Characterization of quantum devices

Using quantum tomography and a single entangled state it is possible to characterize completely a quantum device, a channel, or a measuring apparatus. The method is very robust to imperfections of the tomographers and of the input state (which more generally can be a "faithful" state), and can be made very efficient by max-likelihood methods specially designed for this purpose. Using this method with homodyne detection one can in principle achieve the first absolute characterization of a photocounter.