Implement more decoherence functions/helpers

[TysonRayJones]

The following hypothetical channels offer optimised implementations compatible with QuEST's distribution strategy.

• mixPauliStr(Qureg, qreal p, PauliStr) which effects $\rho \rightarrow (1-p)\rho + p \hat{\sigma}\rho\hat{\sigma}^\dagger$ where $\hat{\sigma}$ is an any-target PauliStr. This corresponds to superoperator $S=(1-p)I\otimes I + p (\hat{\sigma}^* \otimes \hat{\sigma})$ which we would not instantiate (unlike how mixKrausMap() operates). Instead, we trivially prepare $\hat{\sigma}' = \hat{\sigma} \otimes \hat{\sigma}$ (in $O(1)$ bit operations), set $p' = \pm p$ (negative when $\hat{\sigma}$ contains an odd number of $Y$ operators) and require the backend to effect $\alpha_i$ via $\alpha_i \rightarrow p' \beta_i + (1-p)\alpha_i$ where $\beta_i$ is the pair amplitude induced by $\hat{\sigma}'$. Such a backend subroutine already exists as used by Pauli gadgets! 🎉

  We alas cannot extend this to a sum of PauliStr since if any differ in their prefix Paulis (beyond $\hat{X} \leftrightarrow \hat{Y}$ and $\hat{I} \leftrightarrow \hat{Z}$), then each node requires more amplitudes than can fit in its communication buffer.

• setUnitaryKrausMap(KrausMap, qreal* probs, CompMatr* unitaries) which initialises the KrausMap to $\{ \sqrt{1-\sum probs} I, \sqrt{probs[i]} unitaries[i] \}$ such that its subsequent operation via mixKrausMap effects $\rho \rightarrow (1-\sum probs) + \sum_i probs[i] u_i \rho u_i^\dagger $, i.e. a "random unitary channel" (convex combination of unitary maps).

Note a similar-spirited strategy to mixPauliStr for mixing a single unitary (as CompMatr) is not possible. Channel $\rho \rightarrow (1-p)\rho + p \hat{U}\rho\hat{U}^\dagger$ has superoperator $S=(1-p)I\otimes I + p (\hat{U}^* \otimes \hat{U})$. Avoiding instantiating matrix $\hat{U}^* \otimes \hat{U}$ (which would otherwise have similar performance to wrappingmixKrausMap()) while effecting $S$ in one step (so that term $(1-p)I\otimes I$ is handled in-place) requires tediously bespoke, if at all possible.