Quantum mechanics imposes that any amplifier that works independently on the phase of the input signal has to introduce some excess noise. The impossibility of such a noiseless amplifier is rooted into unitarity and linearity of quantum evolution. A possible way to circumvent this limitation is to interrupt such evolution via a measurement, providing a random outcome able to herald a successful - and noiseless - amplification event. Here we show a successful realisation of such an approach; we perform a full characterization of an amplified coherent state using quantum homodyne tomography, and observe a strong heralded amplification, with about 6dB gain and a noise level significantly smaller than the minimal allowed for any ordinary phase-independent device.
@article{arxiv.0912.2065,
title = {Implementation of a non-deterministic optical noiseless amplifier},
author = {Franck Ferreyrol and Marco Barbieri and Remi Blandino and Simon Fossier and Rosa Tualle-Brouri and Philippe Grangier},
journal= {arXiv preprint arXiv:0912.2065},
year = {2015}
}