Fix handling of circuit metadata

qiskit/providers/aer/backends/aerbackend.py

@@ -352,6 +352,11 @@ def _assemble(self, circuits, parameter_binds=None, **run_options):
             circuits, optypes, run_options = self._assemble_noise_model(
                 circuits, optypes, **run_options)
 
+            # Remove metadata from circuits
+            metadata_list = [circuit.metadata for circuit in circuits]
+            for circuit in circuits:
+                circuit.metadata = None
+
             if parameter_binds:
                 # Handle parameter binding
                 parameterizations = self._convert_binds(circuits, parameter_binds)
@@ -366,6 +371,10 @@ def _assemble(self, circuits, parameter_binds=None, **run_options):
             else:
                 qobj = assemble(circuits, backend=self)
 
+            # Recover metadata of circuits
+            for circuit, metadata in zip(circuits, metadata_list):
+                circuit.metadata = metadata
+
             # Add optypes to qobj
             # We convert to strings to avoid pybinding of types
             qobj.config.optypes = [

releasenotes/notes/remove_circuit_metadata_from_qobj-324e7ea9b369ee67.yaml

@@ -0,0 +1,9 @@
+---
+fixes:
+  - |
+    Reduce metadata of circuits in Qobj before simulation and Aer returns
+    results without metadata in ```header``` of ```ExperimentResult```. 
+    Previously, metadata is serialized with JSON format and copied to
+    the result even though metadata is not used in simulation. If metadata
+    can not be serizalized, simulation was failed. This fix works around
+    this issue.

test/terra/backends/aer_simulator/test_metadata.py

@@ -0,0 +1,104 @@
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2018, 2019, 2020, 2021, 2022.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+"""
+AerSimulator Integration Tests
+"""
+from math import sqrt
+from ddt import ddt
+from qiskit import transpile, QuantumCircuit
+from test.terra.reference import ref_algorithms
+
+from test.terra.backends.simulator_test_case import (
+    SimulatorTestCase, supported_methods)
+
+
+@ddt
+class TestMetadata(SimulatorTestCase):
+    """AerSimulator algorithm tests in the default basis"""
+
+    @supported_methods(
+        ['automatic', 'statevector', 'density_matrix',
+         'matrix_product_state', 'extended_stabilizer'])
+    def test_single_circuit_metadata(self, method, device):
+        """Test circuits with object metadata."""
+        backend = self.backend(method=method, device=device)
+        metadata = {1: object}
+        circuit = QuantumCircuit(1, name='circ0', metadata=metadata)
+        result = backend.run(circuit).result()
+        self.assertSuccess(result)
+        self.assertIsNone(result.results[0].header.metadata)
+        self.assertEqual(circuit.metadata, {1: object})
+
+    @supported_methods(
+        ['automatic', 'statevector', 'density_matrix',
+         'matrix_product_state', 'extended_stabilizer'])
+    def test_three_circuit_metadata(self, method, device):
+        """Test circuits with object metadata."""
+        backend = self.backend(method=method, device=device)
+
+        metadata0 = {0: object}
+        circuit0 = QuantumCircuit(1, name='circ0', metadata=metadata0)
+
+        metadata1 = {1: object}
+        circuit1 = QuantumCircuit(1, name='circ1', metadata=metadata1)
+
+        metadata2 = {2: object}
+        circuit2 = QuantumCircuit(1, name='circ2', metadata=metadata2)
+
+        result = backend.run([circuit0, circuit1, circuit2]).result()
+        self.assertSuccess(result)
+        self.assertEqual(len(result.results), 3)
+        self.assertIsNone(result.results[0].header.metadata)
+        self.assertIsNone(result.results[1].header.metadata)
+        self.assertIsNone(result.results[2].header.metadata)
+        self.assertEqual(circuit0.metadata, {0: object})
+        self.assertEqual(circuit1.metadata, {1: object})
+        self.assertEqual(circuit2.metadata, {2: object})
+
+    @supported_methods(
+        ['automatic', 'statevector', 'density_matrix', 'matrix_product_state'])
+    def test_three_parameterized_circuit_metadata(self, method, device):
+        """Test circuits with object metadata."""
+        backend = self.backend(method=method, device=device)
+
+        metadata0 = {0: object}
+        circuit0 = QuantumCircuit(1, name='circ0', metadata=metadata0)
+        circuit0.ry(0.1, 0)
+        circuit0.measure_all()
+
+        metadata1 = {1: object}
+        circuit1 = QuantumCircuit(1, name='circ1', metadata=metadata1)
+        circuit1.ry(0.1, 0)
+        circuit1.measure_all()
+
+        metadata2 = {2: object}
+        circuit2 = QuantumCircuit(1, name='circ2', metadata=metadata2)
+        circuit2.ry(0.1, 0)
+        circuit2.measure_all()
+
+        parameterizations=[[[[0, 0], [0, 1]]],
+                           [[[0, 0], [0, 1, 2]]],
+                           []]
+
+        result = backend.run([circuit0, circuit1, circuit2],
+                             parameterizations=parameterizations).result()
+        self.assertSuccess(result)
+        self.assertEqual(len(result.results), 6)
+        self.assertIsNone(result.results[0].header.metadata)
+        self.assertIsNone(result.results[1].header.metadata)
+        self.assertIsNone(result.results[2].header.metadata)
+        self.assertIsNone(result.results[3].header.metadata)
+        self.assertIsNone(result.results[4].header.metadata)
+        self.assertIsNone(result.results[5].header.metadata)
+        self.assertEqual(circuit0.metadata, {0: object})
+        self.assertEqual(circuit1.metadata, {1: object})
+        self.assertEqual(circuit2.metadata, {2: object})