circuits / quantum-phase-estimation

Quantum Phase Estimation (2+1)

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Estimates the phase θ in U|ψ⟩ = e^{2πiθ}|ψ⟩ using 2 counting qubits and inverse QFT.

quantumcomputer.dev

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qpe phase-estimation subroutinephase-estimation

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Source Code

OPENQASM 2.0;
include "qelib1.inc";
qreg q[3];
creg c[2];
// Eigenstate
x q[2];
// Hadamard counting qubits
h q[0]; h q[1];
// Controlled-U
cp(pi/2) q[1],q[2];
cp(pi) q[0],q[2];
// Inverse QFT
h q[0];
cp(-pi/2) q[1],q[0];
h q[1];
swap q[0],q[1];
measure q[0] -> c[0];
measure q[1] -> c[1];

Cite this circuit

MIT

quantumcomputer.dev. "Quantum Phase Estimation (2+1)". quantumcomputer.dev/circuits/quantum-phase-estimation. 2026.