Fix Initialize.gates_to_uncompute method (backport #12976)

qiskit/circuit/library/data_preparation/initializer.py

@@ -21,6 +21,7 @@
 from qiskit.circuit.quantumcircuit import QuantumCircuit
 from qiskit.circuit.quantumregister import QuantumRegister
 from qiskit.circuit.instruction import Instruction
+from qiskit.circuit.library.generalized_gates import Isometry
 from .state_preparation import StatePreparation
 
 if typing.TYPE_CHECKING:
@@ -86,9 +87,11 @@ def gates_to_uncompute(self) -> QuantumCircuit:
         """Call to create a circuit with gates that take the desired vector to zero.
 
         Returns:
-            Circuit to take ``self.params`` vector to :math:`|{00\\ldots0}\\rangle`
+            QuantumCircuit: circuit to take ``self.params`` vector to :math:`|{00\\ldots0}\\rangle`
         """
-        return self._stateprep._gates_to_uncompute()
+
+        isom = Isometry(self.params, 0, 0)
+        return isom._gates_to_uncompute()
 
     @property
     def params(self):

qiskit/circuit/library/data_preparation/state_preparation.py

@@ -174,7 +174,7 @@ def _define_synthesis_isom(self):
         initialize_circuit = QuantumCircuit(q, name="init_def")
 
         isom = Isometry(self.params, 0, 0)
-        initialize_circuit.append(isom, q[:])
+        initialize_circuit.compose(isom.definition, copy=False, inplace=True)
 
         # invert the circuit to create the desired vector from zero (assuming
         # the qubits are in the zero state)

releasenotes/notes/fix_initialize_gates_to_uncompute-d0dba6a642d07f30.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    Fix a bug that caused the method :meth:`Initialize.gates_to_uncompute()` fail.

test/python/circuit/test_initializer.py

@@ -465,6 +465,14 @@ def test_mutating_params(self):
         self.assertEqual(decom_circ.data[2].operation.name, "state_preparation")
         self.assertEqual(decom_circ.data[2].operation.params, ["0", "0"])
 
+    def test_gates_to_uncompute(self):
+        """Test the gates_to_uncompute() method."""
+        desired_vector = [0.5, 0.5, 0.5, 0.5]
+        initialize = Initialize(desired_vector)
+        qc = initialize.gates_to_uncompute().inverse()
+        vec = Statevector(qc)
+        self.assertTrue(vec == Statevector(desired_vector))
+
 
 class TestInstructionParam(QiskitTestCase):
     """Test conversion of numpy type parameters."""