AerSampler fails with a circuit and split registers

[t-imamichi]

Informations

• Qiskit Aer version: 0.11.2
• Python version: 3.9.15
• Operating system: 12.6.1

What is the current behavior?

#1670 raises an error with a circuit and split registers as follows.

from qiskit import QuantumCircuit, ClassicalRegister
from qiskit.primitives import Sampler as RefSampler
from qiskit_aer.primitives import Sampler as AerSampler

qc = QuantumCircuit(2)
cr1 = ClassicalRegister(1, 'cr1')
cr2 = ClassicalRegister(1, 'cr2')
qc.add_register(cr1)
qc.add_register(cr2)
qc.measure(0, 0)
qc.measure(1, 1)

for sampler in [RefSampler(), AerSampler()]:
    print(sampler.run(qc, shots=100).result())

aer 0.11.1

<qiskit.primitives.sampler.Sampler object at 0x11b93abb0>
SamplerResult(quasi_dists=[{0: 1.0, 1: 0.0, 2: 0.0, 3: 0.0}], metadata=[{'shots': 100}])

<qiskit_aer.primitives.sampler.Sampler object at 0x11b93ab80>
SamplerResult(quasi_dists=[{0: 1.0}], metadata=[{'shots': 100, 'simulator_metadata': {'parallel_state_update': 1, 'parallel_shots': 1, 'sample_measure_time': 9.0047e-05, 'noise': 'ideal', 'batched_shots_optimization': False, 'remapped_qubits': False, 'device': 'CPU', 'active_input_qubits': [0, 1], 'measure_sampling': True, 'num_clbits': 2, 'input_qubit_map': [[1, 1], [0, 0]], 'num_qubits': 2, 'method': 'stabilizer', 'fusion': {'enabled': False}}}])

aer 0.11.2

<qiskit.primitives.sampler.Sampler object at 0x113c32dc0>
SamplerResult(quasi_dists=[{0: 1.0, 1: 0.0, 2: 0.0, 3: 0.0}], metadata=[{'shots': 100}])

<qiskit_aer.primitives.sampler.Sampler object at 0x113c32e20>
Traceback (most recent call last):
  File "/Users/ima/tasks/2_2022/qiskit/aer/tmp/split_cr.py", line 17, in <module>
    print(sampler.run(qc, shots=100).result())
  File "/Users/ima/envs/dev39/lib/python3.9/site-packages/qiskit/primitives/primitive_job.py", line 50, in result
    return self._future.result()
  File "/usr/local/Cellar/python@3.9/3.9.15/Frameworks/Python.framework/Versions/3.9/lib/python3.9/concurrent/futures/_base.py", line 439, in result
    return self.__get_result()
  File "/usr/local/Cellar/python@3.9/3.9.15/Frameworks/Python.framework/Versions/3.9/lib/python3.9/concurrent/futures/_base.py", line 391, in __get_result
    raise self._exception
  File "/usr/local/Cellar/python@3.9/3.9.15/Frameworks/Python.framework/Versions/3.9/lib/python3.9/concurrent/futures/thread.py", line 58, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/Users/ima/envs/dev39/lib/python3.9/site-packages/qiskit_aer/primitives/sampler.py", line 138, in _call
    QuasiDistribution(
  File "/Users/ima/envs/dev39/lib/python3.9/site-packages/qiskit/result/distributions/quasi.py", line 71, in __init__
    raise ValueError(
ValueError: The input keys are not a valid string format, must either be a hex string prefixed by '0x' or a binary string optionally prefixed with 0b

Steps to reproduce the problem

What is the expected behavior?

No error

Suggested solutions

The main reason is Terra's QuasiDistribution cannot accept bitstrings with whitespaces due to split registers.
There are three opitions.

1. Reverts Fix SamplerResult num_clbits #1670
2. Revises QuasiDistribution to accept bitstrings with white spaces
3. Removes white spaces from bitstrings as follows.
   

   qiskit-aer/qiskit_aer/primitives/sampler.py

   Line 139 in 0f98dd4

   {k: v / shots for k, v in counts.items()},

{k.replace(" ", ""): v / shots for k, v in counts.items()},

[ikkoham]

Thank you. I'll fix this until next release (0.12).