Quantum glossary

Quantum phase estimation

Also: qpe

An algorithm that reads out the eigenvalue phase of a unitary into a register of qubits.

It combines controlled unitaries, phase kickback and an inverse QFT. It is the core of Shor’s algorithm and of fault-tolerant quantum chemistry.

See it for yourself

Quantum Fourier transform — The quantum version of the discrete Fourier transform, acting on amplitudes with only O(n²) gates for n qubits.
Phase kickback — When a controlled operation acts on a target in an eigenstate, the eigenvalue’s phase appears on the control qubit instead.
Shor's algorithm — A quantum algorithm that factors integers and computes discrete logarithms in polynomial time.