Function getPartialTrace

tests/utils/linalg.cpp:422–448

qmatrix getPartialTrace(qmatrix in, vector<int> targets) {
    DEMAND( in.size() > getPow2(targets.size()) );

    auto numTargs = targets.size();
    auto numQubits = getLog2(in.size());
    auto numTargVals = getPow2(numTargs);

    qmatrix out = getZeroMatrix(getPow2(numQubits - numTargs));

    for (qindex v=0; v<numTargVals; v++) {

        // prepare { |0>, I, I, |1>, ... }
        vector<qmatrix> matrices(numQubits, getIdentityMatrix(2));
        for (size_t t=0; t<numTargs; t++) {
            int bit = getBitAt(v, t);
            matrices[targets[t]] = {
                {bit? 0.:1.}, 
                {bit? 1.:0.}};
        }

        qmatrix ket = getKroneckerProduct(matrices);
        qmatrix bra = getConjugateTranspose(ket);
        out += bra * in * ket;
    }

    return out;
}


qmatrix getControlledMatrix(qmatrix matrix, int numCtrls) {