Create qpe2.py

Author: mhjensen

doc/src/FuturePlans/programs/qpe2.py

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+import pennylane as qml
+from pennylane import numpy as np
+
+# Number of qubits in the counting register (controls the precision)
+n_counting = 3
+dev = qml.device("default.qubit", wires=n_counting + 1)
+
+# Phase to estimate (should be between 0 and 1)
+phi = 0.125  # exact phase is 1/8
+
+def apply_controlled_unitary(phi, control, target, power):
+    """Apply controlled unitary U^{2^power} = Rz(2πφ * 2^power)"""
+    angle = 2 * np.pi * phi * (2 ** power)
+    qml.ctrl(qml.RZ, control=control)(angle, wires=target)
+
+@qml.qnode(dev)
+def qpe_circuit():
+    # Counting register (first n_counting qubits)
+    for i in range(n_counting):
+        qml.Hadamard(wires=i)
+
+    # Eigenstate |ψ> = |1> on the last qubit
+    qml.PauliX(wires=n_counting)
+
+    # Apply controlled-U^{2^j}
+    for i in range(n_counting):
+        apply_controlled_unitary(phi, control=i, target=n_counting, power=n_counting - 1 - i)
+
+    # Apply inverse QFT on the counting register
+    qml.adjoint(qml.templates.QFT)(wires=range(n_counting))
+
+    # Measurement
+    return qml.probs(wires=range(n_counting))
+
+# Run the circuit
+probs = qpe_circuit()
+estimated_bin = np.argmax(probs)
+estimated_phi = estimated_bin / (2 ** n_counting)
+
+# Print results
+print(f"Exact phase φ: {phi}")
+print(f"Estimated binary outcome: {format(estimated_bin, f'0{n_counting}b')}")
+print(f"Estimated phase φ: {estimated_phi}")