density matrix for batched register

[Xiu-zhe (Roger) Luo]

Hi, I up Yao from v0.6.5 to v0.7.4 today and I found that the density_matrix cannot handle BatchedArrayReg now as you discussed above. I had to go back to v0.6.5 and everything works. Is this an unsolved issue?

Originally posted by @LWKJJONAK in #334 (comment)

[Weikang_Li]

Hi, I posted this issue yesterday and now I've found a way to fix it.

Generally, I need to use density_matrix such as

reg_ = focus!(copy(reg) |> circuit, (nbit)) |> cpu
rdm = density_matrix(reg_).state

Now some properties changed and I don't want to change all my code written before, so I did this:

import YaoAPI.density_matrix

function dens_m(M::DensityMatrix)
    M.state
end
struct density_matrix_
    state::Array{ComplexF64, 3}
end
function density_matrix(reg::BatchedArrayReg)
    temp = density_matrix.(reg)
    density_matrix_(cat(dens_m.(temp)...;dims = 3))
end

In this way, everything works as before, and it even gets faster, from

@btime reg_ = focus!(copy(cu_x_train_yao) |> circuit, (nbit)) |> cpu
@btime temp = (density_matrix(reg_)).state;
931.037 ms (1705 allocations: 9.40 MiB)
671.466 μs (806 allocations: 162.72 KiB)

to

@btime reg_ = focus!(copy(cu_x_train_yao) |> circuit, (nbit)) |> cpu
@btime temp = (density_matrix(reg_)).state;
724.986 ms (2433 allocations: 9.59 MiB)
406.429 μs (1453 allocations: 152.80 KiB)

[Xiu-zhe (Roger) Luo]

yeah, the 0.7 version breaks some of the APIs, that's why it's a new minor version. But you can always pirate the API to add patches for old code. I'm closing this issue then