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Sources<br />
[1] http://www.sciencedirect.com/science/article/pii/0022519373900052<br />
[2] http://www.sciencedirect.com/science/article/pii/S0022519301925044<br />
[3] http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.98.038101<br />
[4] http://www.nature.com/news/2006/061204/full/news061204-10.html