Highlighting Knowledge Gaps in Multipartite Quantum Entanglement that Could Put Quantum Cryptography in Jeopardy

For over 40 years, the Bennett–Brassard quantum key distribution protocol (BB84) has stood as a leading method for 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 a new type of attack that combines quantum and classical strategies. Clones from a quantum cloning machine are used in a way that could slip past BB84's standard defenses. For bi-partite and tri-partite systems, such clones are known to be separable, but it is unclear how this behavior extends to larger systems, such as entanglement between four clones. Our attack hinges on whether and under what conditions this separability persists, underscoring the need to better understand multi-qubit entanglement beyond the two- or three-qubit regime.