title: Experimental eavesdropping based on optimal quantum cloning.
abstract: The security of quantum cryptography is guaranteed by the no-cloning theorem, which implies that an eavesdropper copying transmitted qubits in unknown states causes their disturbance. Nevertheless, in real cryptographic systems some level of disturbance has to be allowed to cover, e.g., transmission losses. An eavesdropper can attack such systems by replacing a noisy channel by a better one and by performing approximate cloning of transmitted qubits which disturb them but below the noise level assumed by legitimate users. We experimentally demonstrate such symmetric individual eavesdropping on the quantum key distribution protocols of Bennett and Brassard (BB84) and the trine-state spherical code of Renes (R04) with two-level probes prepared using a recently developed photonic multifunctional quantum cloner [Lemr et al., Phys. Rev. A 85, 050307(R) (2012)]. We demonstrated that our optimal cloning device with high-success rate makes the eavesdropping possible by hiding it in usual transmission losses. We believe that this experiment can stimulate the quest for other operational applications of quantum cloning.
authors.affiliation: RCPTM, Joint Laboratory of Optics of Palacký University and Institute of Physics of Academy of Sciences of the Czech Republic, Faculty of Science, Palacký University 17. listopadu 12, 771 46 Olomouc, Czech Republic. bartkiewicz@jointlab.upol.cz
authors.forename: Karol
authors.initials: K
authors.lastname: Bartkiewicz
authors.forename: Karel
authors.initials: K
authors.lastname: Lemr
authors.forename: Antonín
authors.initials: A
authors.lastname: Cernoch
authors.forename: Jan
authors.initials: J
authors.lastname: Soubusta
authors.forename: Adam
authors.initials: A
authors.lastname: Miranowicz