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QC Glossary

Optimization

Improving a quantum circuit concerning certain metrics (often depth or gate count) without altering the observable result of the circuit when executed without error.1

Quantum

Full Definition

Optimization aims to minimize (maximize) a cost (utility) function over some domain. Optimization can work on the whole circuit or small parts of it.

Examples

Prominent examples are very local optimizations, so-called “peephole optimizations”. For example:

  • Gate cancellation (two consecutive gates on the same qubit, one of which is the inverse of the other, can be removed)
  • Rotation folding (two successive rotations on the same qubit and axis of rotation can be combined into a single rotation by adding up their angles)

Footnotes

  1. A lower gate count leads to fewer errors introduced by gates, hence the results on real hardware can differ.