Fix handling of simulator backends in BackendV2Converter (Qiskit#9630)

This commit fixes some issues in the BackendV2Converter class around its
usage wrapping BackendV1 based simulators (such as Qiskit Aer with the
current release as of this commit 0.11.x). The BackendV2 converter was
incorrectly handling some edge cases in Target creation with simulators
that have ideal gates. The first issue was that the handling of ideal
gates without any properties was incorrect and could not handle gates
that took > 1 qubit. This has been corrected so the converter will work
as expect. The second issue is the `online_date` field in the
`BackendConfiguration` is optional and may not exist, especially on
simulators which that field never really applies for. With these fixes
the BackendV2Converter can be used to wrap BackendV1 simulators.

Fixes Qiskit#9562

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/providers/backend_compat.py

@@ -138,9 +138,7 @@ def convert_to_target(
     for op in combined_global_ops:
         if op not in target:
             if op in name_mapping:
-                target.add_instruction(
-                    name_mapping[op], {(bit,): None for bit in range(target.num_qubits)}
-                )
+                target.add_instruction(name_mapping[op], name=op)
             else:
                 raise QiskitError(
                     f"Operation name '{op}' does not have a known mapping. Use "
@@ -220,7 +218,7 @@ def __init__(
             provider=backend.provider,
             name=backend.name(),
             description=self._config.description,
-            online_date=self._config.online_date,
+            online_date=getattr(self._config, "online_date", None),
             backend_version=self._config.backend_version,
         )
         self._options = self._backend._options

releasenotes/notes/fix-backendv2-converter-simulator-e8f150d1fd6861fe.yaml

@@ -0,0 +1,9 @@
+---
+fixes:
+  - |
+    Fixed an issue with the :class:`~.BackendV2Converter` class when wrapping
+    a :class:`~.BackendV1` based simulator which would error if either
+    the ``online_date`` field in the :class:`~.BackendConfiguration` for the
+    simulator was not present or if the simulator backend supports ideal
+    implementations of gates that involve more than 1 qubit.
+    Fixed `#9562 <https://github.com/Qiskit/qiskit-terra/issues/9562>`__

test/python/providers/test_fake_backends.py

@@ -14,6 +14,7 @@
 # pylint: disable=missing-module-docstring
 
 import operator
+import unittest
 
 from test import combine
 from ddt import ddt, data
@@ -34,6 +35,30 @@
 from qiskit.providers.backend_compat import BackendV2Converter
 from qiskit.providers.backend import BackendV2
 from qiskit.utils import optionals
+from qiskit.circuit.library import (
+    SXGate,
+    MCPhaseGate,
+    MCXGate,
+    RZGate,
+    RXGate,
+    U2Gate,
+    U1Gate,
+    U3Gate,
+    YGate,
+    ZGate,
+    PauliGate,
+    SwapGate,
+    RGate,
+    MCXGrayCode,
+    RYGate,
+)
+from qiskit.circuit import ControlledGate, Parameter
+from qiskit.quantum_info.operators.channel.quantum_channel import QuantumChannel
+from qiskit.quantum_info.operators.channel.kraus import Kraus
+from qiskit.quantum_info.operators.channel import SuperOp
+from qiskit.extensions import Initialize, UnitaryGate
+from qiskit.extensions.quantum_initializer import DiagonalGate, UCGate
+from qiskit.circuit.controlflow import IfElseOp, WhileLoopOp, ForLoopOp
 
