Stop using circuit metadata to internaly manage simulation results

This fixes `AerSimulator` to use circuit metadata to maintain mapping
from input and output of an executor call. This fixes an issue
Qiskit#1723.

qiskit_aer/backends/aer_compiler.py

@@ -15,7 +15,7 @@
 
 import itertools
 from copy import copy
-from typing import List
+from typing import List, Dict
 
 from qiskit.circuit import QuantumCircuit, Clbit, ParameterExpression
 from qiskit.extensions import Initialize
@@ -364,7 +364,7 @@ def generate_aer_config(
     return config
 
 
-def assemble_circuit(circuit: QuantumCircuit):
+def assemble_circuit(circuit: QuantumCircuit, metadata: Dict):
     """assemble circuit object mapped to AER::Circuit"""
 
     num_qubits = circuit.num_qubits
@@ -391,8 +391,8 @@ def assemble_circuit(circuit: QuantumCircuit):
         global_phase=global_phase,
     )
 
-    if circuit.metadata is not None:
-        header.metadata = circuit.metadata
+    if metadata is not None:
+        header.metadata = metadata
 
     qubit_indices = {qubit: idx for idx, qubit in enumerate(circuit.qubits)}
     clbit_indices = {clbit: idx for idx, clbit in enumerate(circuit.clbits)}
@@ -588,7 +588,7 @@ def _assemble_op(aer_circ, inst, qubit_indices, clbit_indices, is_conditional, c
         raise AerError(f"unknown instruction: {name}")
 
 
-def assemble_circuits(circuits: List[QuantumCircuit]) -> List[AerCircuit]:
+def assemble_circuits(circuits: List[QuantumCircuit], metadata_list: Dict) -> List[AerCircuit]:
     """converts a list of Qiskit circuits into circuits mapped AER::Circuit
 
     Args:
@@ -611,4 +611,6 @@ def assemble_circuits(circuits: List[QuantumCircuit]) -> List[AerCircuit]:
             # Generate AerCircuit from the input circuit
             aer_qc_list = assemble_circuits(circuits=[qc])
     """
-    return [assemble_circuit(circuit) for circuit in circuits]
+    return [
+        assemble_circuit(circuit, metadata) for circuit, metadata in zip(circuits, metadata_list)
+    ]

qiskit_aer/backends/aerbackend.py

@@ -423,19 +423,10 @@ def _execute_circuits_job(self, circuits, noise_model, config, job_id="", format
         # Start timer
         start = time.time()
 
-        # Take metadata from headers of experiments to work around JSON serialization error
-        metadata_list = []
-        for idx, circ in enumerate(circuits):
-            metadata_list.append(circ.metadata)
-            # TODO: we test for True-like on purpose here to condition against both None and {},
-            # which allows us to support versions of Terra before and after QuantumCircuit.metadata
-            # accepts None as a valid value. This logic should be revisited after terra>=0.24.0 is
-            # required.
-            if circ.metadata:
-                circ.metadata = {"metadata_index": idx}
-
         # Run simulation
-        aer_circuits = assemble_circuits(circuits)
+        aer_circuits = assemble_circuits(
+            circuits, [{"metadata_index": idx} for idx in range(len(circuits))]
+        )
         output = self._execute_circuits(aer_circuits, noise_model, config)
 
         # Validate output
@@ -460,10 +451,7 @@ def _execute_circuits_job(self, circuits, noise_model, config, job_id="", format
                 and "metadata_index" in result["header"]["metadata"]
             ):
                 metadata_index = result["header"]["metadata"]["metadata_index"]
-                result["header"]["metadata"] = metadata_list[metadata_index]
-
-        for circ, metadata in zip(circuits, metadata_list):
-            circ.metadata = metadata
+                result["header"]["metadata"] = circuits[metadata_index].metadata
 
         # Add execution time
         output["time_taken"] = time.time() - start

releasenotes/notes/do_not_modify_metadata-60bb4b88707bd021.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Previously :class:`~.AerSimulator` modifies circuit metadata to maintain
+    consistency between input and output of simulation with side effect of
+    unexpected view of metadata from applicatiln in simiulation. This fix
+    avoids using circuit metadata to maintain consistency internaly and then
+    always provides consistent view of metadata to application.

test/terra/backends/aer_simulator/test_circuit.py

@@ -171,3 +171,21 @@ def test_partial_result_a_single_invalid_circuit(self):
         self.assertEqual(result.status, "PARTIAL COMPLETED")
         self.assertTrue(hasattr(result.results[1].data, "counts"))
         self.assertFalse(hasattr(result.results[0].data, "counts"))
+
+    def test_metadata_protected(self):
+        """Test metadata is consitently viewed from users"""
+
+        qc = QuantumCircuit(2)
+        qc.metadata = {"foo": "bar", "object": object}
+
+        circuits = [qc.copy() for _ in range(5)]
+
+        backend = self.backend()
+        job = backend.run(circuits)
+
+        for circuit in circuits:
+            self.assertTrue("foo" in circuit.metadata)
+            self.assertEqual(circuit.metadata["foo"], "bar")
+            self.assertEqual(circuit.metadata["object"], object)
+
+        job.result()