Abstract

In quantum state tomography N identically prepared copies of a quantum state are measured to reconstruct a density matrix describing the single particle state [1]. One purpose of reconstructing a density matrix is to allow the prediction of measurements that could have been made on the initial state. On the other hand, if only one measurement is of interest then performing that measurement on the each of the N copies of the state will yield the most accurate estimate. However, if the measurement choice is unknown the quantum states must be stored in a quantum memory until some later time. The question then becomes: how much memory is required?

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