OpSum definition for new sitetype

ITensor Julia Questions
1

Hi,

I am trying to define a new sitetype which I call “SpinBoson”. It should be used to create sites with dimension 2*d, where d is the boson cutoff (essentially a qudit, where the operators should be Kronecker products of spin and boson operators).

Please see how I define a state and an OpSum:

using ITensors

ITensors.space(::SiteType"SpinBoson", d::Int) = 2*d # space dimension is twice the phonon cutoff
ITensors.state(::StateName"↓0", ::SiteType"SpinBoson", s::Index) = [if i==1 1.0 else 0.0 end for i=1:dim(s)] # ground state

""" Setup parameters """

N = 2 # number of sites
d = 6 # boson cutoff dimension

sites = [siteind("SpinBoson", d) for _ in 1:N] # site indices
states = ["↓0" for _ in 1:N] # ground state
psi = MPS(sites, states); # initialize state in ground state
@show psi

""" apply some operator """
function ITensors.op(::OpName"Ia", ::SiteType"SpinBoson", s::Index)
    d = Int(dim(s)/2)
    mat = zeros(d, d)
    for k in 1:(d - 1)
      mat[k+1, k] = √k
    end
    return kron(Matrix(I,2,2), mat)
end

os = OpSum()
os += 1.0,"Ia",1,"Ia",2
Op = MPO(os, sites)

psi_new = apply(Op, psi)

I get the error:

ERROR: MethodError: Cannot `convert` an object of type Matrix{Float64} to an object of type ITensor

What am I doing wrong here? I can define a single site operator “Ia” and construct an MPO from it, But the OpSum does not work for some reason.

Would be happy for some advice about that.
Thanks

2

If you use an op definition that includes the index s you’ll need to return an ITensor. If you leave out the index, then your example will work but will miss the necessary d value.
To include the d like in the example,

function ITensors.op(::OpName"Ia", ::SiteType"SpinBoson", d::Int)
    mat = zeros(d, d)
    for k in 1:(d - 1)
      mat[k+1, k] = √k
    end
    return kron(Matrix(I,2,2), mat)
end

you will need to build up the sitetype so that the dim d is being passed like the boson states (see for example here: ITensors.jl/src/lib/SiteTypes/src/sitetypes/boson.jl at main · ITensor/ITensors.jl · GitHub )

The quick fix for your operator is to return ITensor(kron(Matrix(I,2,2), mat), s, s')

1 Like
4

Thanks! It works.

1 Like