## Learn to Use ITensor

ITensor version 3 has not been released yet. This documentation is a preview only.

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# Measure a spinless fermion correlator from an MPS wavefunction

Contributed by Jon Spalding, UC Riverside— Oct 12, 2016

For more background on working with fermions in ITensor and Jordan-Wigner string, see the tutorial on fermions.

### Sample code:

```
// Given an MPS or IQMPS called "psi",
// constructed from a Spinless SiteSet "sites"
// Consider a pair of fermionic operators,
// Cdag and C, for spinless fermions.
// If Cdag(i) and C(j) operate at sites separated by some arbitrary
// distance, so that i < j, then in order to represent them
// as bosonic operators Adag(i) and A(j), we need to include
// a chain of "string" (F) operators that preserves the commutation
// relation for Cdag and C.
auto Adag_i = sites.op("Adag",i);
auto A_j = sites.op("A",j);
//'gauge' the MPS to site i
//any 'position' between i and j, inclusive, would work here
psi.position(i);
//index linking i to i+1:
auto ir = commonIndex(psi.A(i),psi.A(i+1),Link);
auto Corr = psi.A(i)*Adag_i*dag(prime(psi.A(i),Site,ir));
for(int k = i+1; k < j; ++k)
{
Corr *= psi.A(k);
Corr *= sites.op("F",k); //Jordan-Wigner string
Corr *= dag(prime(psi.A(k)));
}
Corr *= psi.A(j);
Corr *= A_j;
//index linking j to j-1:
auto jl = commonIndex(psi.A(j),psi.A(j-1),Link);
Corr *= dag(prime(psi.A(j),jl,Site));
auto result = Corr.real(); //or C.cplx() if expecting complex
```

### Notes:

`auto`

is a C++11 keyword that tells the compiler to automatically deduce the correct type from the expression on the right-hand side`Corr`

is the tensor holding our partial result for the correlator- The for loop builds into Corr the MPS tensors making up the MPS "transfer matrices" which carry correlations from i to j
- Since the operators are fermionic, we need to include Jordan-Wigner "string" (F) operators in between them.

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