Quantum glossary

Uncomputation

Running part of a circuit in reverse to return scratch qubits to |0⟩ and disentangle them from the result.

Leftover garbage entangled with your answer acts like a measurement of it and kills interference. Reversibility is what makes clean-up possible.

See it for yourself

ChallengeBorrow a qubit and give it back

Ancilla qubit — An extra qubit used as scratch space during a computation, usually returned to |0⟩ afterwards.
Unitary — A reversible, probability-preserving transformation; every quantum gate is a unitary matrix.