Ancilla bit

Ancilla bits are extra bits (units of information) used in computing paradigms that require reversible operations, such as classical reversible computing and quantum computing. Unlike classical computing where bits can be freely set to 0 or 1, reversible computation requires all operations on computer memory to be invertible. Ancilla bits, whose initial state is known, provide the necessary "workspace" for performing operations that would otherwise erase information. They play a crucial role in implementing complex logic gates and enabling universal computation within these reversible models.

Ancilla bits can simplify complex operations. For example, an ancilla bit can be used to control a Toffoli gate, effectively turning it into a simpler gate like a controlled NOT or a NOT gate.^[1]^: 29

^ Nielsen, Michael A.; Chuang, Isaac L. (2010). Quantum Computation and Quantum Information (2nd ed.). Cambridge: Cambridge University Press. ISBN 978-1-107-00217-3.

[NC-1] Nielsen, Michael A.; Chuang, Isaac L. (2010). Quantum Computation and Quantum Information (2nd ed.). Cambridge: Cambridge University Press. ISBN 978-1-107-00217-3.

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