Efficient use of quantum resources for the transmission of a reference   frame
  We propose a covariant protocol for transmitting reference frames encoded on $N$ spins, achieving sensitivity $N^{-2}$ without the need of a pre-established reference frame and without using entanglement between sender and receiver. The protocol exploits the use of equivalent representations, which were overlooked in the previous literature. 
G. Chiribella, G. M. D'Ariano, P. Perinotti, M. F. Sacchi