Fix bug in QuantumError.to_dict (Qiskit#1420)

qiskit/providers/aer/noise/errors/quantum_error.py

@@ -452,12 +452,21 @@ def error_term(self, position):
 
     def to_dict(self):
         """Return the current error as a dictionary."""
+        # Assemble noise circuits
+        instructions = []
+        for circ in self._circs:
+            circ_inst = []
+            for inst, qargs, _ in circ.data:
+                qobj_inst = inst.assemble()
+                qobj_inst.qubits = [circ.find_bit(q).index for q in qargs]
+                circ_inst.append(qobj_inst.to_dict())
+            instructions.append(circ_inst)
+        # Construct error dict
         error = {
             "type": "qerror",
             "id": self.id,
             "operations": [],
-            "instructions": [[op[0].assemble().to_dict() for op in circ.data]
-                             for circ in self._circs],
+            "instructions": instructions,
             "probabilities": list(self.probabilities)
         }
         return error

releasenotes/notes/fix_qerror_to_dict-13a7683ac4adddd4.yaml

@@ -0,0 +1,10 @@
+---
+fixes:
+  - |
+    Fixes a bug in :meth:`.QuantumError.to_dict` where N-qubit circuit
+    instructions where the assembled instruction always applied to
+    qubits ``[0, ..., N-1]`` rather than the instruction qubits. This
+    bug also affected device and fake backend noise models.
+
+    See `Issue 1415 <https://github.com/Qiskit/qiskit-aer/issues/1415>`__
+    for details.

test/terra/backends/aer_simulator/test_noise.py

@@ -223,19 +223,45 @@ def test_kraus_circuit_noise(self, method, device):
         """Test simulation with Kraus quantum errors in circuit."""
         backend = self.backend(method=method, device=device)
         shots = 1000
-        error1 = noise.amplitude_damping_error(0.05)
-        error2 = error1.tensor(error1)
+        error0 = noise.amplitude_damping_error(0.05)
+        error1 = noise.amplitude_damping_error(0.15)
+        error01 = error1.tensor(error0)
 
-        qc = QuantumCircuit(2)
-        qc.h(0)
-        qc.append(error1, [0])
-        qc.cx(0, 1)
-        qc.append(error2, [0, 1])
-        target_probs = qi.DensityMatrix(qc).probabilities_dict()
+        # Target Circuit 0
+        tc0 = QuantumCircuit(2)
+        tc0.h(0)
+        tc0.append(qi.Kraus(error0), [0])
+        tc0.cx(0, 1)
+        tc0.append(qi.Kraus(error01), [0, 1])
+        target_probs0 = qi.DensityMatrix(tc0).probabilities_dict()
 
-        # Add measurement
-        qc.measure_all()
-        result = backend.run(qc, shots=shots).result()
+        # Sim circuit 0
+        qc0 = QuantumCircuit(2)
+        qc0.h(0)
+        qc0.append(error0, [0])
+        qc0.cx(0, 1)
+        qc0.append(error01, [0, 1])
+        qc0.measure_all()
+
+        # Target Circuit 1
+        tc1 = QuantumCircuit(2)
+        tc1.h(1)
+        tc1.append(qi.Kraus(error0), [1])
+        tc1.cx(1, 0)
+        tc1.append(qi.Kraus(error01), [1, 0])
+        target_probs1 = qi.DensityMatrix(tc1).probabilities_dict()
+
+        # Sim circuit 1
+        qc1 = QuantumCircuit(2)
+        qc1.h(1)
+        qc1.append(error0, [1])
+        qc1.cx(1, 0)
+        qc1.append(error01, [1, 0])
+        qc1.measure_all()
+
+        result = backend.run([qc0, qc1], shots=shots).result()
         self.assertSuccess(result)
-        probs = {key: val / shots for key, val in result.get_counts(0).items()}
-        self.assertDictAlmostEqual(target_probs, probs, delta=0.1)
+        probs = [{key: val / shots for key, val in result.get_counts(i).items()}
+                 for i in range(2)]
+        self.assertDictAlmostEqual(target_probs0, probs[0], delta=0.1)
+        self.assertDictAlmostEqual(target_probs1, probs[1], delta=0.1)