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tokens: We thank Prateek Agrawal, Imre Bartos, Luke Kelley, Frans Pretorius, Jakub Scholtz, and Kip Thorne for extremely useful conversations on several aspects of our paper. L.R. is supported by NSF grants PHY-0855591 and PHY-1216270. Z.-Z.X. is supported in part by the Center of Mathematical Sciences and Applications, Harvard University.