Transpiler errors with Aer 0.13 backends

[chriseclectic]

Informations

• Qiskit Aer version: 0.13.0-0.13.2
• Python version: 3.11
• Operating system: MacOS

What is the current behavior?

Trying to use qiskit pass managers with a user provided coupling map leads to transpiler errors when using Aer 0.13.

Steps to reproduce the problem

from qiskit import QuantumCircuit
from qiskit_aer import AerSimulator
from qiskit.transpiler import CouplingMap
from qiskit.transpiler.preset_passmanagers import generate_preset_pass_manager

qc = QuantumCircuit(3)
qc.cx(0, 1)

pm = generate_preset_pass_manager(2, backend=AerSimulator(), coupling_map=CouplingMap([[0, 2], [1, 2]]))
pm.run([qc])

Raises an exception:

---------------------------------------------------------------------------
TranspilerError                           Traceback (most recent call last)
File ~/mambaforge/envs/qiskit45/lib/python3.11/site-packages/qiskit/transpiler/runningpassmanager.py:161, in _replace_error.<locals>.wrapper(*meth_args, **meth_kwargs)
    160 try:
--> 161     return meth(*meth_args, **meth_kwargs)
    162 except PassManagerError as ex:

File ~/mambaforge/envs/qiskit45/lib/python3.11/site-packages/qiskit/passmanager/base_tasks.py:218, in BaseController.execute(self, passmanager_ir, state, callback)
    217 while True:
--> 218     passmanager_ir, state = next_task.execute(
    219         passmanager_ir=passmanager_ir,
    220         state=state,
    221         callback=callback,
    222     )
    223     try:
    224         # Sending the object through the generator implies the custom controllers
    225         # can always rely on the latest data to choose the next task to run.

File ~/mambaforge/envs/qiskit45/lib/python3.11/site-packages/qiskit/passmanager/base_tasks.py:218, in BaseController.execute(self, passmanager_ir, state, callback)
    217 while True:
--> 218     passmanager_ir, state = next_task.execute(
    219         passmanager_ir=passmanager_ir,
    220         state=state,
    221         callback=callback,
    222     )
    223     try:
    224         # Sending the object through the generator implies the custom controllers
    225         # can always rely on the latest data to choose the next task to run.

File ~/mambaforge/envs/qiskit45/lib/python3.11/site-packages/qiskit/passmanager/base_tasks.py:98, in GenericPass.execute(self, passmanager_ir, state, callback)
     97 if self not in state.workflow_status.completed_passes:
---> 98     ret = self.run(passmanager_ir)
     99     run_state = RunState.SUCCESS

File ~/mambaforge/envs/qiskit45/lib/python3.11/site-packages/qiskit/transpiler/passes/layout/vf2_layout.py:129, in VF2Layout.run(self, dag)
    128 if self.coupling_map is None:
--> 129     raise TranspilerError("coupling_map or target must be specified.")
    130 self.avg_error_map = self.property_set["vf2_avg_error_map"]

TranspilerError: 'coupling_map or target must be specified.'

What is the expected behavior?

The above code runs without error and returns the correct transpiled circuit when using AerSimulator from qiskit-aer 0.12

Suggested solutions

I'm not sure what is going on with Aer, but it seems like it is overriding the user provided coupling map somewhere in the passing of kwargs from the transpiler, and this is only happening in Qiskit Aer 0.13.

This might be related to the target attribute being added to AerSimulator in 0.13? @mtreinish

[doichanj]

I found coupling_map is not set if the target of AerBackend is passed to passmanager_config here

https://github.com/Qiskit/qiskit/blob/3ea3111b10716a0886422bf908d13faab24f5e2e/qiskit/transpiler/passes/layout/vf2_layout.py#L114-L115

We can avoid this by using _skip_target option as following

pm = generate_preset_pass_manager(2, backend=AerSimulator(), coupling_map=CouplingMap([[0, 2], [1, 2]]), _skip_target=True)

But I think the issue in AerBackend is AerBackend.target.build_coupling_map() should return coupling_map when setting coupling_map to AerSimulator as following

backend=AerSimulator(coupling_map=CouplingMap([[0, 2], [1, 2]]))
pm = generate_preset_pass_manager(2, backend=backend)