Customized Operator Error: "Fluxes not all equal"

[VinceNeede]

Hi @HaoyuanShi,

what exactly is your expected behavior?
I’m not sure about this, but I think the idea of conserving quantum numbers is to have operators with specific selection rules, so that when starting from a state with a given flux you can control how this is going to change. Your operator instead can either conserve the flux or increase (or decrease) it by 2.

You can bypass the error by changing the code as follows:

using ITensors, ITensorMPS

const A = [1 0 1; 0 0 0; 1 0 1]

x = siteind("S=1"; conserve_qns = true)  
T = itensor(A, x', dag(x); checkflux = false)

But, as I told you, I’m not sure if it would work later on.

Your operator conserves the parity instead, so you could try to use that as quantum number.

Example for parity qn

I’m not sure this is the proper way but it seems to work

using ITensors, ITensorMPS

alias(::SiteType"ParityQudit") = SiteType"Qudit"()

function ITensors.space(::SiteType"ParityQudit"; dim=2)
    return [QN("P", isodd(n) ? 1 : 0, 2) => 1 for n in 1:dim]
end

function ITensors.op(on::OpName, st::SiteType"ParityQudit", ds::Int...; kwargs...)
    return op(on, alias(st), ds...; kwargs...)
end

function ITensors.op(on::OpName, st::SiteType"ParityQudit", s1::Index, s_tail::Index...; kwargs...)
    rs = reverse((s1, s_tail...))
    ds = dim.(rs)
    opmat = op(on, st, ds...; kwargs...)
    return itensor(opmat, prime.(rs)..., dag.(rs)...)
end

function ITensors.op(::OpName"T", ::SiteType"ParityQudit", d::Int...; kwargs...)
    return [1 0 1;
        0 0 0;
        1 0 1]
end

x = siteind("ParityQudit"; dim=3)
T = op("T", x)

psi = MPS([1, 0, 1], (x,))
println(psi[1])
psi = apply([T], psi)
println(psi[1])

psi = MPS([0, 1, 0], (x,))
println(psi[1])
psi = apply([T], psi)
println(psi[1])

Also, if you are using spin 1 particles, consider using the S=1 site type directly.