Fix `EvolvedOp.to_instruction` (#8384)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/opflow/evolutions/evolved_op.py

@@ -169,7 +169,7 @@ class EvolvedOp(PrimitiveOp):
 
     # pylint: disable=arguments-differ
     def to_instruction(self, massive: bool = False) -> Instruction:
-        mat_op = self.primitive.to_matrix_op(massive=massive)
+        mat_op = self.to_matrix_op(massive=massive)
         if not isinstance(mat_op, MatrixOp):
             raise OpflowError("to_instruction is not allowed for ListOp.")
         return mat_op.to_instruction()

releasenotes/notes/fix-evolvedop-to-instruction-c90c4f1aa6b4232a.yaml

@@ -0,0 +1,18 @@
+---
+fixes:
+  - |
+    Fix :meth:`~.EvolvedOp.to_instruction` which previously tried to create a 
+    :class:`~.UnitaryGate` without exponentiating the operator to evolve.
+    Since this operator is generally not unitary, this raised an error (and if 
+    the operator would have been unitary by chance, it would not have been the expected result).
+
+    Now calling :meth:`~.EvolvedOp.to_instruction` correctly produces a gate 
+    that implements the time evolution of the operator it holds::
+
+      >>> from qiskit.opflow import EvolvedOp, X
+      >>> op = EvolvedOp(0.5 * X)
+      >>> op.to_instruction()
+      Instruction(
+          name='unitary', num_qubits=1, num_clbits=0, 
+          params=[array([[0.87758256+0.j, 0.-0.47942554j], [0.-0.47942554j, 0.87758256+0.j]])]
+      )

test/python/opflow/test_evolution.py

@@ -19,6 +19,7 @@ import scipy.linalg
 
 import qiskit
 from qiskit.circuit import Parameter, ParameterVector
+from qiskit.extensions import UnitaryGate
 from qiskit.opflow import (
     CX,
     CircuitOp,
@@ -364,6 +365,19 @@ class TestEvolution(QiskitOpflowTestCase):
         )
         np.testing.assert_array_almost_equal(evolution.to_matrix(), matrix)
 
+    def test_evolved_op_to_instruction(self):
+        """Test calling `to_instruction` on a plain EvolvedOp.
+
+        Regression test of Qiskit/qiskit-terra#8025.
+        """
+        op = EvolvedOp(0.5 * X)
+        circuit = op.to_instruction()
+
+        unitary = scipy.linalg.expm(-0.5j * X.to_matrix())
+        expected = UnitaryGate(unitary)
+
+        self.assertEqual(circuit, expected)
+
 
 if __name__ == "__main__":
     unittest.main()