Skip to main content
Glossary

Quantum glossary

Variational circuit

Also: parameterized circuit, ansatz, pqc

A circuit with tunable rotation angles that a classical optimiser adjusts to minimise a cost.

The chosen circuit shape is called the ansatz. Too simple and it cannot express the answer; too deep and noise and barren plateaus take over. Choosing it well is most of the art.

See it for yourself

Related terms

  • Variational quantum eigensolver — A hybrid algorithm that estimates a Hamiltonian’s ground-state energy by optimising a variational circuit.
  • QAOA — A variational algorithm for combinatorial optimisation that alternates a problem Hamiltonian with a mixing Hamiltonian.
  • Barren plateau — A region of a variational circuit’s parameter space where the gradient vanishes exponentially with the number of qubits, so training stalls.