Error in executing StateTomography jobs on real device

[HongyeY]

Information

• qiskit-ibm-provider version: 0.3.0
• Python version: 3.10.6
• Operating system: Windows 11

In addition, I'm using the latest version of qiskit_experiments on github with qiskit 0.41.0.

What is the current behavior?

I have an error when running StateTomography job from the qiskit_experiments library. The simulators work well but jobs from real backends keep failing. The error message indicates some I/O errors:

jobs and analysis raised exceptions [Experiment ID: b06fddb7-ffd0-48c4-8420-bc27ed8d5724]:Traceback (most recent call last):

  File "C:\Users\...\AppData\Roaming\Python\Python310\site-packages\qiskit_experiments\framework\experiment_data.py", line 791, in _add_job_data
    job_result = job.result()

  File "C:\Users\...\AppData\Roaming\Python\Python310\site-packages\qiskit_ibm_provider\job\ibm_circuit_job.py", line 247, in result
    self.wait_for_final_state(timeout=timeout)

  File "C:\Users\...\AppData\Roaming\Python\Python310\site-packages\qiskit_ibm_provider\job\ibm_circuit_job.py", line 635, in wait_for_final_state
    status = self.status()

  File "C:\Users\...\AppData\Roaming\Python\Python310\site-packages\qiskit_ibm_provider\job\ibm_circuit_job.py", line 355, in status
    api_response = self._runtime_client.job_get(self.job_id())["state"]

  File "C:\Users\...\AppData\Roaming\Python\Python310\site-packages\qiskit_ibm_provider\api\clients\runtime.py", line 159, in job_get
    response = self._api.program_job(job_id).get()

  File "C:\Users\...\AppData\Roaming\Python\Python310\site-packages\qiskit_ibm_provider\api\rest\program_job.py", line 56, in get
    return self.session.get(self.get_url("self")).json(cls=RuntimeDecoder)

  File "c:\Python\Python310\lib\site-packages\requests\models.py", line 900, in json
    return complexjson.loads(self.text, **kwargs)

  File "c:\Python\Python310\lib\json\__init__.py", line 359, in loads
    return cls(**kw).decode(s)

  File "c:\Python\Python310\lib\json\decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())

  File "c:\Python\Python310\lib\json\decoder.py", line 353, in raw_decode
    obj, end = self.scan_once(s, idx)

  File "C:\Users\...\AppData\Roaming\Python\Python310\site-packages\qiskit_ibm_provider\utils\json.py", line 296, in object_hook
    return _decode_and_deserialize(obj_val, load)[0]

  File "C:\Users\...\AppData\Roaming\Python\Python310\site-packages\qiskit_ibm_provider\utils\json.py", line 126, in _decode_and_deserialize
    return deserializer(buff)

  File "C:\Users\...\AppData\Roaming\Python\Python310\site-packages\qiskit_ibm_provider\qpy\interface.py", line 278, in load
    loader(  # type: ignore[no-untyped-call]

  File "C:\Users\...\AppData\Roaming\Python\Python310\site-packages\qiskit_ibm_provider\qpy\binary_io\circuits.py", line 1012, in read_circuit
    _read_instruction(

  File "C:\Users\...\AppData\Roaming\Python\Python310\site-packages\qiskit_ibm_provider\qpy\binary_io\circuits.py", line 214, in _read_instruction
    struct.unpack(

struct.error: unpack requires a buffer of 5 bytes

Steps to reproduce the problem

1. get the backend

provider = IBMProvider()
backend_auckland = provider.get_backend('ibm_auckland', instance='...')

2. construct an arbitratry circuit

qNum=6
qr = QuantumRegister(qNum)
cr = ClassicalRegister(qNum)
qc = QuantumCircuit(qr,cr)

qc.h(range(qNum))
for i in range(0,qNum-1,2):
    qc.cz(i,i+1)
for i in range(1,qNum-1,2):
    qc.cz(i,i+1)

3. run the state tomography job

from qiskit_experiments.library import StateTomography
initial_layout=[0,1,2,3,5,8]
qstexp = StateTomography(qc,physical_qubits=initial_layout,measurement_indices=[0,1,2])
job=qstexp.run(backend=backend_auckland, shots=32000)

[spratapsi]

I am having a similar issue with the ProcessTomography experiment.

It seems that the circuits are breaking the qpy encoding/decoding.

Minimal example that fails

from qiskit import QuantumCircuit
from qiskit_experiments.library import StateTomography
from qiskit_ibm_provider import qpy

def encode_decode(qc):
    with open('test.qpy', 'wb') as fd:
        qpy.dump(qc, fd)
    with open('test.qpy', 'rb') as fd:
        new_qc = qpy.load(fd)[0]
    return new_qc

# This throws an Exception
qc = QuantumCircuit(2)
tomography = ProcessTomography(circuit=qc)
qc_ = tomography.circuits()[0]
encode_decode(qc_)

The root cause
The problem seems to come from passing the tuples self._prep_indices and self._meas_indices to QuantumCircuit.barrier, in lines 224 and 234 of the definition of TomographyExperiment.circuits.

Here is a simple circuit with the same behaviour.

_prep_indices = (0, 1)

# Fails -- passing a tuple to barrier
qc = QuantumCircuit(2)
qc.barrier(_prep_indices)
encode_decode(qc)

Possible fix
I don't know if this bug belongs here or in the qiskit-experiments repository. If it's the latter, the problem can be solved by unpacking the tuples or converting them to lists.

# Proposed solution 1 -- unpack tuple
qc = QuantumCircuit(2)
qc.barrier(*_prep_indices)
encode_decode(qc)

# Proposed solution 2 -- convert to list
qc = QuantumCircuit(2)
qc.barrier(list(_prep_indices))
encode_decode(qc)

[wshanks]

Thanks, @spratapsi! I would say that this does belong in qiskit-experiments. I will make PR soon to fix this in tomography_experiment.py.