Detecting entanglement between copies in a 1-to-M cloner for quantum cryptography purposes

For over 40 years, the Bennett–Brassard quantum key distribution protocol (BB84) has stood as a well-established safeguard in secure communication, protected by the quantum no-cloning principle: any attempt to copy an unknown quantum state inevitably disturbs it. Given a general quantum circuit (often referred to as an attack), the amount of information that an eavesdropper can gain versus how much disturbance she causes is well known (Fuchs et al., 1997). In this poster, we present an attack based on the Gisin-Massar (Gisin, Massar, 1997) 1-to-M quantum cloner, which lies outside the standard information–disturbance framework. By analyzing the entanglement structure of the cloned states, we show how an absence of multipartite correlations can enable new forms of information leakage in BB84.