Dense coding for continuous variables

A scheme to achieve dense quantum coding for the quadrature amplitudes of the electromagnetic field is presented. The protocol utilizes shared entanglement provided by nondegenerate parametric down-conversion in the limit of large gain to attain high efficiency. For a constraint in the mean number of photons (n) over bar associated with modulation in the signal channel, the channel capacity for dense coding is found to be In(1+(n) over bar+(n) over bar(2)), which always beats coherent-state communication and surpasses squeezed-stare communication fur (n) over bar>1. For (n) over bar >>1, the dense coding capacity approaches twice that of either scheme.