Alignment of reference frames with and without shared entanglement

Full text: A system of spin 1/2 particles can be used to establish a common reference frame between two distant parties. The optimal strategy exploits entanglement much in the same way as dense coding does. It requires that each party has the same number N of spins (2N in total) which have to be prepared beforehand in a specific maximally entangled state. The first part of this talk will review the key points of the optimal strategy. In the second part it will be shown that one can in principle attain the asymptotic fidelity of the optimal strategy with just half the number of spins (N in total) and, moreover, with no shared entanglement. The relation of this work with the estimation of unknown one qubit gates will be also discussed. (author)