Use Operator rather than unitary simulator to convert circuit to unitary matrix (qiskit-community/qiskit-aqua#1224)

* Fix factor of 2 error in converting exp_i to rotation gates

* 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 qiskit-community/qiskit-aqua#1218

* Remove note on reversing order of qubits

Using Operator(QuantumCircuit) gives the same qubit ordering
that the previous unitary-simulator-based code did. I removed
the comment entirely because the behavior depends only on
Operator(QuantumCircuit), which is not obscure.

Author: jlapeyre

test/aqua/operators/test_evolution.py

@@ -18,6 +18,7 @@
 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,
@@ -29,6 +30,13 @@
 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

@@ -17,6 +17,7 @@
 
 from test.aqua import QiskitAquaTestCase
 import itertools
+import scipy
 from scipy.stats import unitary_group
 import numpy as np
 from ddt import ddt, data
@@ -201,6 +202,14 @@ 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()