Fig. 6

The optical set-up for the state comparison experiments. A coherent laser source is coupled into a single-mode optical-fiber, then attenuated down to single-photon level. The experiment interferometers are constructed of polarization maintaining fiber, therefore the polarization of the coherent light is aligned with the fast axis of the polarization maintaining fiber. The initial 50:50 beamsplitter splits the light intensity into two paths. Path 1 is a reference copy of the quantum coherent state, used to perform state tomography of the amplified state on the final beamsplitter. The signal and guess quantum coherent states, required for state comparison amplification, are created by splitting Path 2 into a further two paths. One path remains unchanged, simulating the unknown signal state, while the guess state is modified using a phase modulator, simulating a guess state from the amplifier. Gray beamsplitter cubes denote 50:50 splitting ratios, while the red denote 90:10 ratios (90% transmission). The faint beamsplitter is the second subtraction stage. The additional channel noise used in some of the experiments was provided by a light-emitting diode (LED) coupled into the signal arm