title: Security of quantum key distribution using d-level systems.
abstract: We consider two quantum cryptographic schemes relying on encoding the key into qudits, i.e., quantum states in a d-dimensional Hilbert space. The first cryptosystem uses two mutually unbiased bases (thereby extending the BB84 scheme), while the second exploits all d+1 available such bases (extending the six-state protocol for qubits). We derive the information gained by a potential eavesdropper applying a cloning-based individual attack, along with an upper bound on the error rate that ensures unconditional security against coherent attacks.
authors.affiliation: Ecole Polytechnique, CP 165, Université Libre de Bruxelles, 1050 Bruxelles, Belgium.
authors.forename: Nicolas J
authors.initials: NJ
authors.lastname: Cerf
authors.forename: Mohamed
authors.initials: M
authors.lastname: Bourennane
authors.forename: Anders
authors.initials: A
authors.lastname: Karlsson
authors.forename: Nicolas
authors.initials: N
authors.lastname: Gisin