[Unitary Hack]Partial Trace of the stabilizer state

[hongyehu]

The task of this bounty is to implement a function p_trace(state, positions) to return a partial trace (potentially a mixed state) of the stabilizer state, and the positions are the qubits that are not being traced out. References and preliminary code can be provided upon request.
A successful solution should include testing results

[Youngcius]

I would like to try to solve this issue.
Shall I implement this functionality in both PyClifford/utils.py and torchclifford.utils.py, right?

[Youngcius]

Hi @hongyehu Should the input state be in type of StabilizerState or an ndarray?

[hongyehu]

Hi @Youngcius, if you can implement the ptrace in pyclifford, and test it, I will consider you have the bounty :-) Also, there are already some implementations or hints in the repo. Please let me know if you need help. The input state is in type of StabilizerState

[atomgardner]

@hongyehu, is the idea here to adapt PTRACE from https://arxiv.org/pdf/quant-ph/0505036 to the stabilizer+destabilizer architecture?

[hongyehu]

Hi @atomgardner, yes, exactly! Btw, there is a branch called partial trace, where the canonicalization is implemented. And feel free to test and use it!