make final_density_matrix work with classical control

[0x177]

Fixes #5915

Largely a direct implementation of the idea of @daxfohl in #5915.

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[0x177]

i appear to have missed handling unused qubits

[daxfohl]

looks good so far, just a couple points

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.50%. Comparing base (9672295) to head (5382b11).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7610   +/-   ##
=======================================
  Coverage   97.50%   97.50%           
=======================================
  Files        1103     1103           
  Lines       99709    99732   +23     
=======================================
+ Hits        97222    97246   +24     
+ Misses       2487     2486    -1     

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[daxfohl]

You can use check/format-incremental --apply to autoformat, and check/pylint-changed-files to see if there are linting errors locally (but there's no auto-fix flag on that one).

[daxfohl]

lgtm % a couple nits.

[0x177]

there is one more issue that I am not sure how to handle properly: in the case of classical control, if the qubit_order has 2 qubits, but the circuit only uses 1, the function will fail because the circuit_like will not take the unused qubit into account.

[daxfohl]

there is one more issue that I am not sure how to handle properly: in the case of classical control, if the qubit_order has 2 qubits, but the circuit only uses 1, the function will fail because the circuit_like will not take the unused qubit into account.

Hmm, so what happens? Does the simulation output tensor include a dimension for the 2nd qubit even though it's not in the circuit? I'd say that sounds like a bug in the simulate method, not a bug here. I think it's fine to leave that case unhandled. If anyone encounters that problem, then we can figure out how they expect it to be handled at that point.

[0x177]

there is one more issue that I am not sure how to handle properly: in the case of classical control, if the qubit_order has 2 qubits, but the circuit only uses 1, the function will fail because the circuit_like will not take the unused qubit into account.

Hmm, so what happens? Does the simulation output tensor include a dimension for the 2nd qubit even though it's not in the circuit? I'd say that sounds like a bug in the simulate method, not a bug here. I think it's fine to leave that case unhandled. If anyone encounters that problem, then we can figure out how they expect it to be handled at that point.

yes, something like that. It eventually results in a reshapring error

[daxfohl]

lgtm cc @pavoljuhas

[pavoljuhas]

LGTM with a couple of tweaks in 2828ba9 and 2940ee8