Fix noise model construction for a backend with no T2 value for a qub…

…it (Qiskit#1912)

* Fix a bug failing to create device gate errors when t2==None while t1!=None for a qubit.

* Add reno

---------

Co-authored-by: Jun Doi <doichan@jp.ibm.com>

qiskit_aer/noise/device/models.py

@@ -360,8 +360,10 @@ def _device_thermal_relaxation_error(
 
 def _truncate_t2_value(t1, t2):
     """Return t2 value truncated to 2 * t1 (for t2 > 2 * t1)"""
-    if t1 is None or t2 is None:
+    if t1 is None:
         return t2
+    elif t2 is None:
+        return 2 * t1
     return min(t2, 2 * t1)
 
 

releasenotes/notes/fix-none-t2-handling-in-noise-model-5b712adf3569eccd.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fixed a bug where :meth:`~.NoiseModel.from_backend` raises an error when
+    a backend reports no T2 value (while it reports T1 value) for a qubit,
+    which was an oversight in the previous fix on None handling:
+    `#1818 <https://github.com/Qiskit/qiskit-aer/issues/1818>`__.
+    Fixed `#1896 <https://github.com/Qiskit/qiskit-aer/issues/1896>`__.

test/terra/noise/test_device_models.py

@@ -63,3 +63,10 @@ def test_basic_device_gate_errors_from_target_and_properties(self):
             self.assertEqual(name1, name2)
             self.assertEqual(tuple(qargs1), qargs2)
             self.assertEqual(err1, err2)
+
+    def test_basic_device_gate_errors_from_target_with_no_t2_value(self):
+        """Test if gate errors are successfully created from a target with qubits not reporting T2.
+        See https://github.com/Qiskit/qiskit-aer/issues/1896 for the details."""
+        target = FakeNairobiV2().target
+        target.qubit_properties[0].t2 = None
+        basic_device_gate_errors(target=target)