01113nas a2200169 4500008004100000022001400041245009200055210006900147260003900216300001100255490000700266520055700273100001700830700001700847700002400864856005500888 2004 eng d a1050-294700aQuantum reverse engineering and reference-frame alignment without nonlocal correlations0 aQuantum reverse engineering and referenceframe alignment without bAmerican Physical Societyc09/2004 a0303010 v703 aThe estimation of unknown qubit elementary gates and the alignment of reference frames are formally the same problem. Using quantum states made out of N qubits, we show that the theoretical precision limit for both problems, which behaves as 1∕N2, can be asymptotically attained with a covariant protocol that exploits the quantum correlation of internal degrees of freedom instead of the more fragile entanglement between distant parties. This cuts by half the number of qubits needed to achieve the precision of the dense covariant coding protocol.1 aBagan, Emili1 aBaig, Marià1 aMuñoz-Tapia, Ramon uhttp://link.aps.org/doi/10.1103/PhysRevA.70.030301