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International Conference on Advances in Computing, Communications and Informatics (ICACCI)

Reversible circuit optimization using modified toffoli templates

Mridula J Deshpande, H. V. Jayashree, V.K. Agrawal

Published Sep 1, 2017• 0 citations
Publisher

Abstract

Reversible gates are gaining popularity in the field of circuit synthesis due to their property of having lower power loss and reduced dissipation of power as heat. These low power design models need to have better optimization properties. In this paper, we have ventured into the template and rule based optimization process. While most reversible netlists are built using the standard Toffoli gates, the proposed Modified Toffoli Gate has been used to achieve optimization. In this paper, the quantum realization of mixed control Modified Toffoli Gate and negative control Modified Toffoli Gates are proposed. Few templates and rules for transforming the original Toffoli network to the Modified Toffoli network are also proposed. This optimization yields a better quantum cost and gate count when compared to that of it's Toffoli counterpart.

DOI
10.1109/icacci.2017.8125864

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Reversible gates are gaining popularity in the field of circuit synthesis due to their property of having lower power loss and reduced dissipation of power as heat. These low power design models need to have better optimization properties. In this paper, we have ventured into the template and rule based optimization process. While most reversible netlists are built using the standard Toffoli gates, the proposed Modified Toffoli Gate has been used to achieve optimization. In this paper, the quantum realization of mixed control Modified Toffoli Gate and negative control Modified Toffoli Gates are proposed. Few templates and rules for transforming the original Toffoli network to the Modified Toffoli network are also proposed. This optimization yields a better quantum cost and gate count when compared to that of it's Toffoli counterpart.
Optimal synthesis of reversible circuits is a very hard task. In 2010, a method and a tool of practical usage for finding optimal circuits built from multi-control Toffoli gates for any 4-bit reversible specification were finally developed. In 2011 we published an extension to the algorithm, which allows to reduce the quantum cost of the resulting circuits. In this paper we present another extension to this approach. Namely, we have extended the reversible gate library to mixed-polarity multi-control Toffoli gates (i.e. with both positive and negative controls). Our experimental results for the known reversible benchmarks show that using mixed-polarity Toffoli gates gives significant savings in gate count. The paper presents standard computational experiments for circuit synthesis. These results have implications in testing synthesis algorithms for reversible mixed-polarity circuits and quantum circuits.
Optimal synthesis of reversible circuits is a very hard task. In 2010, a method and a tool of practical usage for finding optimal circuits built from multi-control Toffoli gates for any 4-bit reversible specification were finally developed. In 2011 we published an extension to the algorithm, which allows to reduce the quantum cost of the resulting circuits. In this paper we present another extension to this approach. Namely, we have extended the reversible gate library to mixed-polarity multi-control Toffoli gates (Le. with both positive and negative controls). Our experimental results for the known reversible benchmarks show that using mixed-polarity Toffoli gates gives significant savings in gate count. The paper presents standard computational experiments for circuit synthesis. These results have implications in testing synthesis algorithms for reversible mixed-polarity circuits and quantum circuits.
The problem of synthesis and optimization of reversible and quantum circuits have drawn the attention of researchers in recent years. The typical design flow for such circuits first carries out the step of synthesis in terms of reversible gates, and then maps (decomposes) each reversible gate into equivalent set of quantum gates (e.g. from the NCV library). Since its initial proposal in realizing a Toffoli gate using quantum gates, several structural modifications have been proposed in the literature in order to reduce the cost of the generated netlist. The most recently introduced approach produces improved NCV cascade successively by partitioning the control lines of the given Toffoli gate in all possible ways, and replacing intermediate Toffoli gates with optimized equivalent NCV cascades form a catalog. In this approach the complexity of decomposing large Toffoli gates is high. In the present paper an efficient heuristic for partitioning the control lines of a given Toffoli gate is proposed. This heuristic is used for decomposition of the Toffoli gate without using any NCV catalog, after applying a template based optimization step. The cost of resultant circuit is similar to or sometimes better than the one reported by previous approaches.
Research affinity towards Reversible Computing is increasing day to day due to its lower energy dissipation computing capability. To evaluate any reversible function it is necessary to build the system with reversible gates. In view of this there are many new reversible logic gates proposed by many authors. Quantum cost of few reversible gates are unpublished. Simplified version of transformation technique to synthesize new reversible gates with Fredkin and Toffoli gates network are presented in this paper. Post synthesis optimization with the new set of templates are carried out to reduce the Quantum cost.
Optimization of reversible circuits is one of key problems of reversible logic synthesis.To solve the problem of high algorithm complexity and poor scalability of reversible Toffoli circuits,the paper analyzed and summarized relationship between adjacent Toffoli gates,proposed and proven the moving and simplification rules of sub-sequence of reversible circuit cascaded by Toffoli gates,and gave the optimization algorithm based on these rules.The algorithm examines the circuit bidirectionally according to the moving rules,searches the sub-sequence satisfying simplification rules,performs corresponding optimization,and the process is repeated until the circuit does' t change.The algorithm is irrelevant to the number of input lines,does' t need to store extra information,is suitable to different Toffoli circuit synthesis methods,and its algorithm complexity is O(s3),which is superior to O(n!t2s3)of generally used template optimization technique.Results of examples and experiments on all 3-bit reversible functions show that the number of gates and the control bits can be reduced effectively and the cost of reversible circuit is decreased.
Quantum cost is the most important criteria to evaluate reversible and quantum circuits. Also the fundamental building blocks of reversible and quantum circuits are Multiple-Control Toffoli (MCT) gates. The synthesis of MCT based reversible circuits are usually conducted into two steps. First, MCT circuits are decomposed into quantum circuits and then they are optimized using various techniques such as template matching, moving rules to reduce the quantum cost of reversible circuits. In this paper, we propose new techniques to decompose the Toffoli gates, in which MCT based circuits are mapped into a corresponding quantum realization. The main improvement is that the resulting quantum realization of MCT based circuits makes significantly better realization than those achieved in the earlier approaches and further reduction is possible using some other optimization techniques. Experimental results show that our new techniques enable to get sub-optimal realization of the MCT based reversible circuits in decomposition stage and quantum cost reduction of the reversible circuits is achieved by using that sub-optimal realization.