Shashank Pandey, Zhang Jiang, Joshua Combes, Carlton M. Caves
An ideal phase-preserving linear amplifier is a deterministic device that adds the minimal amount of noise to an input signal consistent with the constraints imposed by quantum mechanics. A nondeterministic noiseless linear amplifier takes an input coherent state to an amplified coherent state, but only works part of the time. Such a device is better than noiseless, since the output has less noise than the amplified noise of the input coherent state; for this reason we refer to such devices as {\em immaculate}. Here we bound the working probabilities probabilistic and approximate immaculate amplifiers and construct theoretical models that achieve some of these bounds. Our chief conclusion is that the working probability of a phase-insensitive immaculate amplifier is very small in any phase-plane region where the device works with high fidelity.
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http://arxiv.org/abs/1304.3901
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