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A simple formula for pooling knowledge about a quantum system

Kurt Jacobs
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Abstract

When various observers obtain information in an independent fashion about a classical system, there is a simple rule which allows them to pool their knowledge, and this requires only the states-of-knowledge of the respective observers. Here we derive an equivalent quantum formula. While its realm of applicability is necessarily more limited, it does apply to a large class of measurements, and we show explicitly for a single qubit that it satisfies the intuitive notions of what it means to pool knowledge about a quantum system. This analysis also provides a physical interpretation for the trace of the product of two density matrices.

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Kurt Jacobs. 2005-03-24. A simple formula for pooling knowledge about a quantum system. https://doi.org/10.1103/physreva.72.044101

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