How to build an MPO using 2-sites local operators?

[ryanlevy]

MPO construction from multi-site OpSum

I am interested in creating Hamiltonians built from multi-site terms which are not easily expressed as sums of (tensor products of) single-site basis operators. OpSum() supports multisite operators, but I am not sure about how to turn the resulting OpSum into an MPO, as the naive method—as in the following code—fails: sites = siteinds("Qubit",2) opsum = OpSum() add!(opsum,"CNOT",1,2) H = MPO(opsum,sites) with the last line producing an error. What is the correct syntax here?

It’s not directly supported, try to expand your operator into sums of local terms (Pauli/etc)

An example of the CNOT gate:

Question on creating an MPO

A very convenient way to make a controlled gate like CNOT as an MPO using ITensor is to use our “OpSum” system. Here’s an example: n = 10 # 10 qubits, no problem to do more s = siteinds("Qubit",n) i = 3 # i will be the "control" site j = 7 # j will be the site where the gate acts cnot = OpSum() cnot += "Proj0",i,"Id",j cnot += "Proj1",i,"X",j CNOT = MPO(cnot,s) The above is using the following way of writing the CNOT gate: CNOT_{i,j} = \ket{0}_i \bra{0}_i \otimes I_j + \ket…