Measuring the MPS

function entropy(psi::MPS, bond::Int)

Compute von Neumann Entropy of a given MPS psi at bond.

function entropy(psi::MPS; kwargs...)

Compute von Neumann Entropies of a given MPS psi at all the bonds.

Optionally, for specific bonds, keyword argument bonds can be specified, e.g., bonds = [1, 2, 3].

function measure(::Type{ElT} = ComplexF64, psi::MPS, opten::ITensor)

Returns (complex / real) local expectation value (::ComplexF64 / ::Float64) of a given MPS psi::MPS. The operator opten::ITensor must be single-site operator.

For ElT = Float64, if the expectation value is complex, raises a warning!

function measure(::Type{ElT} = ComplexF64, psi::MPS, opstr::String, pos::Int)

Returns (complex / real) local expectation value (::ComplexF64 / ::Float64) of a given MPS psi::MPS for a given operator name (opstr::String) and a site index (pos::Int)

For ElT = Float64, if the expectation value is complex, raises a warning!

function measure(::Type{ElT} = ComplexF64, psi::MPS, opstr::String; kwargs...)

Returns (complex / real) local expectation values (::Vector{ComplexF64} / ::Vector{Float64}) of a given MPS psi::MPS for a given operator name (opstr::String) at all the sites.

Optionally, for specific sites, keyword argument sites can be specified, e.g., sites = [1, 2, 3].

For ElT = Float64, if the expectation value is complex, raises a warning!

function measure(::Type{ElT} = ComplexF64, psi::MPS, optens::Vector{ITensor})

Returns (complex / real) multi-site expectation value (::ComplexF64 / ::Float64) of a given MPS psi::MPS for a given vector of single-site operators (optens::Vector{ITensor}).

For ElT = Float64, if the expectation value is complex, raises a warning!

function measure(::Type{ElT} = ComplexF64, psi::MPS, oppairs::Vector{Pair{String, Int}};
                 isfermions::Bool = true)

Returns (complex / real) multi-site expectation value (::ComplexF64 / ::Float64) of a given MPS psi::MPS. oppairs::Vector{Pair{String, Int}} contains pairs of operator names (String) and site locations (Int). E.g., for <ψ|OᵢOⱼOₖ... |ψ>, oppairs = ["O" => i, "O" => j, "O" => k,...].

Fermionic JW strings are added automatically for fermionic operators if isfermions::Bool = true (default).

For ElT = Float64, if the expectation value is complex, raises a warning!

Example:

valueC = measure(psi, ["Cdag" => 2, "C" => 6, "Cdag" => 9, "C" => 12])
valueR = measure(Float64, psi, ["Cdag" => 2, "C" => 6, "Cdag" => 9, "C" => 12])