title: General theory of measurement with two copies of a quantum state.
abstract: We analyze the results of the most general measurement on two copies of a quantum state. We show that by using two copies of a quantum state it is possible to achieve an exponential improvement with respect to known methods for quantum state tomography. We demonstrate that mu can label a set of outcomes of a measurement on two copies if and only if there is a family of maps C_{micro} such that the probability Prob(micro) is the fidelity of each map, i.e., Prob(micro) = Tr[rhoC_{micro}(rho)]. Here, the map C_{micro} must be completely positive after being composed with the transposition (these are called completely copositive, or CCP, maps) and must add up to the fully depolarizing map. This implies that a positive operator valued measure on two copies induces a measure on the set of CCP maps (i.e., a CCP map valued measure).
authors: Departamento de Física, Juan José Giambiagi, FCEyN UBA, Pabellón 1, Ciudad Universitaria, 1428 Buenos Aires, Argentina.
authors: Ariel
authors: A
authors: Bendersky
authors: Juan Pablo
authors: JP
authors: Paz
authors: Marcelo Terra
authors: MT
authors: Cunha