On the optimality of individual entangling-probe attacks against BB84 quantum key distribution I. M. Herbauts, S. Bettelli, H. Hübel and M. Peev Eur. Phys. J. D, 46 2 (2008) 395-406 Published online: 09 January 2008 DOI: 10.1140/epjd/e2008-00002-x