Fix ComposedOp.to_matrix (#9316)

* Fix ComposedOp.to_matrix

* lint

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

qiskit/opflow/list_ops/composed_op.py

@@ -15,6 +15,7 @@
 from functools import partial, reduce
 from typing import List, Optional, Union, cast, Dict
 
+from numbers import Number
 import numpy as np
 
 from qiskit import QuantumCircuit
@@ -64,6 +65,20 @@ def settings(self) -> Dict:
     #     """ Tensor product with Self Multiple Times """
     #     raise NotImplementedError
 
+    def to_matrix(self, massive: bool = False) -> np.ndarray:
+        OperatorBase._check_massive("to_matrix", True, self.num_qubits, massive)
+
+        mat = self.coeff * reduce(
+            np.dot, [np.asarray(op.to_matrix(massive=massive)) for op in self.oplist]
+        )
+
+        # Note: As ComposedOp has a combo function of inner product we can end up here not with
+        # a matrix (array) but a scalar. In which case we make a single element array of it.
+        if isinstance(mat, Number):
+            mat = [mat]
+
+        return np.asarray(mat, dtype=complex)
+
     def to_circuit(self) -> QuantumCircuit:
         """Returns the quantum circuit, representing the composed operator.
 

qiskit/opflow/list_ops/list_op.py

@@ -13,7 +13,6 @@
 """ ListOp Operator Class """
 
 from functools import reduce
-from numbers import Number
 from typing import Any, Callable, Dict, Iterator, List, Optional, Set, Sequence, Union, cast
 
 import numpy as np
@@ -365,10 +364,6 @@ def to_matrix(self, massive: bool = False) -> np.ndarray:
                 [op.to_matrix(massive=massive) * self.coeff for op in self.oplist], dtype=object
             )
         )
-        # Note: As ComposedOp has a combo function of inner product we can end up here not with
-        # a matrix (array) but a scalar. In which case we make a single element array of it.
-        if isinstance(mat, Number):
-            mat = [mat]
         return np.asarray(mat, dtype=complex)
 
     def to_spmatrix(self) -> Union[spmatrix, List[spmatrix]]:

releasenotes/notes/fix-composedop-08e14db184c637c8.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixed two bugs in the :class:`.ComposedOp` where the :meth:`.ComposedOp.to_matrix`
+    method did not provide the correct results for compositions with :class:`.StateFn`
+    and for compositions with a global coefficients.
+    See also `#9283 <https://github.com/Qiskit/qiskit-terra/issues/9283>_`.

test/python/opflow/test_op_construction.py

@@ -1084,6 +1084,30 @@ def test_empty_listops(self):
         with self.subTest("eval empty TensoredOp "):
             self.assertEqual(TensoredOp([]).eval(), 0.0)
 
+    def test_composed_op_to_matrix_with_coeff(self):
+        """Test coefficients are properly handled.
+
+        Regression test of Qiskit/qiskit-terra#9283.
+        """
+        x = MatrixOp(X.to_matrix())
+        composed = 0.5 * (x @ X)
+
+        expected = 0.5 * np.eye(2)
+
+        np.testing.assert_almost_equal(composed.to_matrix(), expected)
+
+    def test_composed_op_to_matrix_with_vector(self):
+        """Test a matrix-vector composed op can be cast to matrix.
+
+        Regression test of Qiskit/qiskit-terra#9283.
+        """
+        x = MatrixOp(X.to_matrix())
+        composed = x @ Zero
+
+        expected = np.array([0, 1])
+
+        np.testing.assert_almost_equal(composed.to_matrix(), expected)
+
 
 class TestOpMethods(QiskitOpflowTestCase):
     """Basic method tests."""