title: Secret quantum communication of a reference frame.
abstract: We propose quantum-cryptographic protocols to secretly communicate a reference frame--unspeakable information in the sense it cannot be encoded into a string of bits. Two distant parties can secretly align their Cartesian axes by exchanging N spin-1/2 particles, achieving the optimal accuracy 1/N. A possible eavesdropper cannot gain any information without being detected.
authors.affiliation: QUIT-Quantum Information Theory Group, Dipartimento di Fisica A. Volta Università di Pavia, via A. Bassi 6, I-27100 Pavia, Italy.
authors.forename: Giulio
authors.initials: G
authors.lastname: Chiribella
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Maccone
authors.forename: Paolo
authors.initials: P
authors.lastname: Perinotti