Set the number of qubits with the coupling map (#1825)

* Set n_qubits from coupling map

* Add a test and release notes

* Fix lint

---------

Co-authored-by: Hiroshi Horii <hhorii@users.noreply.github.com>

qiskit_aer/backends/aer_simulator.py

@@ -940,5 +940,9 @@ class AerSimulator(AerBackend):
             # Clear options to default
             description = None
             n_qubits = None
+
+        if self._configuration.coupling_map:
+            n_qubits = max(list(map(max, self._configuration.coupling_map))) + 1
+
         self._set_configuration_option("description", description)
         self._set_configuration_option("n_qubits", n_qubits)

releasenotes/notes/fix-number-qubits-a417ca6afa64264f.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fix an issue even if the number of qubits is set by a coupling map 
+    or device's configuration, when the simulation method is configured, 
+    the number of qubits is overwritten in accordance with the method. 
+    Fixed `#1769 <https://github.com/Qiskit/qiskit-aer/issues/1769>`__

test/terra/backends/aer_simulator/test_options.py

@@ -19,6 +19,9 @@ from qiskit_aer.noise import NoiseModel
 from test.terra.backends.simulator_test_case import SimulatorTestCase, supported_methods
 from qiskit.quantum_info.random import random_unitary
 from qiskit.quantum_info import state_fidelity
+from qiskit.providers.fake_provider import FakeMontreal
+
+from qiskit_aer import AerSimulator
 
 
 @ddt
@@ -282,3 +285,20 @@ class TestOptions(SimulatorTestCase):
             "Required memory: {}".format(2 ** (n - 20) * 16) in result.results[0].status
         )
         self.assertTrue("max memory: {}".format(max_memory_mb) in result.results[0].status)
+
+    @data(
+        "automatic",
+        "stabilizer",
+        "statevector",
+        "density_matrix",
+        "matrix_product_state",
+        "extended_stabilizer",
+        "unitary",
+        "superop",
+    )
+    def test_num_qubits(self, method):
+        """Test number of qubits is correctly checked"""
+
+        num_qubits = FakeMontreal().configuration().num_qubits
+        backend = AerSimulator.from_backend(FakeMontreal(), method=method)
+        self.assertGreaterEqual(backend.configuration().num_qubits, num_qubits)