WIP: Feature propagatable error gens

[sserita]

This is a draft PR for integrating @ashenmill's work on end-of-circuit error generators, which will probably be paired with some sort of STIM forward simulator support.

I'm opening this PR early so that I can comment on specific lines of code.

[sserita]

@ashenmill Is there a specific version of STIM that is required for this? I tried subtracting PauliString objects like this on stim 1.13.0 and got unsupported operand type. Negation and addition do exist, but addition seems to just be concatenation. Am I missing something?

[sserita]

Should this not be ErG2.getP1()? Hamiltonian error generators should only have one basis element, right?

[sserita]

This line may fail if there is a complex phase because you would need to remove 2 characters.

[coreyostrove]

Quick note to future us to remove this mutable default arg for MultiGateDict.

[sserita]

Just noting some discrepancies between the notes and code. It doesn't really matter if we know the code is correct, but we will eventually want some documentation for this so ensuring the notes match is probably a good step.

[sserita]

Notes have this as

$$\large [H_P, C_{A,B}] = iC_{[A,P],B} + iC_{[P,B],A} $$

but what is coded is

$$\large [H_P, C_{A,B}] = iC_{[A,P],B} + iC_{\textcolor{red}{[B,P]},A} $$

I think the swapped order of the commutator introduces a sign difference, right? Which is correct?

[sserita]

Similar as [H,C]. The notes have

$$\large [H_P, A_{A,B}] = -iA_{[A,P],B} - iA_{A,[P,B]} $$

but coded is

$$\large [H_P, A_{A,B}] = -iA_{\textcolor{red}{[P,A]},B} - iA_{A,[P,B]} $$

Again, we have comm order swap in the first term with a potential sign difference. Which is correct?

[sserita]

Why do we add what looks like a dummy H label?

[sserita]

Notes have

$$\large [S_P, C_{A,B}] = -iA_{AP,BP} -iA_{PB,AP} -\frac{i}{2}\big(A_{{A,B}P,P} + A_{P,P{A,B}}\big) $$

but code is

$$\large [S_P, C_{A,B}] = -iA_{\textcolor{red}{PA},BP} -iA_{PB,\textcolor{red}{PA}} -\frac{i}{2}\big(A_{{A,B}P,P} + A_{P,P{A,B}}\big) $$

Similar to the commutators, this change in multiplication could come with a sign change (depending on whether $[P,A] = 0$ and $[P,B] = 0$).
(Also we are computing ${A,B}$ twice).

[sserita]

Note has $\frac{i}{2}$ for this weight but here it is just $\frac{1}{2}$.

[sserita]

First, I'll just say that this is way more readable with the intermediate variables. It would be even more readable if pVec* were named things like AP or acomAB or things like that.

Note has

$$\large \begin{align*} [C_{A,B}, C_{P,Q}] &= -i\big(A_{AP,QB} + A_{AQ,BP} - A_{BP,QA} - A_{BQ,PA}\big)\\ &-\frac{i}{2}\big(A_{[P,{A,B}],Q} + A_{[Q,{A,B}],P} + A_{[{P,Q},A],B} + A_{[{P,Q},B],A}\big)\\ &+\frac{i}{4}H_{[{A,B},{P,Q}]} \end{align*} $$

but coded up is

$$\large \begin{align*} [C_{A,B}, C_{P,Q}] &= -i\big(A_{AP,QB} + A_{AQ,\textcolor{red}{PB}} \textcolor{red}{+} A_{BP,QA} \textcolor{red}{+} A_{BQ,PA}\big)\\ &-\frac{i}{2}\big(A_{[P,{A,B}],Q} + A_{[Q,{A,B}],P} + A_{[{P,Q},A],B} + A_{[{P,Q},B],A}\big)\\ &+\frac{i}{4}H_{[{A,B},{P,Q}]} \end{align*} $$

Which is correct? (Also we are recomputing anticommutators)

[sserita]

Notes have a factor of $\frac{i}{4}$ but this is just $\frac{1}{4}$.

[sserita]

Notes have $-\frac{1}{2}$ but @ashenmill confirmed that the coded $-\frac{i}{2}$ is correct.

[sserita]

FYI, I have split off another branch from this called feature-nonstim-BCH-tools. The key difference is that the BCH tools in the branch will a) not rely on STIM and b) use existing label classes + defaultdicts to avoid the need to subclass out PropagatableErrorgen.

I'm not planning to make tools that cover all of the functionality Ashe covered, so likely some merge will be required, but I think my branch will have a pretty good idea of what these tools might look like when they are finally merged. I hope this will be at least a good discussion point for us in a week. :)