Ancilla bit

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.

[2] While additional ancilla bits aren't strictly required, they can provide extra working space, leading to simpler circuit constructions using fewer logic gates.

[3] One notable example for the use of ancilla qubits in quantum computing is the Deutsch–Jozsa algorithm.

Quantum catalysis uses ancilla qubits to store entangled states that enable tasks that would not normally be possible with local operations and classical communication (LOCC).

Using three ancilla bits and four Toffoli gates to construct a NOT gate with 5 controls. The ancilla bits end up trashed because the effects on them were not uncomputed .