An Extension to DNA Based Fredkin Gate Circuits: Design of Reversible   Sequential Circuits using Fredkin Gates
  In recent years, reversible logic has emerged as a promising computing paradigm having its applications in low power computing, quantum computing, nanotechnology, optical computing and DNA computing. The classical set of gates such as AND, OR, and EXOR are not reversible. Recently, it has been shown how to encode information in DNA and use DNA amplification to implement Fredkin gates. Furthermore, in the past Fredkin gates have been constructed using DNA, whose outputs are used as inputs for other Fredkin gates. Thus, it can be concluded that arbitrary circuits of Fredkin gates can be constructed using DNA. This paper provides the initial threshold to building of more complex system having reversible sequential circuits and which can execute more complicated operations. The novelty of the paper is the reversible designs of sequential circuits using Fredkin gate. Since, Fredkin gate has already been realized using DNA, it is expected that this work will initiate the building of complex systems using DNA. The reversible circuits designed here are highly optimized in terms of number of gates and garbage outputs. The modularization approach that is synthesizing small circuits and thereafter using them to construct bigger circuits is used for designing the optimal reversible sequential circuits. 
Himanshu Thapliyal and M.B Srinivas