Kaonan Micadei, Kavan Modi, Lucas C. Celeri, Roberto M. Serra
Quantum mechanical systems can be used to out perform classical ones in several tasks. For instance, quantum correlations can be employed to beat the shot-noise limit in metrology protocols. Such parameter estimation methods are crucial for both advances in science and the development of technologies. Almost all quantum technologies operate with some level of noise and how quantum-enhancement fares in the presence of noise is still unclear. Here we show that, when a system in mixed multipartite quantum state is used to estimate a parameter, the information must be decoded by coherent interactions between the parts of the probe. Strangely, we find that the quantum Fisher information is adaptively additive for any pure entangled states, that is, no coherent processing is necessary. This leads to an operational interpretation for the ability of performing coherent interactions in parameter estimation and highlights a fundamental difference between mixed and pure states. This result has fundamental importance in the search for the source of quantum advantage and it also has practical relevance for the designing of new high-precision-measurement devices.
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http://arxiv.org/abs/1209.2731
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