Secret Quantum Communication of a Reference Frame
Giulio Chiribella, Lorenzo Maccone, Paolo Perinotti · Physical Review Letters · 2007
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.