speciation.nv - the rate of 75 kilobits per second with a...

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Unformatted text preview: the rate of 75 kilobits per second with a transmission loss of slightly more than 50%; the rate reached 1.7 megabits for lossless transmission. One of the things that makes quantum cryptography work is that quantum infor- mation (that is, information stored in a quantum system) cannot be exactly copied. This is known as the no-cloning theorem 3 . A consequence is that the most obvious action for an eavesdropper who has managed to intercept a message containing key bits — to make a copy of it and send the original on to the intended party — is not an option. Although it is impossible to construct per- fect copies, approximate copies are allowed, but there are limits on how good the copies can be 4 . Grosshans et al. have explicitly demonstrated that their quantum crypto- graphic system is secure against an attack using the best possible coherent-state cloner. The use of continuous variables, rather than qubits, in quantum information and computing is an expanding area of research and shows great promise 5 . Until recently, results in this area had been purely theoretical, but with the experimental demonstration of quantum key distribution and teleportation using continuous vari- ables 6 , this field of quantum information has entered the laboratory, and may soon arrive at practical applications. ■ Mark Hillery is in the Department of Physics, Hunter College, City University of New York, 695 Park Avenue, New York, New York 10021, USA. e-mail: [email protected] 1. Grosshans, F. et al. Nature 421, 238–241 (2003). 2. Bennett, C. & Brassard, G. in Proc. IEEE Conf. Computers, Systems and Signal Processing 175–179 (IEEE, New York, 1984). 3. Wooters, W. K. & Zurek, W. H. Nature 299, 802–803 (1982). 4. Cerf, N. J., Ipe, A. & Rottenberg, X. Phys. Rev. Lett. 85, 1754–1757 (2000). 5. Braunstein, S. L. & Pati, A. K. Quantum Information Theory with Continuous Variables (Kluwer, Dordrecht, in the press). 6. Furusawa, A. et al. Science 282, 706–709 (1998). U ntil recently, the overriding credo for explaining how new species are formed has run as follows: first, a population of organisms splits into several subpopulations; once isolated from other members of their own kind, these subpopulations become adapted to local conditions; so, over millions of years, their descendants evolve into new species. This is ‘allopatric speciation’, a concept in which spatial separation comes first and genetic divergence follows, and which has dominated biological thinking for many decades. The alternative, ‘sympatric speciation’, in which new species are created within a single population, has long been seen as a heresy — to the extent that young biologists would risk their careers if they pro- posed that such a mechanism could occur 1 ....
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