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Glossary

Quantum glossary

Barren plateau

A region of a variational circuit’s parameter space where the gradient vanishes exponentially with the number of qubits, so training stalls.

Deep, randomly initialised circuits tend to land on these plateaus, and no optimiser can find a slope that is exponentially small. It is one of the main reasons variational quantum machine learning has proved harder than it first looked.

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Related terms

  • Variational quantum eigensolver — A hybrid algorithm that estimates a Hamiltonian’s ground-state energy by optimising a variational circuit.
  • Variational circuit — A circuit with tunable rotation angles that a classical optimiser adjusts to minimise a cost.