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Ancilla bits are some extra bits being used to achieve some specific goals in computation. In classical computation, any memory bit can be turned on or off at will, requiring no prior knowledge or extra gadgetry. However, this is not the case in quantum computing or classical reversible computing. In these models of computing, all operations on computer memory must be reversible, and toggling a bit on or off would lose the information about the initial value of that bit. For this reason, in a quantum algorithm there is no way to deterministically put bits in a specific prescribed state unless one is given access to bits whose original state is known in advance. Such bits, whose values are known a priori, are known as ancilla bits in a quantum or reversible computing task.
A trivial use for ancilla bits is downgrading complicated quantum gates into simple gates. For example, by placing controls on ancilla bits, a Toffoli gate can be used as a controlled NOT gate or a NOT gate.
For classical reversible computation it is known that a constant number O of ancilla bits is necessary and sufficient for universal computation. Additional ancilla bits are not necessary, but the extra workspace can allow for simpler circuit constructions that use fewer gates.
Extra bits required in reversible and quantum computation, as bits cannot be modified arbitrarily