Function applyMultiQubitMeasurementAndGetProb

quest/src/api/operations.cpp:1663–1696

}

qindex applyMultiQubitMeasurementAndGetProb(Qureg qureg, int* qubits, int numQubits, qreal* probability) {
    validate_quregFields(qureg, __func__);
    validate_targets(qureg, qubits, numQubits, __func__);

    // find the probability of all possible outcomes...
    qindex numProbs = powerOf2(numQubits);

    // by allocating a temp vector, and validating successful (since exponentially big!)
    vector<qreal> probs;
    auto callback = [&]() { validate_tempAllocSucceeded(false, numProbs, sizeof(qreal), __func__); };
    util_tryAllocVector(probs, numProbs, callback);

    // populate probs
    calcProbsOfAllMultiQubitOutcomes(probs.data(), qureg, qubits, numQubits); // harmlessly re-validates

    // we cannot meaningfully sample these probs if not normalised
    validate_measurementProbsAreNormalised(probs, __func__);

    // randomly choose an outcome
    qindex outcome = rand_getRandomMultiQubitOutcome(probs);
    *probability = probs[outcome];

    // map outcome to individual qubit outcomes
    auto qubitVec = util_getVector(qubits, numQubits);
    auto outcomeVec = vector<int>(numQubits);
    getBitsFromInteger(outcomeVec.data(), outcome, numQubits);

    // project to the outcomes, renormalising the surviving states
    (qureg.isDensityMatrix)?
        localiser_densmatr_multiQubitProjector(qureg, qubitVec, outcomeVec, *probability):
        localiser_statevec_multiQubitProjector(qureg, qubitVec, outcomeVec, *probability);

    return outcome;
}

qreal applyForcedMultiQubitMeasurement(Qureg qureg, int* qubits, int* outcomes, int numQubits) {
    validate_quregFields(qureg, __func__);