Learn to Use ITensor

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Measure Local Properties of an MPS

For matrix product states (MPS) representing the wavefunction of a quantum system, a common task is to measure expected values of local observables acting on each site or physical degree of freedom. This might be done, for example, after a DMRG calculation or at each step of a time-evolution code. To see how to do this in ITensor, let's start from the following example of measuring the "Sz" operator for each site of an MPS psi:

for j=1:length(psi)

  s = siteind(psi,j)
  val = scalar(psi[j]*op(s,"Sz")*dag(prime(psi[j],s)))

  println("$j $val")

Now let's discuss each part of the code above. The for loop runs from 1 up to the number of sites of the MPS (number of tensors of the MPS).

The next line orthogonalize!(psi,j) shifts the orthogonality center of the MPS to site number j. Practically speaking, this step is what lets us only use the tensor psi[j] when doing the measurement, ignoring the other MPS tensors. But to be more precise, we are not actually ignoring the other MPS tensors, it's just that because orthogonalize! brings the MPS into a canonical form, the other MPS tensors obey left- and right-canonical conditions so would cancel anyway even if we included them in the expectation value calculation.

The line s = siteind(psi,j) retrieves the site, or physical index of the jth MPS tensor. If we already have access to this Index such as through an array obtained from the siteinds function then we could also obtain it that way.

The line

val = scalar(psi[j]*op(s,"Sz")*dag(prime(psi[j],s)))

actually performs the computation of the expected value of the operator "Sz". The call to op(s,"Sz") makes the ITensor for the "Sz" operator, where here we are assuming that the Index s carries a physical tag type such that "Sz" is defined for this Index. (Examples could include the tag "S=1/2" or the tag "Electron".) The returned operator has two indices: s and s'. Contracting the operator with psi[j] contracts over the Index s. Then contracting with dag(prime(psi[j],s)) contracts over s' and the two bond indices of the MPS conecting to the jth MPS tensor. Finally, the call to scalar converts the resulting scalar-valued ITensor into a number which is stored into the variable val (either Float64 or ComplexF64).

Finally, the line println("$j $val") just prints the site number and the variable val.

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