Function applyMultiStateControlledPauliGadget

quest/src/api/operations.cpp:1278–1306

}

void applyMultiStateControlledPauliGadget(Qureg qureg, int* controls, int* states, int numControls, PauliStr str, qreal angle) {
    validate_quregFields(qureg, __func__);
    validate_controlsAndPauliStrTargets(qureg, controls, numControls, str, __func__);
    validate_controlStates(states, numControls, __func__); // permits states==nullptr

    // a non-controlled str=I effects a global phase change (of -angle/2) which does not 
    // at all change a density matrix; the subsequent dagger operation would undo it,
    // which we avoid to preserve numerical accuracy
    if (paulis_isIdentity(str) && numControls == 0 && qureg.isDensityMatrix)
        return;

    // when numControls >= 1, all amps satisfying the control condition undergo a phase 
    // change of -angle/2, as if all non-control-qubits were targeted by exp(-angle/2)I,
    // which is sufficiently efficient using the existing gadget backend function

    qreal phase = util_getPhaseFromGateAngle(angle);
    auto ctrlVec = util_getVector(controls, numControls);
    auto stateVec = util_getVector(states, numControls); // empty if states==nullptr
    localiser_statevec_anyCtrlPauliGadget(qureg, ctrlVec, stateVec, str, phase);

    if (!qureg.isDensityMatrix)
        return;

    // conj(e^(i a P)) = e^(-i s a P)
    phase *= - paulis_getSignOfPauliStrConj(str);
    ctrlVec = util_getBraQubits(ctrlVec, qureg);
    str = paulis_getShiftedPauliStr(str, qureg.numQubits);
    localiser_statevec_anyCtrlPauliGadget(qureg, ctrlVec, stateVec, str, phase);
}

} // end de-mangler