  A tripartite entangled state of bright optical field is experimentally
produced using an Einstein-Podolsky-Rosen entangled state for continuous
variables and linear optics. The controlled dense coding among a sender, a
receiver and a controller is demonstrated by exploiting the tripartite
entanglement. The obtained three-mode position correlation and relative
momentum correlation between the sender and the receiver and thus the
improvements of the measured signal to noise ratios of amplitude and phase
signals with respect to the shot noise limit are 3.28dB and 3.18dB
respectively. If the mean photon number $\bar{n}$ equals 11 the channel
capacity can be controllably inverted between 2.91 and 3.14. When $\bar{n}$ is
larger than 1.0 and 10.52 the channel capacities of the controlled dense coding
exceed the ideal single channel capacities of coherent and squeezed state light
communication.
