Fix handling of None in noise model construction from BackendV2 (#1818)

Fixes `NoiseModel.from_backend` not to fail when calling it with a V2 backend having
faulty qubits such that T1 and T2 are `None` in the `target.qubit_properties`. 

* fix handling of None in noise model construction from BackendV2
* add reno
* simplify a bit
* update docs

qiskit_aer/noise/device/models.py

@@ -106,21 +106,26 @@ def basic_device_gate_errors(
     Args:
         properties (BackendProperties): device backend properties.
         gate_error (bool): Include depolarizing gate errors (Default: True).
-        thermal_relaxation (Bool): Include thermal relaxation errors
-                                   (Default: True).
+        thermal_relaxation (Bool): Include thermal relaxation errors (Default: True).
+                If no ``t1`` and ``t2`` values are provided (i.e. None) in ``target`` for a qubit,
+                an identity ``QuantumError` (i.e. effectively no thermal relaxation error)
+                will be added to the qubit even if this flag is set to True.
+                If no ``frequency`` is not defined (i.e. None) in ``target`` for a qubit,
+                no excitation is considered in the thermal relaxation error on the qubit
+                even with non-zero ``temperature``.
         gate_lengths (list): Override device gate times with custom
                              values. If None use gate times from
                              backend properties. (Default: None).
-        gate_length_units (str): Time units for gate length values in gate_lengths.
+        gate_length_units (str): Time units for gate length values in ``gate_lengths``.
                                  Can be 'ns', 'ms', 'us', or 's' (Default: 'ns').
         temperature (double): qubit temperature in milli-Kelvin (mK)
                               (Default: 0).
         warnings (bool): DEPRECATED, Display warnings (Default: None).
         target (Target): device backend target (Default: None). When this is supplied,
                          several options are disabled:
-                         `properties`, `gate_lengths` and `gate_length_units` are not used
+                         ``properties``, ``gate_lengths`` and ``gate_length_units`` are not used
                          during the construction of gate errors.
-                         Default values are always used for `warnings`.
+                         Default values are always used for ``warnings``.
 
     Returns:
         list: A list of tuples ``(label, qubits, QuantumError)``, for gates
@@ -339,6 +344,10 @@ def _device_thermal_relaxation_error(
     for qubit in qubits:
         t1, t2, freq = relax_params[qubit]
         t2 = _truncate_t2_value(t1, t2)
+        if t1 is None:
+            t1 = inf
+        if t2 is None:
+            t2 = inf
         population = _excited_population(freq, temperature)
         if first:
             error = thermal_relaxation_error(t1, t2, gate_time, population)
@@ -351,14 +360,15 @@ def _device_thermal_relaxation_error(
 
 def _truncate_t2_value(t1, t2):
     """Return t2 value truncated to 2 * t1 (for t2 > 2 * t1)"""
-    new_t2 = t2
-    if t2 > 2 * t1:
-        new_t2 = 2 * t1
-    return new_t2
+    if t1 is None or t2 is None:
+        return t2
+    return min(t2, 2 * t1)
 
 
 def _excited_population(freq, temperature):
     """Return excited state population from freq [GHz] and temperature [mK]."""
+    if freq is None or temperature is None:
+        return 0
     population = 0
     if freq != inf and temperature != 0:
         # Compute the excited state population from qubit frequency and temperature

qiskit_aer/noise/noise_model.py

@@ -435,8 +435,8 @@ def from_backend(
                 t1s = [prop.t1 for prop in all_qubit_properties]
                 t2s = [_truncate_t2_value(prop.t1, prop.t2) for prop in all_qubit_properties]
                 delay_pass = RelaxationNoisePass(
-                    t1s=t1s,
-                    t2s=t2s,
+                    t1s=[np.inf if x is None else x for x in t1s],  # replace None with np.inf
+                    t2s=[np.inf if x is None else x for x in t2s],  # replace None with np.inf
                     dt=dt,
                     op_types=Delay,
                     excited_state_populations=excited_state_populations,
@@ -474,10 +474,14 @@ def from_backend_properties(
         Args:
             backend_properties (BackendProperties): The property of backend.
             gate_error (Bool): Include depolarizing gate errors (Default: True).
-            readout_error (Bool): Include readout errors in model
-                                  (Default: True).
-            thermal_relaxation (Bool): Include thermal relaxation errors
-                                       (Default: True).
+            readout_error (Bool): Include readout errors in model (Default: True).
+            thermal_relaxation (Bool): Include thermal relaxation errors (Default: True).
+                If no ``t1`` and ``t2`` values are provided (i.e. None) in ``target`` for a qubit,
+                an identity ``QuantumError` (i.e. effectively no thermal relaxation error)
+                will be added to the qubit even if this flag is set to True.
+                If no ``frequency`` is not defined (i.e. None) in ``target`` for a qubit,
+                no excitation is considered in the thermal relaxation error on the qubit
+                even with non-zero ``temperature``.
             temperature (double): qubit temperature in milli-Kelvin (mK) for
                                   thermal relaxation errors (Default: 0).
             gate_lengths (Optional[list]): Custom gate times for thermal relaxation errors.

