Function cpu_densmatr_multiQubitProjector_sub

quest/src/cpu/cpu_subroutines.cpp:2393–2429

template <int NumQubits>
void cpu_densmatr_multiQubitProjector_sub(Qureg qureg, vector<int> qubits, vector<int> outcomes, qreal prob) {

    // this function is merely an optimisation to avoid calling the above
    // cpu_statevec_multiQubitProjector_sub() twice upon a density matrix;
    // pre- and post-multiply projector versions DO just call above.

    // qubits are unconstrained, and can include prefix qubits
    assert_numTargsMatchesTemplateParam(qubits.size(), NumQubits);

    // visit every amp, setting most to zero and multiplying the remainder by renorm
    qindex numIts = qureg.numAmpsPerNode;
    qreal renorm = 1 / prob;

    // binary value of targeted qubits in basis states which are to be retained
    qindex retainValue = getIntegerFromBits(outcomes.data(), outcomes.size());

    // use template param to compile-time unroll loops in getValueOfBits()
    SET_VAR_AT_COMPILE_TIME(int, numBits, NumQubits, qubits.size());

    #pragma omp parallel for if(qureg.isMultithreaded)
    for (qindex n=0; n<numIts; n++) {

        // i = global index of nth local amp
        qindex i = concatenateBits(qureg.rank, n, qureg.logNumAmpsPerNode);

        // r, c = global row and column indices of nth local amp
        qindex r = getBitsRightOfIndex(i, qureg.numQubits);
        qindex c = getBitsLeftOfIndex(i, qureg.numQubits-1);

        qindex v1 = getValueOfBits(r, qubits.data(), numBits);
        qindex v2 = getValueOfBits(c, qubits.data(), numBits);

        // multiply amp with renorm or zero if values disagree with given outcomes
        qcomp fac = renorm * (v1 == v2) * (retainValue == v1);
        qureg.cpuAmps[n] *= fac;
    }
}


INSTANTIATE_FUNC_OPTIMISED_FOR_NUM_TARGS( void, cpu_statevec_multiQubitProjector_sub, (Qureg qureg, vector<int> qubits, vector<int> outcomes, qreal prob) )