Fix from backend of V2 primitives to use existing AerSimulator (#2173)

* Fix from_backend of V2 primitives to use existing AerSimulator

* add reno, fix format

* delete print for debug

---------

Co-authored-by: Hiroshi Horii <[email protected]>

Author: doichanj

qiskit_aer/backends/aer_simulator.py

@@ -855,7 +855,9 @@ def from_backend(cls, backend, **options):
                 open_pulse=False,
                 memory=False,
                 max_shots=int(1e6),
-                coupling_map=list(backend.coupling_map.get_edges()),
+                coupling_map=(
+                    None if backend.coupling_map is None else list(backend.coupling_map.get_edges())
+                ),
                 max_experiments=backend.max_circuits,
                 description=description,
             )

qiskit_aer/primitives/estimator_v2.py

@@ -76,7 +76,10 @@ def __init__(
     def from_backend(cls, backend, **options):
         """make new sampler that uses external backend"""
         estimator = cls(**options)
-        estimator._backend = AerSimulator.from_backend(backend)
+        if isinstance(backend, AerSimulator):
+            estimator._backend = backend
+        else:
+            estimator._backend = AerSimulator.from_backend(backend)
         return estimator
 
     @property

qiskit_aer/primitives/sampler_v2.py

@@ -97,7 +97,10 @@ def __init__(
     def from_backend(cls, backend, **options):
         """make new sampler that uses external backend"""
         sampler = cls(**options)
-        sampler._backend = AerSimulator.from_backend(backend)
+        if isinstance(backend, AerSimulator):
+            sampler._backend = backend
+        else:
+            sampler._backend = AerSimulator.from_backend(backend)
         return sampler
 
     @property

releasenotes/notes/fix_from_backend-a885d43caf390873.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    `from_backend` function of V2 primitives are fixed to accept AerSimulator
+    objects as input so that user can define simulator objects outside of
+    primitives