Remove TrotterCircuit cache

[stavros11]

Fixes #1357.

I believe the TrotterCircuit class is implementing a cache of the circuit implementing the Trotter decomposition returned by hamiltonian.circuit(dt) , to avoid recreating it if the users decides to use a different dt. However, this creates a ("secret") link between the circuits returned by hamiltonian.circuit that can confuse users, such as in #1357, and also complicates the code.

If allocating a circuit is not considered very costly, this cache probably does not provide much in terms of performance, therefore I decided to remove it to have cleaner code (and behavior).

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.84%. Comparing base (ad50a20) to head (784db2c).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1371      +/-   ##
==========================================
- Coverage   99.84%   99.84%   -0.01%     
==========================================
  Files          74       74              
  Lines       10806    10788      -18     
==========================================
- Hits        10789    10771      -18     
  Misses         17       17              

Flag	Coverage Δ
unittests	99.84% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[alecandido]

Thanks, I agree with your analysis, which is confirmed by the test you added.

[marekgluza]

If allocating a circuit is not considered very costly

• In my applications it was not costly and I was doing deepcopy all the time.

[marekgluza]

Thank you @stavros11 for this implementation of a solution.

When I was reading the code before I was wondering what was the use of TrotterCircuit and removing it resolves that question too.

[stavros11]

Thank you for reviewing. I updated the test in 784db2c. I will merge after the CI passes.