Fixed IQAE bug mentioned in issue 9280 (#9887)

* Fixed IQAE bug mentioned in issue 9280 * Update releasenotes/notes/iterative-phase-estimation-bugfix-b676ffc23cea8251.yaml Co-authored-by: Julien Gacon <[email protected]> * Added new line to release notes * Update releasenotes/notes/iterative-phase-estimation-bugfix-b676ffc23cea8251.yaml Co-authored-by: Elena Peña Tapia <[email protected]> --------- Co-authored-by: Julien Gacon <[email protected]> Co-authored-by: Steve Wood <[email protected]> Co-authored-by: Elena Peña Tapia <[email protected]>
Qiskit · Apr 5, 2023 · 2bb15ed · 2bb15ed
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diff --git a/qiskit/algorithms/phase_estimators/ipe.py b/qiskit/algorithms/phase_estimators/ipe.py
@@ -118,7 +118,7 @@ def construct_circuit(
         # For example, it may be desirable to compute the power via Trotterization, if
         # we are doing Trotterization anyway.
         unitary_power = unitary.power(2 ** (k - 1)).control()
-        qc = qc.compose(unitary_power, list(range(1, unitary.num_qubits + 1)) + [0])
+        qc = qc.compose(unitary_power, [unitary.num_qubits] + list(range(0, unitary.num_qubits)))
         qc.p(omega, phase_register[0])
         # hadamard on phase_register[0]
         qc.h(phase_register[0])

diff --git a/releasenotes/notes/iterative-phase-estimation-bugfix-b676ffc23cea8251.yaml b/releasenotes/notes/iterative-phase-estimation-bugfix-b676ffc23cea8251.yaml
@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixed a bug where :class:`.IterativePhaseEstimation` was generating the wrong circuit, causing the
+    algorithm to fail for simple cases. Fixed `#9280 <https://github.com/Qiskit/qiskit-terra/issues/9280>`__.
diff --git a/test/python/algorithms/test_phase_estimator.py b/test/python/algorithms/test_phase_estimator.py
@@ -28,7 +28,18 @@
 )
 import qiskit
 from qiskit import QuantumCircuit
-from qiskit.opflow import H, X, Y, Z, I, StateFn, PauliTrotterEvolution, MatrixEvolution, PauliSumOp
+from qiskit.opflow import (
+    H,
+    X,
+    Y,
+    Z,
+    I,
+    T,
+    StateFn,
+    PauliTrotterEvolution,
+    MatrixEvolution,
+    PauliSumOp,
+)
 from qiskit.test import slow_test
 
 
@@ -515,6 +526,23 @@ def test_qpe_RZ_sampler(self, state_preparation, expected_phase, phase_estimator
         )
         self.assertEqual(phase, expected_phase)
 
+    @data(
+        ((X ^ X).to_circuit(), 0.25, IterativePhaseEstimation),
+        ((I ^ X).to_circuit(), 0.125, IterativePhaseEstimation),
+        ((X ^ X).to_circuit(), 0.25, PhaseEstimation),
+        ((I ^ X).to_circuit(), 0.125, PhaseEstimation),
+    )
+    @unpack
+    def test_qpe_two_qubit_unitary(self, state_preparation, expected_phase, phase_estimator):
+        """two qubit unitary T ^ T"""
+        unitary_circuit = (T ^ T).to_circuit()
+        phase = self.one_phase_sampler(
+            unitary_circuit,
+            state_preparation,
+            phase_estimator,
+        )
+        self.assertEqual(phase, expected_phase)
+
     def test_check_num_iterations_sampler(self):
         """test check for num_iterations greater than zero"""
         unitary_circuit = QuantumCircuit(1).compose(XGate())