ProcessTomography analysis gives UnboundLocalError: local variable 'num_cond' referenced before assignment

[kevinsung]

Informations

• Qiskit Experiments version: 0ce3f85
• Python version: 3.10.11
• Operating system: Arch Linux

What is the current behavior?

Steps to reproduce the problem

1. Run a ProcessTomography experiment and save the data to the database service

from qiskit import QuantumCircuit, QuantumRegister
from qiskit_experiments.library import ProcessTomography
from qiskit_ibm_provider import IBMProvider

hub = 'ibm-q-internal'
group = 'deployed'
project ='default'

provider = IBMProvider(instance=f"{hub}/{group}/{project}")
backend = provider.get_backend("ibm_sherbrooke")

register = QuantumRegister(2, name="q")
target_circuit = QuantumCircuit(register)
a, b = register
target_circuit.h(a)
target_circuit.cnot(a, b)

experiment = ProcessTomography(target_circuit)
device_data = experiment.run(backend, shots=10000)
device_data.save()

2. Load the data from the service and attempt to produce analysis results

from qiskit_experiments.framework.experiment_data import ExperimentData

service = ExperimentData.get_service_from_backend(backend)
data = ExperimentData.load("4f939335-6fe0-4711-b47e-e7c6a14afb0c", service)
experiment = ProcessTomography(target_circuit)
data = experiment.analysis.run(data)
data.block_for_results()

3. Observe error

Analysis callback failed [Experiment ID: 1ccd621b-cdb8-4c16-b575-db350cbf99d2][Analysis Callback ID: d313440656894aecb8c2338ac6f23ecd]:
Traceback (most recent call last):
  File "/home/kjs/projects/qiskit-experiments/qiskit_experiments/framework/experiment_data.py", line 992, in _run_analysis_callback
    callback(self, **kwargs)
  File "/home/kjs/projects/qiskit-experiments/qiskit_experiments/framework/base_analysis.py", line 169, in run_analysis
    results, figures = analysis._run_analysis(expdata)
  File "/home/kjs/projects/qiskit-experiments/qiskit_experiments/library/tomography/tomography_analysis.py", line 192, in _run_analysis
    outcome_data, shot_data, meas_data, prep_data = tomography_fitter_data(
  File "/home/kjs/projects/qiskit-experiments/qiskit_experiments/library/tomography/fitters/fitter_data.py", line 122, in tomography_fitter_data
    if num_cond:
UnboundLocalError: local variable 'num_cond' referenced before assignment

What is the expected behavior?

Should work.

Suggested solutions

Weird thing I noticed is that if I add the following line, which should be unnecessary, then there is no error, but also no analysis results are produced (data.analysis_results() is an empty list)

data.add_jobs([provider.retrieve_job(job_id) for job_id in data.job_ids])

[kevinsung]

Ok specifying the provider fixes the issue. The error message should be made more helpful though.

from qiskit_experiments.framework.experiment_data import ExperimentData

service = ExperimentData.get_service_from_backend(backend)
data = ExperimentData.load("4f939335-6fe0-4711-b47e-e7c6a14afb0c", service, provider=provider)
experiment = ProcessTomography(target_circuit)
data = experiment.analysis.run(data)
data.block_for_results()

[ItamarGoldman]

the data is retrieved using _retrieve_data() in ExperimentData class. If provider is not passed when loading the data, the method _retrieve_data() return nothing.
I will add a meaningful message so the user will know.

Edit:
From what I can see there is no check to see if the ExperimentData object contain any data before running an analysis. I think we should check if there is data before calling the analysis.