Fix: Initialize.inverse() now raises CircuitError immediately (#15782)

* Fix: Initialize.inverse() now raises CircuitError immediately

* move importations to the beggining

* Fix: apply black formatting

* Apply suggestions from code review by Cryoris

Co-authored-by: Julien Gacon <gaconju@gmail.com>

---------

Co-authored-by: Julien Gacon <gaconju@gmail.com>
Co-authored-by: Shelly Garion <46566946+ShellyGarion@users.noreply.github.com>

Author: 3 people

qiskit/circuit/library/data_preparation/initializer.py

@@ -23,6 +23,7 @@
 from qiskit.circuit.instruction import Instruction
 from qiskit.circuit.library.generalized_gates import Isometry
 from .state_preparation import StatePreparation
+from qiskit.circuit.exceptions import CircuitError
 
 if typing.TYPE_CHECKING:
     from qiskit.quantum_info.states.statevector import Statevector
@@ -106,3 +107,10 @@ def params(self, parameters: Statevector | Sequence[complex] | str | int) -> Non
 
     def broadcast_arguments(self, qargs, cargs):
         return self._stateprep.broadcast_arguments(qargs, cargs)
+
+    def inverse(self, annotated: bool = False):
+        """Raises an error as Initialize cannot be inverted."""
+        raise CircuitError(
+            "Initialize is not unitary thus can not be inverted. "
+            "If you want an invertible state preparation, use StatePreparation instead."
+        )

releasenotes/notes/fix-initialize.inverse()-late-error-1846fbbae82ec011.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixed an unclear error message in :meth:`.Initialize.inverse`, which now immediately 
+    raises an error when called since the instruction is not invertible.
+
+    See `#15595 <https://github.com/Qiskit/qiskit/issues/15595>`__.

test/python/circuit/test_initializer.py

@@ -30,6 +30,7 @@
 from qiskit.exceptions import QiskitError
 from qiskit.circuit.library import Initialize
 from test import QiskitTestCase
+from qiskit.circuit.exceptions import CircuitError
 
 
 @ddt
@@ -514,6 +515,13 @@ def test_repeat(self):
         statevector = Statevector(qc)
         self.assertTrue(np.allclose(statevector, desired_vector))
 
+    def test_irreversible_initialize(self):
+        """Test failure of the initialize.inverse() method"""
+        circuit = QuantumCircuit(4, 0)
+        circuit.initialize(0, circuit.qubits)
+        with self.assertRaises(CircuitError):
+            _ = circuit.inverse()
+
 
 class TestInstructionParam(QiskitTestCase):
     """Test conversion of numpy type parameters."""