Fix Initialize.gates_to_uncompute method

qiskit/circuit/library/data_preparation/state_preparation.py

@@ -243,6 +243,20 @@ def validate_parameter(self, parameter):
     def _return_repeat(self, exponent: float) -> "Gate":
         return Gate(name=f"{self.name}*{exponent}", num_qubits=self.num_qubits, params=[])
 
+    ### helper functions for initializer.gates_to_uncompute() method ###
+
+    def _gates_to_uncompute(self):
+        """Call to create a circuit with gates that take the desired vector to zero.
+        Returns:
+            QuantumCircuit: circuit to take self.params vector to :math:`|{00\\ldots0}\\rangle`
+        """
+        q = QuantumRegister(self.num_qubits)
+        circuit = QuantumCircuit(q, name="disentangler")
+
+        isom = Isometry(self._params_arg, 0, 0)
+        circuit.append(isom, q[:])
+        return circuit.inverse()
+
 
 class UniformSuperpositionGate(Gate):
     r"""Implements a uniform superposition 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."""