Function rightapplyPauliStrSum

quest/src/api/multiplication.cpp:660–683

}

void rightapplyPauliStrSum(Qureg qureg, PauliStrSum sum, Qureg workspace) {
    validate_quregFields(qureg, __func__);
    validate_quregFields(workspace, __func__);
    validate_quregIsDensityMatrix(qureg, __func__);
    validate_quregCanBeWorkspace(qureg, workspace, __func__);
    validate_pauliStrSumFields(sum, __func__);
    validate_pauliStrSumTargets(sum, qureg, __func__);

    // clone qureg to workspace, set qureg to blank
    localiser_statevec_setQuregToClone(workspace, qureg);
    localiser_statevec_initUniformState(qureg, 0);

    // post-multiply each term in-turn, mixing into output qureg, then undo using idempotency
    for (qindex i=0; i<sum.numTerms; i++) {
        PauliStr str =  paulis_getShiftedPauliStr(sum.strings[i], qureg.numQubits);
        qcomp factor = paulis_getSignOfPauliStrConj(str); // undoes transpose

        localiser_statevec_anyCtrlPauliTensor(workspace, {}, {}, str, factor);
        localiser_statevec_setQuregToWeightedSum(qureg, {1, sum.coeffs[i]}, {qureg, workspace});
        localiser_statevec_anyCtrlPauliTensor(workspace, {}, {}, str, factor);
    }

    // workspace -> qureg, and qureg -> sum * qureg
}

} // end de-mangler