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Glossary

Quantum glossary

Phase kickback

When a controlled operation acts on a target in an eigenstate, the eigenvalue’s phase appears on the control qubit instead.

It sounds backwards and it is the trick behind Deutsch–Jozsa, Bernstein–Vazirani, phase estimation and Shor. The target is unchanged; the information moves to the control.

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Related terms

  • Oracle — A subroutine that encodes the problem, often by flipping the phase of the states that are solutions.
  • Quantum phase estimation — An algorithm that reads out the eigenvalue phase of a unitary into a register of qubits.