modified 11/22/00