Experimental logical gates in a reaction-diffusion medium: the XOR gate and beyond.

We exploit the particulars of diffusive wave front interactions in certain types of two-reactant reaction-diffusion medium to construct a laboratory prototype of an XOR gate. In the design, the values of the logic variables are represented by the presence or absence of a precipitate, "wires" are constructed of a substrate loaded gel, and the computation is based on diffusive wave dynamics. We also discuss implementation of AND gate and study a three-valued composition, derived from the gate dynamic, and discuss possible logics that could be derived from this composition.