Fix barrier instructions in tomography experiments

qiskit_experiments/library/tomography/tomography_experiment.py

@@ -221,7 +221,7 @@ def circuits(self):
                 prep_circ = self._prep_circ_basis.circuit(prep_element, self._prep_physical_qubits)
                 circ.reset(self._prep_indices)
                 circ.compose(prep_circ, self._prep_indices, inplace=True)
-                circ.barrier(self._prep_indices)
+                circ.barrier(*self._prep_indices)
 
             # Add target circuit
             # Have to use compose since circuit.to_instruction has a bug
@@ -231,7 +231,7 @@ def circuits(self):
             # Add tomography measurement
             if meas_element:
                 meas_circ = self._meas_circ_basis.circuit(meas_element, self._meas_physical_qubits)
-                circ.barrier(self._meas_indices)
+                circ.barrier(*self._meas_indices)
                 circ.compose(meas_circ, self._meas_indices, meas_clbits, inplace=True)
 
             # Add metadata

releasenotes/notes/tomo-barriers-aae4aafedaca5c3d.yaml

@@ -0,0 +1,10 @@
+---
+fixes:
+  - |
+    Fix qpy serialization and deserialization of tomography experiments. The
+    barrier instructions in tomography experiments were created with the wrong
+    Python type which qpy did not support. This issue was most acute when using
+    `qiskit-ibm-provider` which submits circuits to the provider using qpy.
+    There could have been subtler issues with circuit timing using a different
+    provider if the barriers were not separating important circuit
+    instructions. See `#1060 <https://github.com/Qiskit/qiskit-experiments/issues/1060>`_.

test/library/tomography/test_process_tomography.py

@@ -13,18 +13,24 @@
 """
 ProcessTomography experiment tests
 """
+import io
 from test.base import QiskitExperimentsTestCase
+
 import ddt
 import numpy as np
-from qiskit import QuantumCircuit
+
+import qiskit.quantum_info as qi
+from qiskit import QuantumCircuit, qpy
 from qiskit.circuit.library import XGate, CXGate
 from qiskit.result import LocalReadoutMitigator
-import qiskit.quantum_info as qi
+
 from qiskit_aer import AerSimulator
 from qiskit_aer.noise import NoiseModel
+
 from qiskit_experiments.database_service import ExperimentEntryNotFound
 from qiskit_experiments.library import ProcessTomography, MitigatedProcessTomography
 from qiskit_experiments.library.tomography import ProcessTomographyAnalysis, basis
+
 from .tomo_utils import (
     FITTERS,
     filter_results,
@@ -90,6 +96,23 @@ def test_full_qpt_analysis_none(self):
         self.assertExperimentDone(expdata)
         self.assertFalse(expdata.analysis_results())
 
+    def test_circuit_serialization(self):
+        """Test simple circuit serialization"""
+        circ = QuantumCircuit(2)
+        circ.h(0)
+        circ.s(0)
+        circ.cx(0, 1)
+
+        exp = ProcessTomography(circ)
+        circs = exp.circuits()
+
+        qpy_file = io.BytesIO()
+        qpy.dump(circs, qpy_file)
+        qpy_file.seek(0)
+        new_circs = qpy.load(qpy_file)
+
+        self.assertEqual(circs, new_circs)
+
     def test_cvxpy_gaussian_lstsq_cx(self):
         """Test fitter with high fidelity threshold"""
         seed = 1234

test/library/tomography/test_state_tomography.py

@@ -13,14 +13,18 @@
 """
 StateTomography experiment tests
 """
+import io
 from test.base import QiskitExperimentsTestCase
 from math import sqrt
+
 import ddt
 import numpy as np
-from qiskit import QuantumCircuit
+
+import qiskit.quantum_info as qi
+from qiskit import QuantumCircuit, qpy
 from qiskit.circuit.library import XGate
 from qiskit.result import LocalReadoutMitigator
-import qiskit.quantum_info as qi
+
 from qiskit_aer import AerSimulator
 from qiskit_aer.noise import NoiseModel
 
@@ -191,6 +195,23 @@ def test_exp_circuits_measurement_indices(self, meas_qubits):
         fid = qi.state_fidelity(target_state, qi.Statevector(target_circ))
         self.assertGreater(fid, 0.99, msg="target_state is incorrect")
 
+    def test_circuit_serialization(self):
+        """Test simple circuit serialization"""
+        circ = QuantumCircuit(2)
+        circ.h(0)
+        circ.s(0)
+        circ.cx(0, 1)
+
+        exp = StateTomography(circ)
+        circs = exp.circuits()
+
+        qpy_file = io.BytesIO()
+        qpy.dump(circs, qpy_file)
+        qpy_file.seek(0)
+        new_circs = qpy.load(qpy_file)
+
+        self.assertEqual(circs, new_circs)
+
     @ddt.data([0], [1], [2], [0, 1], [1, 0], [0, 2], [2, 0], [1, 2], [2, 1])
     def test_full_exp_measurement_indices(self, meas_qubits):
         """Test subset state tomography generation"""