  We propose and experimentally demonstrate an optimal non-unity gain Gaussian
scheme for partial measurement of an unknown coherent state that causes minimal
disturbance of the state. The information gain and the state disturbance are
quantified by the noise added to the measurement outcomes and to the output
state, respectively. We derive the optimal trade-off relation between the two
noises and we show that the trade-off is saturated by non-unity gain
teleportation. Optimal partial measurement is demonstrated experimentally using
a linear optics scheme with feed-forward.
