Ancilla Qubit

An ancilla qubit (from Latin "ancilla" — servant) is an auxiliary qubit used to assist in a quantum computation without being part of the primary data register. Ancilla qubits have several important uses: (1) Syndrome measurement in quantum error correction — ancilla qubits are entangled with data qubits to detect errors without directly measuring the data; (2) Phase kickback — ancilla qubits in superposition "kick back" phase information from controlled operations (used in Shor's and Grover's algorithms); (3) Temporary computation — storing intermediate results that are later uncomputed (erased) to avoid polluting the computation with garbage qubits. After their purpose is served, ancilla qubits are typically reset to |0⟩ for reuse. The number of ancilla qubits required is a key resource measure for quantum algorithms.