Entanglement entropy and the determination of an unknown quantum state
  An initial unknown quantum state can be determined with a single measurement apparatus by letting it interact with an auxiliary, "Ancilla", system as proposed by Allahverdyan, Balian and Nieuwenhuizen [Phys. Rev. Lett. 92, 120402 (2004)]. In the case of two qubits, this procedure allows to reconstruct the initial state of the qubit of interest S by measuring three commuting observables and therefore by means of a single apparatus, for the total system S + A at a later time. The determinant of the matrix of the linear transformation connecting the measurements of three commuting observables at time t > 0 to the components of the polarization vector of S at time t = 0 is used as an indicator of the reconstructability of the initial state of the system S. We show that a connection between the entanglement entropy of the total system S + A and such a determinant exists, and that for a pure state a vanishing entanglement individuates, without a need for any measurement, those intervals of time for which the reconstruction procedure is least efficient. This property remains valid for a generic dimension of S. In the case of a mixed state this connection is lost. 
Gerardo Aquino and Filippo Giraldi