Function localiser_statevec_calcExpecPauliStr

quest/src/core/localiser.cpp:2026–2062

qcomp localiser_statevec_calcExpecPauliStr(Qureg qureg, PauliStr str) {
    assert_localiserGivenStateVec(qureg);

    qcomp value = 0;

    // partition non-Id Paulis into prefix and suffix, since...
    // - prefix X,Y determine communication, because they apply bit-not to rank
    // - prefix Y,Z determine node-wide coefficient, because they contain rank-determined !=1 elements
    // - suffix X,Y,Z determine local amp coefficients
    // noting that when !qureg.isDistributed, all paulis will be in suffix
    auto [targsX, targsY, targsZ] = paulis_getSeparateInds(str);
    auto [prefixX, suffixX] = util_getPrefixAndSuffixQubits(targsX, qureg);
    auto [prefixY, suffixY] = util_getPrefixAndSuffixQubits(targsY, qureg);
    auto [prefixZ, suffixZ] = util_getPrefixAndSuffixQubits(targsZ, qureg);

    // embarrassingly parallel when there is only Z's in prefix (which themselves are handled afterward)
    if (prefixX.empty() && prefixY.empty()) {
        value = getStateVecExpecAllSuffixPauliStr(qureg, suffixX, suffixY, suffixZ);

    // otherwise, communication is pairwise exchange
    } else {
        auto prefixXY = util_getConcatenated(prefixX, prefixY);
        int pairRank = util_getRankWithQubitsFlipped(prefixXY, qureg);
        comm_exchangeAmpsToBuffers(qureg, pairRank);

        value = accel_statevec_calcExpecPauliStr_subB(qureg, suffixX, suffixY, suffixZ);
    }

    // apply this node's pre-factor
    value *= paulis_getPrefixPaulisElem(qureg, prefixY, prefixZ);

    // combine contributions from each node
    if (qureg.isDistributed)
        comm_reduceAmp(&value);

    return value;
}


qcomp localiser_densmatr_calcExpecPauliStr(Qureg qureg, PauliStr str) {