Function partialTraceOnPrefix

quest/src/core/localiser.cpp:1798–1818

void partialTraceOnPrefix(Qureg inQureg, Qureg outQureg, vector<int> ketTargs) {

    // all ketTargs (pre-sorted) are in the suffix, but one or more braTargs are in the prefix
    auto braTargs = util_getBraQubits(ketTargs, inQureg); // sorted
    auto allTargs = util_getSorted(ketTargs, braTargs);   // sorted
    auto sufTargs = getQubitsSwappedToMaxSuffix(inQureg, allTargs); // arbitrarily ordered

    // swap iniQureg's prefix bra-qubits into suffix, invoking communication
    anyCtrlMultiSwapBetweenPrefixAndSuffix(inQureg, {}, {}, sufTargs, allTargs);

    // use the second half of sufTargs as the pair targs, which are now all in the suffix,
    // to perform embarrassingly parallel overwriting of outQureg
    vector<int> pairTargs(sufTargs.begin() + ketTargs.size(), sufTargs.end()); // arbitrarily ordered
    accel_densmatr_partialTrace_sub(inQureg, outQureg, ketTargs, pairTargs);

    // restore the relative order of outQureg's remaining qubits using SWAPs
    reorderReducedQureg(inQureg, outQureg, allTargs, sufTargs);

    // undo the swaps on inQureg
    anyCtrlMultiSwapBetweenPrefixAndSuffix(inQureg, {}, {}, sufTargs, allTargs);
}


void localiser_densmatr_partialTrace(Qureg inQureg, Qureg outQureg, vector<int> targs) {