Batched block

[GiggleLiu]

We have already got batched register for solving the sampling problem.

Also, I find pretty useful to have batched blocks, especially in simulating kernel quantum algorithms in

Schuld, Maria, and Nathan Killoran. "Quantum machine learning in feature Hilbert spaces." Physical review letters 122.4 (2019): 040504.

A possible prototype

using Yao

struct Batched{N, T, BT<:AbstractBlock{N, T}} <: CompositeBlock{N, T}
    blocks::Vector{<:BT}
end

Base.broadcastable(bb::Batched) = bb.blocks

function Yao.apply!(reg::AbstractRegister{B}, bb::Batched) where B
    B == length(bb.blocks) || error()
    Yao.apply!.(reg, bb)
    reg
end

zero_state(4, nbatch=10) |> bb |> state

The problem is how do we dispatch a Batched block? Or just disallow dispatch! a scalar for such a block?

[Roger-luo]

I think this is similar to what I proposed "Array" block before.

[GiggleLiu]

Let's add it after summer school and add some beautiful examples!

[Roger-luo]

1. I don't think we should let it be broadcastable, having two similar things apply! and apply!. is dangerous, especially when a simple for loop will do the job.
2. I was think about share type annotation, so the a batch of same block with parameters are contiguous on memory which improves cache efficiency and simplifies the update process of variational circuits, and it will fallback to the only static type Any which will not hoist the type annotation.

[GiggleLiu]

Just do it in a simple way. But please notice, apply!. gives you the reasonable result if and only if Batched is broadcatable.

Anyway, let’s not discuss good or bad before implementing one.