MPO divided by a number

ITensor Julia Questions
,
1

Hi,

I just encounter a small problem about MPO. Suppose I use a MPO to represent an operator A and I want to divide it by a number likeA/k. Then I have

using ITensors
let 
  n = 4
  s = siteinds("Qubit",n)
  rho = MPO(s,"Id") 
  rho = rho/2
end

Then I got the error

ERROR: MethodError: no method matching /(::MPO, ::Int64)
Closest candidates are:
  /(::Any, ::ChainRulesCore.AbstractThunk) at C:\Users\jisut\.julia\packages\ChainRulesCore\ksvfu\src\tangent_types\thunks.jl:35
  /(::Union{Int128, Int16, Int32, Int64, Int8, UInt128, UInt16, UInt32, UInt64, UInt8}, ::Union{Int128, Int16, Int32, Int64, Int8, UInt128, UInt16, UInt32, UInt64, UInt8}) at D:\Julia-1.7.2\share\julia\base\int.jl:93
  /(::StridedArray{P}, ::Real) where P<:Dates.Period at D:\Julia-1.7.2\share\julia\stdlib\v1.7\Dates\src\deprecated.jl:44
  ...

But if I use

using ITensors
let 
  n = 4
  s = siteinds("Qubit",n)
  rho = MPO(s,"Id") 
  rho = rho *0.5
end

It seems that there is no error. Is only multiplication of MPO with a number allowed?

Thanks

2

Yes, but probably only because we did not think to add that function. If you think it would be useful to have, please file an issue on our Github issue tracker and it would remind us to add it in the future, or you could submit a pull requst (PR) if you’d like to add this function yourself. Thanks!

1 Like
3

What version of ITensor are you using? That should be defined in the latest version:

julia> using ITensors

julia> s = siteinds("Qubit", 4);

julia> rho = MPO(s, "Id");

julia> rho / 2
MPO
[1] ((dim=2|id=149|"Qubit,Site,n=1")', (dim=2|id=149|"Qubit,Site,n=1"), (dim=1|id=522|"Link,l=1"))
[2] ((dim=2|id=854|"Qubit,Site,n=2")', (dim=2|id=854|"Qubit,Site,n=2"), (dim=1|id=984|"Link,l=2"), (dim=1|id=522|"Link,l=1"))
[3] ((dim=2|id=215|"Qubit,Site,n=3")', (dim=2|id=215|"Qubit,Site,n=3"), (dim=1|id=460|"Link,l=3"), (dim=1|id=984|"Link,l=2"))
[4] ((dim=2|id=689|"Qubit,Site,n=4")', (dim=2|id=689|"Qubit,Site,n=4"), (dim=1|id=460|"Link,l=3"))


julia> norm(rho)
4.0

julia> norm(rho / 2)
2.0

julia> using Pkg

julia> Pkg.status("ITensors")
      Status `~/.julia/environments/v1.7/Project.toml`
  [9136182c] ITensors v0.3.10
2 Likes
4

Oh I just found that I am still using the v0.2.15 version. I will update it to the latest version. Thanks for reminding me.

1 Like