Preserve StatePreparation width in inverse (#16431) (#16681)

* Preserve StatePreparation width in inverse

* Strengthen StatePreparation inverse regression

* Address StatePreparation inverse review

* Respect StatePreparation inverse input contract

(cherry picked from commit d94420a)

Co-authored-by: Peter Chen J. <34339487+peter941221@users.noreply.github.com>

Author: mergify[bot]

qiskit/circuit/library/data_preparation/state_preparation.py

@@ -207,7 +207,10 @@ def inverse(self, annotated: bool = False):
             None if self._label in ("State Preparation", "State Preparation Dg") else self._label
         )
 
-        return StatePreparation(self._params_arg, inverse=not self._inverse, label=label)
+        num_qubits = self.num_qubits if self._from_int else None
+        return StatePreparation(
+            self._params_arg, num_qubits=num_qubits, inverse=not self._inverse, label=label
+        )
 
     def broadcast_arguments(self, qargs, cargs):
         flat_qargs = [qarg for sublist in qargs for qarg in sublist]

releasenotes/notes/stateprep-inverse-preserve-int-width-3e184153a463f3dd.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixed :meth:`.StatePreparation.inverse` for integer-bitmap inputs with an
+    explicit ``num_qubits`` so the inverse gate preserves the original qubit
+    width.
+    Fixed `#16430 <https://github.com/Qiskit/qiskit/issues/16430>`__.

test/python/circuit/library/test_state_preparation.py

@@ -81,6 +81,31 @@ def test_inverse(self, state):
         qc.append(stateprep.inverse(), [0, 1])
         self.assertTrue(np.allclose(Operator(qc).data, np.identity(2**qc.num_qubits)))
 
+    def test_inverse_preserves_integer_bitmap_width(self):
+        """Test inverse preserves explicit width for integer-bitmap inputs."""
+        stateprep = StatePreparation(1, num_qubits=2)
+        inverse = stateprep.inverse()
+        qc = QuantumCircuit(2)
+        qc.append(stateprep, [0, 1])
+        qc.append(inverse, [0, 1])
+
+        self.assertEqual(inverse.num_qubits, 2)
+        self.assertTrue(np.allclose(Operator(qc).data, np.identity(2**qc.num_qubits)))
+
+    def test_inverse_integer_bitmap_matches_vector_form(self):
+        """Test integer-bitmap inverse matches the equivalent vector-form semantics."""
+        int_stateprep = StatePreparation(1, num_qubits=2)
+        vector_stateprep = StatePreparation([0, 1, 0, 0])
+
+        for stateprep, inverse in (
+            (int_stateprep, vector_stateprep.inverse()),
+            (vector_stateprep, int_stateprep.inverse()),
+        ):
+            qc = QuantumCircuit(2)
+            qc.append(stateprep, [0, 1])
+            qc.append(inverse, [0, 1])
+            self.assertTrue(Statevector(qc) == Statevector.from_label("00"))
+
     def test_double_inverse(self):
         """Test twice inverse of StatePreparation"""
         desired_sv = Statevector([1 / math.sqrt(2), 0, 0, 1 / math.sqrt(2)])
@@ -96,6 +121,11 @@ def test_incompatible_state_and_qubit_args(self):
         with self.assertRaises(QiskitError):
             qc.prepare_state("11")
 
+    def test_num_qubits_with_non_integer_params(self):
+        """Test number of qubits is rejected for non-integer state arguments."""
+        with self.assertRaises(QiskitError):
+            StatePreparation([1, 0], num_qubits=1)
+
     def test_incompatible_int_state_and_qubit_args(self):
         """Test error raised if number of qubits not compatible with  integer state arg"""
         with self.assertRaises(QiskitError):