ITensorCorrelators

This is a package for efficiently measuring n-point correlators of matrix product states (MPS), built on top of the ITensors.jl and ITensorMPS.jl libraries.

This package is currently not registered. You can install it with:

julia> using Pkg

julia> Pkg.add(url="https://github.com/ITensor/ITensorCorrelators.jl.git")

As an example, to compute the correlator <Sz_i Sz_j Sz_k Sz_l> on some set of sites i, j, k, l, you can use the correlator function:

using ITensors, ITensorMPS
using ITensorCorrelators

s = siteinds("S=1/2", 10)
psi = random_mps(s; linkdims=10)

c = correlator(psi, ("Sz", "Sz", "Sz", "Sz"), [(1, 3, 4, 5), (2, 3, 4, 5), (3, 4, 5, 10)])
c[(2, 3, 4, 5)]

This outputs a dictionary mapping the set of sites i, j, k, l to the resulting correlator <Sz_i Sz_j Sz_k Sz_l>, so in the last line above we are accessing the result for the correlator <Sz_2 Sz_3 Sz_4 Sz_5>.

Note that currently, when fermionic sites are being used, this package does not handle the case of mixed A- and C-operators acting on the same sites properly (i.e. it doesn't handle inputs (2, 3, 3, 5) properly for ("Adagdn", "Adagup", "Cup", "Cdn"), say compared to how that operator would be constructed in the OpSum to MPO converter in ITensorMPS.jl). This is not very common, and generally users should just use C-operators if they want to compute correlators involving fermionic creation and annhilation operators. When C-operators are used with fermionic sites the correlator function will automatically insert the correct Jordan-Wigner strings to handle fermionic statistics.