Measurements

Functions for effecting destructive measurements. More...

Functions
qreal	applyForcedMultiQubitMeasurement (Qureg qureg, int *qubits, int *outcomes, int numQubits)
qreal	applyForcedQubitMeasurement (Qureg qureg, int target, int outcome)
qindex	applyMultiQubitMeasurement (Qureg qureg, int *qubits, int numQubits)
qindex	applyMultiQubitMeasurementAndGetProb (Qureg qureg, int *qubits, int numQubits, qreal *probability)
int	applyQubitMeasurement (Qureg qureg, int target)
int	applyQubitMeasurementAndGetProb (Qureg qureg, int target, qreal *probability)

Detailed Description

Functions for effecting destructive measurements.

Function Documentation

applyForcedMultiQubitMeasurement()

qreal applyForcedMultiQubitMeasurement	(	Qureg	qureg,
		int *	qubits,
		int *	outcomes,
		int	numQubits )

Note

Documentation for this function or struct is under construction!

Definition at line 1709 of file operations.cpp.

                                                                                               {
    validate_quregFields(qureg, __func__);
    validate_targets(qureg, qubits, numQubits, __func__);
    validate_measurementOutcomesAreValid(outcomes, numQubits, __func__);
 
    auto qubitList = lists_getList64(qubits, numQubits);
    auto outcomeList = lists_getList64(outcomes, numQubits);
 
    // ensure probability of the forced measurement outcome is not negligible
    qreal prob = calcProbOfMultiQubitOutcome(qureg, qubits, outcomes, numQubits); // harmlessly re-validates
    validate_measurementOutcomesProbNotZero(outcomes, numQubits, prob, __func__);
 
    // project to the outcome, renormalising the surviving states
    (qureg.isDensityMatrix)?
        localiser_densmatr_multiQubitProjector(qureg, qubitList, outcomeList, prob):
        localiser_statevec_multiQubitProjector(qureg, qubitList, outcomeList, prob);
 
    return prob;
}

Referenced by applyForcedMultiQubitMeasurement().

applyForcedQubitMeasurement()

qreal applyForcedQubitMeasurement	(	Qureg	qureg,
		int	target,
		int	outcome )

Note

Documentation for this function or struct is under construction!

Definition at line 1642 of file operations.cpp.

                                                                        {
    validate_quregFields(qureg, __func__);
    validate_target(qureg, target, __func__);
    validate_measurementOutcomeIsValid(outcome, __func__);
 
    // note that we do not merely invoke applyForcedMultiQubitMeasurement()
    // because we must validate the renormalising probability and
    // report this function's name during the error message
    qreal prob = calcProbOfQubitOutcome(qureg, target, outcome); // harmlessly re-validates
    validate_measurementOutcomeProbNotZero(outcome, prob, __func__);
 
    auto targList    = lists_getList64({target});
    auto outcomeList = lists_getList64({outcome});
 
    // project to the outcome, renormalising the surviving states
    (qureg.isDensityMatrix)?
        localiser_densmatr_multiQubitProjector(qureg, targList, outcomeList, prob):
        localiser_statevec_multiQubitProjector(qureg, targList, outcomeList, prob);
 
    return prob;
}

applyMultiQubitMeasurement()

qindex applyMultiQubitMeasurement	(	Qureg	qureg,
		int *	qubits,
		int	numQubits )

Note

Documentation for this function or struct is under construction!

Definition at line 1664 of file operations.cpp.

                                                                           {
    validate_quregFields(qureg, __func__);
    validate_targets(qureg, qubits, numQubits, __func__);
 
    qreal prob = 0; // ignored
 
    // below validates post-measurement and would report 'AndGetProb' function suffix. Eh!
    return applyMultiQubitMeasurementAndGetProb(qureg, qubits, numQubits, &prob);
}

Referenced by applyMultiQubitMeasurement().

applyMultiQubitMeasurementAndGetProb()

qindex applyMultiQubitMeasurementAndGetProb	(	Qureg	qureg,
		int *	qubits,
		int	numQubits,
		qreal *	probability )

Note

Documentation for this function or struct is under construction!

Definition at line 1674 of file operations.cpp.

                                                                                                         {
    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_tempListAllocSucceeded(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 qubitList = lists_getList64(qubits, numQubits);
    auto outcomeList = util_getConstantList(-1, numQubits);
    setToBitsOfInteger(outcomeList.data(), outcome, numQubits);
 
    // project to the outcomes, renormalising the surviving states
    (qureg.isDensityMatrix)?
        localiser_densmatr_multiQubitProjector(qureg, qubitList, outcomeList, *probability):
        localiser_statevec_multiQubitProjector(qureg, qubitList, outcomeList, *probability);
 
    return outcome;
}

Referenced by applyMultiQubitMeasurement(), and applyMultiQubitMeasurementAndGetProb().

applyQubitMeasurement()

int applyQubitMeasurement	(	Qureg	qureg,
		int	target )

Note

Documentation for this function or struct is under construction!

Definition at line 1606 of file operations.cpp.

                                                   {
    validate_quregFields(qureg, __func__);
    validate_target(qureg, target, __func__);
 
    qreal prob = 0; // ignored
    return applyQubitMeasurementAndGetProb(qureg, target, &prob); // harmlessly re-validates
}

applyQubitMeasurementAndGetProb()

int applyQubitMeasurementAndGetProb	(	Qureg	qureg,
		int	target,
		qreal *	probability )

Note

Documentation for this function or struct is under construction!

Definition at line 1614 of file operations.cpp.

                                                                                 {
    validate_quregFields(qureg, __func__);
    validate_target(qureg, target, __func__);
 
    // we do not assume state normalisation (that is posteriori checked),
    // so we must perform two reductions; one for each outcome. We choose
    // to re-enumerate the state (potentially doubling caching costs) to
    // avoid the nuisances/race-cons of parallel adding to two scalars.
    vector<qreal> probs(2);
    probs[0] = calcProbOfQubitOutcome(qureg, target, 0); // harmlessly re-validates
    probs[1] = calcProbOfQubitOutcome(qureg, target, 1); // " "
    validate_measurementProbsAreNormalised(probs, __func__);
 
    // randomly choose the outcome
    int outcome = rand_getRandomSingleQubitOutcome(probs[0]);
    *probability = probs[outcome];
 
    auto targList    = lists_getList64({target});
    auto outcomeList = lists_getList64({outcome});
 
    // collapse to the outcome
    (qureg.isDensityMatrix)?
        localiser_densmatr_multiQubitProjector(qureg, targList, outcomeList, *probability):
        localiser_statevec_multiQubitProjector(qureg, targList, outcomeList, *probability);
 
    return outcome;
}

Referenced by applyQubitMeasurement().