  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.
