  We investigate the optimal tradeoff between information gained about an
unknown coherent state and the state disturbance caused by the measurement
process. We propose several optical schemes that can enable this task, and we
implement one of them, a scheme which relies on only linear optics and homodyne
detection. Experimentally we reach near optimal performance, limited only by
detection inefficiencies. In addition we show that such a scheme can be used to
enhance the transmission fidelity of a class of noisy channels.
