QuantumAlgorithms.aiA citable library

Glossary

Ansatz

A parameterized family of quantum states, chosen in advance, inside which a hybrid algorithm searches for a good solution.

An ansatz is a family of states specified by a finite list of parameters. A variational quantum algorithm searches that family. It does not search the whole Hilbert space, and it cannot return a state the family does not contain.

Peruzzo and coauthors use the word for the state preparation in VQE. Two families show up in the papers this library cites. Unitary coupled cluster is built from the structure of the electronic Hamiltonian. A hardware-efficient ansatz is built from the gates a given device can perform. QAOA is an ansatz of a third shape: p alternations of a cost unitary and a mixer, with 2p angles. In every case the approximation is only as good as the family. Saying "the ansatz was optimized" reports a search inside a choice someone already made.

References

  1. Peruzzo, A., McClean, J., Shadbolt, P., Yung, M.-H., Zhou, X.-Q., Love, P. J., Aspuru-Guzik, A., & O'Brien, J. L. (2014). A variational eigenvalue solver on a photonic quantum processor. Nature Communications, 5, 4213. DOI

Updated 7 Oct 2026.