Entanglement detection with bounded reference frames

Full text: Quantum experiments usually assume the existence of perfect, classical, reference frames, which allow for the specification of measurement settings (e.g. orientation of the Stern Gerlach magnet in spin measurements) with arbitrary precision. If the reference frames are 'bounded' (i.e. are quantum systems themselves, with a finite number of degrees of freedom), only limited precision can be attained. Using spin coherent states as bounded reference frames we have found the minimal size needed to violate local realism for entangled spin systems. For composite systems of spin-1/2 particles reference frames of very small size are sufficient for the violation; however, to see this violation for macroscopic entangled spins, the size of the reference frame must be at least quadratically larger than that of the spins. This gives a possible explanation for the non-observance of entanglement at macroscopic scales, that is, the emergence of classicality. (author)