Typicality of pure states randomly sampled according to the Gaussian adjusted projected measure
Consider a mixed quantum mechanical state, describing a statistical ensemble in terms of an arbitrary density operator rho of low purity, tr rho(2) << 1, and yielding the ensemble averaged expectation value tr(rho A) for any observable A. Assuming that the given statistical ensemble rho is generated by randomly sampling pure states vertical bar psi > according to the corresponding so-called Gaussian adjusted projected measure (Goldstein et al. in J. Stat. Phys. 125:1197, 2006), the expectation value <psi vertical bar A vertical bar psi > is shown to be extremely close to the ensemble average tr(rho A) for the overwhelming majority of pure states vertical bar psi > and any experimentally realistic observable A. In particular, such a 'typicality' property holds whenever the Hilbert space H of the system contains a high dimensional subspace H+ subset of H with the property that all vertical bar psi > epsilon H+ are realized with equal probability and all other vertical bar psi > epsilon H are excluded.
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