Make CircuitOp use Operator rather than unitary simulator to get matrix

The unitary simulator does not account for the global phase in a
circuit. qiskit.quantum_info.Operator does account for global phase
when converting (when possible) a circuit to a unitary matrix.

Closes #1218

qiskit/aqua/operators/primitive_ops/circuit_op.py

@@ -18,7 +18,8 @@
 import logging
 import numpy as np
 
-from qiskit import QuantumCircuit, BasicAer, execute
+import qiskit
+from qiskit import QuantumCircuit
 from qiskit.circuit.library import IGate
 from qiskit.circuit import Instruction, ParameterExpression
 
@@ -141,14 +142,13 @@ def to_matrix(self, massive: bool = False) -> np.ndarray:
                 ' in this case {0}x{0} elements.'
                 ' Set massive=True if you want to proceed.'.format(2 ** self.num_qubits))
 
+        # TODO: Check order of qubits. The note below refers to using the unitary simulator.
+        # But, we have replaced this code with qiskit.quantum_info.Operator
         # NOTE: not reversing qubits!! We generally reverse endianness when converting between
         # circuit or Pauli representation and matrix representation, but we don't need to here
         # because the Unitary simulator already presents the endianness of the circuit unitary in
         # forward endianness.
-        unitary_backend = BasicAer.get_backend('unitary_simulator')
-        unitary = execute(self.to_circuit(),
-                          unitary_backend,
-                          optimization_level=0).result().get_unitary()
+        unitary = qiskit.quantum_info.Operator(self.to_circuit()).data
         # pylint: disable=cyclic-import
         from ..operator_globals import EVAL_SIG_DIGITS
         return np.round(unitary * self.coeff, decimals=EVAL_SIG_DIGITS)

test/aqua/operators/test_evolution.py

@@ -20,6 +20,7 @@
 import numpy as np
 import scipy.linalg
 
+import qiskit
 from qiskit.circuit import ParameterVector, Parameter
 
 from qiskit.aqua.operators import (X, Y, Z, I, CX, H, ListOp, CircuitOp, Zero, EvolutionFactory,
@@ -31,6 +32,13 @@
 class TestEvolution(QiskitAquaTestCase):
     """Evolution tests."""
 
+    def test_exp_i(self):
+        """ exponential of Pauli test """
+        op = Z.exp_i()
+        gate = op.to_circuit().data[0][0]
+        self.assertIsInstance(gate, qiskit.circuit.library.RZGate)
+        self.assertEqual(gate.params[0], 2)
+
     def test_pauli_evolution(self):
         """ pauli evolution test """
         op = (-1.052373245772859 * I ^ I) + \

test/aqua/operators/test_op_construction.py

@@ -19,6 +19,7 @@
 
 from test.aqua import QiskitAquaTestCase
 import itertools
+import scipy
 from scipy.stats import unitary_group
 import numpy as np
 from ddt import ddt, data
@@ -203,6 +204,14 @@ def test_to_matrix(self):
         np.testing.assert_array_almost_equal(
             op6.to_matrix(), op5.to_matrix() + Operator.from_label('+r').data)
 
+    def test_circuit_op_to_matrix(self):
+        """ test CircuitOp.to_matrix """
+        qc = QuantumCircuit(1)
+        qc.rz(1.0, 0)
+        qcop = CircuitOp(qc)
+        np.testing.assert_array_almost_equal(
+            qcop.to_matrix(), scipy.linalg.expm(-0.5j * Z.to_matrix()))
+
     def test_matrix_to_instruction(self):
         """Test MatrixOp.to_instruction yields an Instruction object."""
         matop = (H ^ 3).to_matrix_op()