 FAKE_PROVIDER_FOR_BACKEND_V2 = FakeProviderForBackendV2()
 FAKE_PROVIDER = FakeProvider()
@@ -193,3 +218,243 @@ def test_converter_delay_circuit(self):
         qc.measure_all()
         res = transpile(qc, backend_v2)
         self.assertIn("delay", res.count_ops())
+
+    @unittest.skipUnless(optionals.HAS_AER, "Aer required for this test")
+    def test_converter_simulator(self):
+        class MCSXGate(ControlledGate):
+            def __init__(self, num_ctrl_qubits, ctrl_state=None):
+                super().__init__(
+                    "mcsx",
+                    1 + num_ctrl_qubits,
+                    [],
+                    None,
+                    num_ctrl_qubits,
+                    ctrl_state=ctrl_state,
+                    base_gate=SXGate(),
+                )
+
+        class MCYGate(ControlledGate):
+            def __init__(self, num_ctrl_qubits, ctrl_state=None):
+                super().__init__(
+                    "mcy",
+                    1 + num_ctrl_qubits,
+                    [],
+                    None,
+                    num_ctrl_qubits,
+                    ctrl_state=ctrl_state,
+                    base_gate=YGate(),
+                )
+
+        class MCZGate(ControlledGate):
+            def __init__(self, num_ctrl_qubits, ctrl_state=None):
+                super().__init__(
+                    "mcz",
+                    1 + num_ctrl_qubits,
+                    [],
+                    None,
+                    num_ctrl_qubits,
+                    ctrl_state=ctrl_state,
+                    base_gate=ZGate(),
+                )
+
+        class MCRXGate(ControlledGate):
+            def __init__(self, theta, num_ctrl_qubits, ctrl_state=None):
+                super().__init__(
+                    "mcrx",
+                    1 + num_ctrl_qubits,
+                    [theta],
+                    None,
+                    num_ctrl_qubits,
+                    ctrl_state=ctrl_state,
+                    base_gate=RXGate(theta),
+                )
+
+        class MCRYGate(ControlledGate):
+            def __init__(self, theta, num_ctrl_qubits, ctrl_state=None):
+                super().__init__(
+                    "mcry",
+                    1 + num_ctrl_qubits,
+                    [theta],
+                    None,
+                    num_ctrl_qubits,
+                    ctrl_state=ctrl_state,
+                    base_gate=RYGate(theta),
+                )
+
+        class MCRZGate(ControlledGate):
+            def __init__(self, theta, num_ctrl_qubits, ctrl_state=None):
+                super().__init__(
+                    "mcrz",
+                    1 + num_ctrl_qubits,
+                    [theta],
+                    None,
+                    num_ctrl_qubits,
+                    ctrl_state=ctrl_state,
+                    base_gate=RZGate(theta),
+                )
+
+        class MCRGate(ControlledGate):
+            def __init__(self, theta, phi, num_ctrl_qubits, ctrl_state=None):
+                super().__init__(
+                    "mcr",
+                    1 + num_ctrl_qubits,
+                    [theta, phi],
+                    None,
+                    num_ctrl_qubits,
+                    ctrl_state=ctrl_state,
+                    base_gate=RGate(theta, phi),
+                )
+
+        class MCU1Gate(ControlledGate):
+            def __init__(self, theta, num_ctrl_qubits, ctrl_state=None):
+                super().__init__(
+                    "mcu1",
+                    1 + num_ctrl_qubits,
+                    [theta],
+                    None,
+                    num_ctrl_qubits,
+                    ctrl_state=ctrl_state,
+                    base_gate=U1Gate(theta),
+                )
+
+        class MCU2Gate(ControlledGate):
+            def __init__(self, theta, lam, num_ctrl_qubits, ctrl_state=None):
+                super().__init__(
+                    "mcu2",
+                    1 + num_ctrl_qubits,
+                    [theta, lam],
+                    None,
+                    num_ctrl_qubits,
+                    ctrl_state=ctrl_state,
+                    base_gate=U2Gate(theta, lam),
+                )
+
+        class MCU3Gate(ControlledGate):
+            def __init__(self, theta, lam, phi, num_ctrl_qubits, ctrl_state=None):
+                super().__init__(
+                    "mcu3",
+                    1 + num_ctrl_qubits,
+                    [theta, phi, lam],
+                    None,
+                    num_ctrl_qubits,
+                    ctrl_state=ctrl_state,
+                    base_gate=U3Gate(theta, phi, lam),
