Add global phase to qobj in assembly (#4822)

This commit adds the global phase circuit property to the experiment
headers in the assembled qobj. Currently we do not pass the global phase
to backends so simulators can not take it into account. The experiment
header is used since it's a free form field for properties of the
circuit that a backend can use if they want, but there are no
requirements on it. With this in place it should enable #4805 and
Qiskit/qiskit-aer#847 to be fixed.

Related to #4805

qiskit/assembler/assemble_circuits.py

@@ -47,7 +47,8 @@ def _assemble_circuit(circuit):
                                   clbit_labels=clbit_labels,
                                   memory_slots=memory_slots,
                                   creg_sizes=creg_sizes,
-                                  name=circuit.name)
+                                  name=circuit.name,
+                                  global_phase=circuit.global_phase)
     # TODO: why do we need n_qubits and memory_slots in both the header and the config
     config = QasmQobjExperimentConfig(n_qubits=num_qubits, memory_slots=memory_slots)
 

test/python/compiler/test_assembler.py

@@ -342,6 +342,19 @@ def test_init_qubits_false(self):
         qobj = assemble(self.circ, init_qubits=False)
         self.assertEqual(qobj.config.init_qubits, False)
 
+    def test_circuit_with_global_phase(self):
+        """Test that global phase for a circuit is handled correctly."""
+        circ = QuantumCircuit(2)
+        circ.h(0)
+        circ.cx(0, 1)
+        circ.measure_all()
+        circ.global_phase = .3 * np.pi
+        qobj = assemble([circ, self.circ])
+        self.assertEqual(getattr(qobj.experiments[1].header, 'global_phase'),
+                         0)
+        self.assertEqual(getattr(qobj.experiments[0].header, 'global_phase'),
+                         .3 * np.pi)
+
 
 class TestPulseAssembler(QiskitTestCase):
     """Tests for assembling schedules to qobj."""