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

* Set n_qubits from coupling map * Add a test and release notes * Fix lint --------- Co-authored-by: Hiroshi Horii <[email protected]>
Madhurima176 · May 31, 2023 · 0fc19fd · 0fc19fd
1 parent 3954278
commit 0fc19fd
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diff --git a/qiskit_aer/backends/aer_simulator.py b/qiskit_aer/backends/aer_simulator.py
@@ -940,5 +940,9 @@ def _set_method_config(self, method=None):
             # 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)
diff --git a/releasenotes/notes/fix-number-qubits-a417ca6afa64264f.yaml b/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>`__
diff --git a/test/terra/backends/aer_simulator/test_options.py b/test/terra/backends/aer_simulator/test_options.py
@@ -19,6 +19,9 @@
 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 @@ def test_statevector_memory(self):
             "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)