Function createMyHamiltonian

examples/extended/dynamics.cpp:26–64

 */

PauliStrSum createMyHamiltonian(int numQubits) {

    // we prepare a Heisenberg XYZ spin-ring Hamiltonian,
    // i.e. H = -1/2 sum( Jx XX + Jy YY + Jz ZZ + h Z )
    // upon all nearest neighbour qubits, with periodicity.
    // The coefficients must be real for H to be Hermitian
    // and ergo its time-evolution operator to be unitary,
    // although they must be represented with a qcomp type.
    vector<string> operators = {"XX", "YY", "ZZ", "Z"};
    vector<qcomp> coefficients = {.1, .2, .3, .4}; // Jx,Jy,Jz,h

    // we will populate the below vectors with 4*numQubits
    // elements which we could pre-allocate with .reserve,
    // but we might incur Donald Knuth's justified wrath.
    vector<PauliStr> allStrings;
    vector<qcomp> allCoeffs;

    // prepare all XX + YY + ZZ
    for (int p=0; p<3; p++) {
        for (int i=0; i<numQubits; i++) {

            // A_i, A_i+1
            vector<int> targs = {i, (i+1)%numQubits};
            PauliStr str = getPauliStr(operators[p], targs);

            allStrings.push_back(str);
            allCoeffs.push_back(coefficients[p]);
        }
    }

    // prepare Z
    for (int i=0; i<numQubits; i++) {
        allStrings.push_back(getPauliStr(operators[3], {i}));
        allCoeffs.push_back(coefficients[3]);
    }

    // must be freed by caller
    return createPauliStrSum(allStrings, allCoeffs);
}