We propose a scheme for enhancing the information capacity to more than 2 b
its in dense coding quantum communication that involves transmitting a pola
rization entangled twin photon and a subsequent joint measurement with a Be
ll-state analyzer. Although direct modulation of the timing position and op
tical frequency of the photon play no role in dense coding quantum communic
ation, we show that the frequency-dependent phase correlation of polarizati
on entangled twin photons can be used for this enhancement if photon twins
are generated through optical parametric down-conversion. With our proposed
Bell-state analyzer containing a nonlinear optical gate, the capacity can
be enhanced from the reported 2 bits to 3 bits by the local operation of a
frequency-dependent phase shift on the transmitted photon. [S1050-2947(99)0
4802-7].