Update BaseEstimator import to support qiskit 2.0

[ElePT]

Summary

Qiskit/qiskit#13877 removes the non-versioned primitive V1 base class aliases. It looks like the AerSampler was already using the versioned base class, but AerEstimator wasn't. This PR updates the BaseEstimator import to BaseEstimatorV1.

Details and comments

The PR has been extended to handle more incompatibilities with 2.0.

[kt474]

LGTM, this is also causing issues in the qiskit-ibm-runtime unit tests

[wshanks]

In my testing with Aer and Qiskit 2.0, this line is also problematic. Do you want to address it here as well?

qiskit-aer/qiskit_aer/primitives/sampler.py

Line 27 in 0f2ff71

from qiskit.primitives.utils import final_measurement_mapping, init_circuit

The problem addressed by the fix in this PR and the line I pointed out are both problems with trying to do import qiskit_aer.primitives with Qiskit 2.0.

[wshanks]

Ah, there is a helpful test run that now picks up the Qiskit rc and encounters further issues:

   File "/opt/hostedtoolcache/Python/3.10.16/x64/lib/python3.10/site-packages/qiskit_aer/primitives/sampler.py", line 24, in <module>
    from qiskit.circuit.bit import Bit
ModuleNotFoundError: No module named 'qiskit.circuit.bit'

[ElePT]

A couple of unit tests with fractional gates were failing because of Qiskit/qiskit#14002. Given that this PR is blocking other efforts, I added a conditional skip for these tests until the issue is addresed.

[ElePT]

The estimator unit tests are currently failing because of another bug in qiskit: Qiskit/qiskit#14003. A fix is on the way. In the meantime, I have chosen to skip transpilation in that specific test, as it's the part of the code that triggers the qiskit bug.

[AlbertJP]

Try run_options = {'shots': None} to get exact values for the circuits that result in 0.

Alternative: change qc2 to give -1 instead of 0, then you won't have shot noise either. This can be done by changing np.pi / 2 to np.pi in the circuit, or [1] to [2] in the parameter.
(Estimator has no shot noise when the expectation value is exactly -1 or 1)

[ElePT]

Thanks! I haven't used the aer estimator in a while and forgot how to set these.

[AlbertJP]

This option is documented at

qiskit-aer/qiskit_aer/primitives/estimator.py

Lines 68 to 75 in 076fe00

	When combined with the approximation option, we get the expectation values as follows:
	* shots is None and approximation=False: Return an expectation value with sampling-noise w/
	warning.
	* shots is int and approximation=False: Return an expectation value with sampling-noise.
	* shots is None and approximation=True: Return an exact expectation value.
	* shots is int and approximation=True: Return expectation value with sampling-noise using a
	normal distribution approximation.

. Honestly, I don't understand why it did not work. I suspect a bug in the Estimator class in combination with skip_transpilation=True.

[ElePT]

It's likely. Something to look at in a follow-up.