A quantum ancilla is an auxiliary quantum system used to assist preparation, evolution or measurement of another system. It can carry temporary information, mediate an interaction or record an outcome. Its initial state and whether it is retained, measured or discarded are part of the protocol.
For example, two controlled-NOT operations from data qubits into an ancilla initially in copy their computational-basis parity into the ancilla. Measuring it reveals the parity without distinguishing the two basis states within either parity sector. This is useful in quantum error correction, where a syndrome must be learned without reading the logical amplitudes. An ancilla entangled with a channel input also tests complete positivity, which cannot be checked using isolated input states alone.
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