releasenotes/notes/fix-none-handling-in-noise-model-34fcc9a3e3cbdf6f.yaml

@@ -0,0 +1,10 @@
+---
+fixes:
+  - |
+    Fixed a bug in :meth:`~.NoiseModel.from_backend` that raised an error when
+    the backend has no T1 and T2 values (i.e. None) for a qubit in its qubit properties.
+    This commit updates :meth:`NoiseModel.from_backend` and :func:`basic_device_gate_errors`
+    so that they add an identity ``QuantumError`` (i.e. effectively no thermal relaxation error)
+    to a qubit with no T1 and T2 values for all gates acting on qubits including the qubit.
+    Fixed `#1779 <https://github.com/Qiskit/qiskit-aer/issues/1779>`__
+    and `#1815 <https://github.com/Qiskit/qiskit-aer/issues/1815>`__.

test/terra/noise/test_device_models.py

@@ -16,6 +16,7 @@
 
 from test.terra.common import QiskitAerTestCase
 
+from qiskit.providers import QubitProperties
 from qiskit.providers.fake_provider import FakeNairobi, FakeNairobiV2
 from qiskit_aer.noise.device.models import basic_device_gate_errors
 
@@ -33,6 +34,24 @@ def test_basic_device_gate_errors_from_target(self):
         self.assertEqual(len(errors_on_reset), 7)
         self.assertEqual(len(gate_errors), 40)
 
+    def test_basic_device_gate_errors_from_target_with_non_operational_qubits(self):
+        """Test if no thermal relaxation errors are generated for qubits with undefined T1 and T2."""
+        target = FakeNairobiV2().target
+        # tweak target to have non-operational qubits
+        faulty_qubits = (1, 2)
+        for q in faulty_qubits:
+            target.qubit_properties[q] = QubitProperties(t1=None, t2=None, frequency=0)
+        # build gate errors with only relaxation errors i.e. without depolarizing errors
+        gate_errors = basic_device_gate_errors(target=target, gate_error=False)
+        errors_on_sx = {qubits: error for name, qubits, error in gate_errors if name == "sx"}
+        errors_on_cx = {qubits: error for name, qubits, error in gate_errors if name == "cx"}
+        self.assertEqual(len(gate_errors), 40)
+        # check if no errors are added on sx gates on qubits without T1 and T2 definitions
+        for q in faulty_qubits:
+            self.assertTrue(errors_on_sx[(q,)].ideal())
+        # check if no error is added on cx gate on a qubit pair without T1 and T2 definitions
+        self.assertTrue(errors_on_cx[faulty_qubits].ideal())
+
     def test_basic_device_gate_errors_from_target_and_properties(self):
         """Test if the device same gate errors are produced both from target and properties"""
         errors_from_properties = basic_device_gate_errors(properties=FakeNairobi().properties())

test/terra/noise/test_noise_model.py

@@ -33,6 +33,7 @@
 from qiskit.circuit.library.generalized_gates import PauliGate
 from qiskit.circuit.library.standard_gates import IGate, XGate
 from qiskit.compiler import transpile
+from qiskit.providers import QubitProperties
 from qiskit.providers.fake_provider import (
     FakeBackend,
     FakeAlmaden,
@@ -258,6 +259,25 @@ def test_noise_model_from_lagos_v2(self):
         result = AerSimulator().run(circ, noise_model=noise_model).result()
         self.assertTrue(result.success)
 
+    def test_noise_model_from_backend_v2_with_non_operational_qubits(self):
+        """Test if possible to create a noise model from backend with non-operational qubits.
+        See issues #1779 and #1815 for the details."""
+        backend = FakeLagosV2()
+        # tweak target to have non-operational qubits
+        faulty_qubits = [0, 1]
+        for qubit in faulty_qubits:
+            backend.target.qubit_properties[qubit] = QubitProperties(t1=None, t2=None, frequency=0)
+
+        noise_model = NoiseModel.from_backend(backend)
+
+        circ = QuantumCircuit(2)
+        circ.h(0)
+        circ.cx(0, 1)
+        circ.measure_all()
+        circ = transpile(circ, backend, scheduling_method="alap")
+        result = AerSimulator().run(circ, noise_model=noise_model).result()
+        self.assertTrue(result.success)
+
     def test_noise_model_from_invalid_t2_backend(self):
         """Test if silently truncate invalid T2 values when creating a noise model from backend"""
         from qiskit.providers.models.backendproperties import BackendProperties, Gate, Nduv