Dense coding for continuous variables - art. no. 042302

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