A most compendious and facile quantum de Finetti theorem

In its most basic form, the finite quantum de Finetti theorem states that the reduced k -partite density operator of an n -partite symmetric state can be approximated by a convex combination of k -fold product states. Variations of this result include Renner's "exponential" approximation by "almost-product" states, a theorem which deals with certain triples of representations of the unitary group, and the result of D'Cruz [e-print quant-ph/0606139; Phys. Rev. Lett. 98, 160406 (2007)] for infinite-dimensional systems. We show how these theorems follow from a single, general de Finetti theorem for representations of symmetry groups, each instance corresponding to a particular choice of symmetry group and representation of that group. This gives some insight into the nature of the set of approximating states and leads to some new results, including an exponential theorem for infinite-dimensional systems.