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Glossary

Quantum glossary

Unitary

Also: unitary operator, unitary matrix

A reversible, probability-preserving transformation; every quantum gate is a unitary matrix.

Unitarity means UU† = I, so every gate can be undone. That is why quantum circuits are reversible and why measurement, which is not, is the odd one out.

U†U = UU† = I

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Related terms

  • Hamiltonian — The operator for a system’s total energy; it determines how the state evolves in time.
  • Uncomputation — Running part of a circuit in reverse to return scratch qubits to |0⟩ and disentangle them from the result.