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Quantifying entanglement in macroscopic systems


Traditionally, entanglement was considered to be a quirk of microscopic objects that defied a common-sense explanation. Now, however, entanglement is recognized to be ubiquitous and robust. With the realization that entanglement can occur in macroscopic systems — and with the development of experiments aimed at exploiting this fact — new tools are required to define and quantify entanglement beyond the original microscopic framework.

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Figure 1: A way of generating entangled photons by using down conversion.
Figure 2: Separable states.
Figure 3: Susceptibility as a macroscopic witness of entanglement.


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I am grateful for funding from the Engineering and Physical Sciences Research Council, the Wolfson Foundation, the Royal Society and the European Union. My work is also supported by the National Research Foundation (Singapore) and the Ministry of Education (Singapore). I thank J. A. Dunningham, A. J. Leggett, D. Markham, E. Rieper, W. Son and M. Williamson for discussions of this and related subjects. W. Son's help with illustrations is also gratefully acknowledged.

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Vedral, V. Quantifying entanglement in macroscopic systems. Nature 453, 1004–1007 (2008).

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