Support dynamic circuit in BackendEstimator

[ikkoham]

Summary

This PR makes BackendEstimator supports dynamic circuit.

Details and comments

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[coveralls]

Pull Request Test Coverage Report for Build 5821920454

• 10 of 11 (90.91%) changed or added relevant lines in 1 file are covered.
• 288 unchanged lines in 18 files lost coverage.
• Overall coverage increased (+1.4%) to 87.256%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/primitives/backend_estimator.py	10	11	90.91%

Files with Coverage Reduction	New Missed Lines	%
crates/accelerate/src/sabre_swap/mod.rs	1	97.53%
qiskit/visualization/circuit/_utils.py	1	93.79%
qiskit/qpy/binary_io/circuits.py	2	93.05%
qiskit/pulse/library/waveform.py	3	93.75%
qiskit/transpiler/passes/utils/block_to_matrix.py	3	83.33%
crates/qasm2/src/lex.rs	4	90.89%
qiskit/transpiler/preset_passmanagers/builtin_plugins.py	4	97.04%
qiskit/transpiler/preset_passmanagers/level0.py	4	93.75%
qiskit/transpiler/preset_passmanagers/level1.py	4	96.49%
qiskit/transpiler/passes/optimization/consolidate_blocks.py	5	95.04%

Totals
Change from base Build 5684162640:	1.4%
Covered Lines:	74290
Relevant Lines:	85140

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💛 - Coveralls

[garrison]

Actually, it is not clear to me why circuit.num_qubits == observable.num_qubits should be relaxed to circuit.num_qubits >= observable.num_qubits in order to support dynamic circuits. Can you clarify?

[ikkoham]

@garrison Thanks. The intent of this relaxed condition is to support circuits with auxiliary qubits like the test example https://github.com/Qiskit/qiskit-terra/pull/9700/files#diff-ec629567417753086ce09aba87c8dc9f34ab946d903fe809200ba692a7ed8e2dR364

[garrison]

I see. But in that example, why not just specify the observable as "ZI", i.e., such that it is over all the qubits?

Otherwise, you presumably need some logic to determine what is an "auxiliary" qubit or not. For instance, is it considered an auxiliary qubit if it contains a Measure instruction somewhere? If so, what if the Measure is immediately followed by a Reset? Would one be prevented from taking expectation values over such qubits?

Another way of phrasing my question: With this PR, you could pass the same circuit multiple times along with observables of different numbers of qubits. How are these different observables expected to be handled within an Estimator implementation?

[ikkoham]

You are right. Certainly there is no way to distinguish if they are auxiliary qubits or not, so it may be a good idea to have the user explicitly enter I.

[qiskit-bot]

One or more of the the following people are requested to review this:

• @Qiskit/terra-core
• @ajavadia
• @ikkoham
• @levbishop
• @t-imamichi

[t-imamichi]

I come up with an idea to make a separate classical register for measurements. Could you take a look at it, @ikkoham?
ikkoham#78

[ikkoham]

I initially made this PR with the intention of supporting dynamic circuits, but I noticed that it is bug for circuits with classical registers, even if they are not dynamic. I hope it will be merged as soon as possible.

[t-imamichi]

OK. I review it asap

[t-imamichi]

When I ran the test locally, I found the following warning in test_job_size_limit_v1 and test_job_size_limit_v2. The denom value gets 0. It would be better to check zero division before expvals /= denom.

./Users/ima/tasks/3_2023/qiskit/terra/qiskit/primitives/backend_estimator.py:454: RuntimeWarning: invalid value encountered in divide
  expvals /= denom

[t-imamichi]

How is the final measurement treated?

1. Should the final measurement order be same as the observable order?
2. What if final measurements are applied to only q0 and q2, and q1 is left not measured? Should two-qubit observables be passed with the circuit?

[t-imamichi]

minor update of tests. ikkoham#79

[ikkoham]

Discussing internally.

[t-imamichi]

LGTM