Allow StandardRB to handle a V2 backend without a coupling map (#1293)

`StandardRB` handles directed two-qubit gates differently from
bi-directional ones. For `BackendV2`, it used the `coupling_map` to
check for directionality but did not handle the case where the coupling
map was `None` which implies all to all connectivity. With this change,
`StandardRB` treats gates with no coupling map as bidirectional.

This issue first surfaced with qiskit-aer 0.13.0 which made
`AerSimulator` a `BackendV2` backend
(#1292).

Author: wshanks

qiskit_experiments/library/randomized_benchmarking/standard_rb.py

@@ -241,6 +241,9 @@ def _get_basis_gates(self) -> Optional[Tuple[str, ...]]:
             return tuple(sorted(basis_gates)) if basis_gates else None
 
         def is_bidirectional(coupling_map):
+            if coupling_map is None:
+                # None for a coupling map implies all-to-all coupling
+                return True
             return len(coupling_map.reduce(self.physical_qubits).get_edges()) == 2
 
         # 2 qubits case: Return all basis gates except for one-way directed 2q-gates.

releasenotes/notes/rb-v2-none-coupling-fda2b22afdef507b.yaml

@@ -0,0 +1,12 @@
+---
+fixes:
+  - |
+    Changed :class:`.StandardRB` to treat two qubit operations in the
+    :class:`qiskit.transpiler.Target` as having all-to-all connectivity if
+    there is no set of specific pairs of coupled qubits. Most importantly, this
+    change allows :class:`.StandardRB` to work with
+    :class:`qiskit_aer.AerSimulator` for multi-qubit benchmarking after
+    ``qiskit-aer`` 0.13.0. Version 0.13.0 of ``qiskit-aer`` changed
+    the default :class:`qiskit_aer.AerSimulator` to have such a
+    :class:`qiskit.transpiler.Target` without specific coupled pairs.
+    See `#1292 <https://github.com/Qiskit-Extensions/qiskit-experiments/issues/1292>`__.