01607nas a2200181 4500008004100000245002800041210002800069300001000097490000700107520115700114653001601271653001301287653001601300653001901316653002001335100001401355856005601369 2005 eng d00aMacroscopic Observables0 aMacroscopic Observables a221020 v713 aWe study macroscopic observables defined as the total value of a physical quantity over a collection of quantum systems. We show that previous results obtained for infinite ensemble of identically prepared systems lead to incorrect conclusions for finite ensembles. In particular, exact measurement of a macroscopic observable significantly disturbs the state of any finite ensemble. However, we show how this disturbance can be made arbitrarily small when the measurement are of finite accuracy. We demonstrate a general tradeoff between state disturbance and measurement coarseness as a function of the size of the ensemble. Using this tradeoff, we show that the histories generated by any sequence of finite accuracy macroscopic measurements always generate a consistent family in the absence of large scale entanglement, for sufficiently large ensembles. Hence, macroscopic observables behave ``classically" provided that their accuracy is coarser than the quantum correlation length-scale of the system. The role of these observable is also discussed in the context of NMR quantum information processing and bulk ensemble quantum state tomography.10aDecoherence10aEmergent10aFoundations10aQuantum Theory10aRenormalization1 aPoulin, D uhttps://www.physique.usherbrooke.ca/pages/node/6214