Perfect quantum amplification is not possible. Either noise must be added, or we must use a scheme which does not work all of the time. One such scheme is the State Comparison Amplifer (SCAMP) [1,2]. This provides almost perfect amplification in a limited basis at a high rate and could form the basis of a repeater for communication systems.


[1] E. Eleftheriadou et al.,  Phys. Rev. Lett. 111, 213601 (2013) [ArXiv:1311.5730]
[2] R. J. Donaldson et al., Experimental Implementation of a Quantum Optical State Comparison Amplifier, Submitted.