+                )
+
+        class MCUGate(ControlledGate):
+            def __init__(self, theta, lam, phi, num_ctrl_qubits, ctrl_state=None):
+                super().__init__(
+                    "mcu",
+                    1 + num_ctrl_qubits,
+                    [theta, phi, lam],
+                    None,
+                    num_ctrl_qubits,
+                    ctrl_state=ctrl_state,
+                    base_gate=U3Gate(theta, phi, lam),
+                )
+
+        class MCSwapGate(ControlledGate):
+            def __init__(self, num_ctrl_qubits, ctrl_state=None):
+                super().__init__(
+                    "mcswap",
+                    2 + num_ctrl_qubits,
+                    [],
+                    None,
+                    num_ctrl_qubits,
+                    ctrl_state=ctrl_state,
+                    base_gate=SwapGate(),
+                )
+
+        from qiskit_aer import AerSimulator
+        from qiskit_aer.library import (
+            SaveExpectationValue,
+            SaveAmplitudes,
+            SaveStatevectorDict,
+            SaveSuperOp,
+            SaveClifford,
+            SaveMatrixProductState,
+            SaveDensityMatrix,
+            SaveProbabilities,
+            SaveStatevector,
+            SetDensityMatrix,
+            SetUnitary,
+            SaveState,
+            SetMatrixProductState,
+            SaveUnitary,
+            SetSuperOp,
+            SaveExpectationValueVariance,
+            SaveStabilizer,
+            SetStatevector,
+            SetStabilizer,
+            SaveAmplitudesSquared,
+            SaveProbabilitiesDict,
+        )
+        from qiskit_aer.noise.errors import ReadoutError
+        from qiskit_aer.noise.noise_model import QuantumErrorLocation
+
+        sim = AerSimulator()
+        phi = Parameter("phi")
+        lam = Parameter("lam")
+        backend = BackendV2Converter(
+            sim,
+            name_mapping={
+                "mcsx": MCSXGate,
+                "mcp": MCPhaseGate,
+                "mcphase": MCPhaseGate,
+                "quantum_channel": QuantumChannel,
+                "initialize": Initialize,
+                "save_expval": SaveExpectationValue,
+                "diagonal": DiagonalGate,
+                "save_amplitudes": SaveAmplitudes,
+                "roerror": ReadoutError,
+                "mcrx": MCRXGate,
+                "kraus": Kraus,
+                "save_statevector_dict": SaveStatevectorDict,
+                "mcx": MCXGate,
+                "mcu1": MCU1Gate,
+                "mcu2": MCU2Gate,
+                "mcu3": MCU3Gate,
+                "save_superop": SaveSuperOp,
+                "multiplexer": UCGate,
+                "mcy": MCYGate,
+                "superop": SuperOp,
+                "save_clifford": SaveClifford,
+                "save_matrix_product_state": SaveMatrixProductState,
+                "save_density_matrix": SaveDensityMatrix,
+                "save_probabilities": SaveProbabilities,
+                "if_else": IfElseOp,
+                "while_loop": WhileLoopOp,
+                "for_loop": ForLoopOp,
+                "save_statevector": SaveStatevector,
+                "mcu": MCUGate,
+                "set_density_matrix": SetDensityMatrix,
+                "qerror_loc": QuantumErrorLocation,
+                "unitary": UnitaryGate,
+                "mcz": MCZGate,
+                "pauli": PauliGate,
+                "set_unitary": SetUnitary,
+                "save_state": SaveState,
+                "mcswap": MCSwapGate,
+                "set_matrix_product_state": SetMatrixProductState,
+                "save_unitary": SaveUnitary,
+                "mcr": MCRGate,
+                "mcx_gray": MCXGrayCode,
+                "mcrz": MCRZGate,
+                "set_superop": SetSuperOp,
+                "save_expval_var": SaveExpectationValueVariance,
+                "save_stabilizer": SaveStabilizer,
+                "set_statevector": SetStatevector,
+                "mcry": MCRYGate,
+                "set_stabilizer": SetStabilizer,
+                "save_amplitudes_sq": SaveAmplitudesSquared,
+                "save_probabilities_dict": SaveProbabilitiesDict,
+                "cu2": U2Gate(phi, lam).control(),
+            },
+        )
+        self.assertIsInstance(backend, BackendV2)
+        res = transpile(self.circuit, backend)
+        job = backend.run(res)
+        result = job.result()
+        counts = result.get_counts()
+        max_count = max(counts.items(), key=operator.itemgetter(1))[0]
+        self.assertEqual(max_count, "11")