In the last step to get result from a quantum computer, measurement is required to collapse the quantum state. This can be done by the measuring a large ensemble average of same computed result. One realization I know is to use NMR to measure billions of spin. So, what is the minimum number of ensembles are required to get a 'good' result? Furthermore, suppose the number of one qubit requires O(1) ensemble, how many ensembles are required for n qubits system to have same accuracy? I suppose the answer should be O(n), but if it is O(2^n), then the realization of quantum computer still seems useless to me. Any explanation of the answer?