replace flatten() by ravel()

src/operators/matrix_product_operator.py

@@ -135,7 +135,7 @@ class MatrixProductOperator(EndomorphicOperator):
                     res = res.reshape(np.flip(res.shape))
 
             else:
-                res = m.dot(x.val.flatten()).reshape(self._domain.shape)
+                res = m.dot(x.val.ravel()).reshape(self._domain.shape)
             return Field(self._domain, res)
 
         mat_axes = self._mat_last_n if times else np.flip(self._mat_last_n)