Macroscopic observables

We 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 an 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 measurements are of finite accuracy. We demonstrate a general trade-off between state disturbance and measurement coarseness as a function of the size of the ensemble. Using this trade-off, 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.