  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 [K. 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.
