Single observable concurrence measurement without simultaneous copies
  We present a protocol that allows us to obtain the concurrence of any two qubit pure state by performing a minimal and optimal tomography of one of the subsystems through measuring a single observable of an ancillary four dimensional qudit. An implementation for a system of trapped ions is also proposed, which can be achieved with present day experimental techniques. 
A. Salles, F. de Melo, J. C. Retamal, R. L. de Matos Filho, and N.   Zagury