量子測定の量子情報理論的研究

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The accessible information quantifies the amount of classical information that can be extracted from an ensemble of quantum states. Analogously, the informational power quantifies the amount of classical information that can be extracted by a quantum measurement.For both quantities, we provided upper and lower bounds that depend only on the dimension of the system, and we proved their tightness. In the case of symmetric informationally complete (SIC) ensembles and measurements, we provided stronger bounds and we proved their tightness for qubits and qutrits. From our upper bounds, we noticed, perhaps surprisingly, that the statistics generated by SIC ensembles or measurements in arbitrary dimension, though optimal for tomographic purposes, in fact never contains more than just one bit of information, the rest being constituted by completely random bits. On the other hand, from our lower bounds, we obtained an explicit strategy beating the so-called "pretty-good" one for the extraction of mutual information in the case of SIC ensembles and measurements.