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JUCS - Journal of Universal Computer Science

A reversible circuit synthesis algorithm with progressive increase of controls in generalized Toffoli gates

Edinelço Dalcumune, Luis Antônio Brasil Kowada, André da Cunha Ribeiro, Celina M.H. de Figueiredo, Franklin de Lima Marquezino

Published Jun 28, 2021• 5 citations

Abstract

We present a new algorithm for synthesis of reversible circuits for arbitrary n-bit bijective functions. This algorithm uses generalized Toffoli gates, which include positive and negative controls. Our algorithm is divided into two parts. First, we use partially controlled gen- eralized Toffoli gates, progressively increasing the number of controls. Second, exploring the properties of the representation of permutations in disjoint cycles, we apply generalized Toffoli gates with controls on all lines except for the target line. Therefore, new in the method is the fact that the obtained circuits use first low cost gates and consider increasing costs towards the end of the synthesis. In addition, we employ two bidirectional synthesis strategies to improve the gate count, which is the metric used to compare the results obtained by our algorithm with the results presented in the literature. Accordingly, our experimental results consider all 3-bit bijective functions and twenty widely used benchmark functions. The results obtained by our synthesis algorithm are competitive when compared with the best results known in the literature, considering as a complexity metric just the number of gates, as done by alternative best heuristics found in the literature. For example, for all 3-bit bijective functions using generalized Toffoli gates library, we obtained the best so far average count of 5.23.

DOI: 10.3897/jucs.69617

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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.

With increase in emphasis on low-power design and quantum computation, research in reversible logic synthesis has gained great attention. Various methods of synthesis exist in the literature, like exact synthesis, heuristic approaches, methods based on group theory, and those based on higher level representation of functions like Binary Decision Diagram (BDD). In this paper a group theory based synthesis approach for reversible circuits is presented, using both positive-control and negative-control Toffoli gates. The method uses group theoretic rules for factoring permutation cycles for synthesis. The use of both positive and negative control Toffoli gates results in reduction of total number of gates required as compared to previous methods based on group theory. Several worked out examples illustrate the advantage of the proposed approach.

The problem of synthesis and optimization of reversible and quantum circuits have drawn the attention of researchers for the last two decades due to increasing interest in quantum computing. Although lot of works have been done on the synthesis of binary reversible circuits, very less works have been reported on the synthesis of ternary reversible circuits. Ternary circuits have lower cost of implementation as compared to their binary counterparts. However, the synthesis approaches that exist for ternary reversible circuits either use too many circuit lines (qutrits) or too many gates. Only one prior work has discussed the problem of generating cost-optimal ternary reversible circuits, but for a very restrictive gate library, which limits the approach to a specific subset of ternary reversible functions and often the solution becomes sub-optimal due to the imposed restrictions. The present paper overcomes that restriction, and uses multiple control ternary Toffoli gates with all possible ternary target operations as the gate library. This gate library is functionally complete and can be used to synthesize any arbitrary function. The proposed SAT-based synthesis approach provides low cost solutions in terms of the number of gates for any arbitrary ternary reversible function. Experimental results on various randomly generated permutations as well as standard ternary benchmarks establish this claim. The results can be used as template for other synthesis approaches by observing how far they deviate from the optimal solutions.

The development of synthesis approaches for reversible circuits is an active research area. Besides heuristic methods, also exact synthesis received significant attention. Here, circuits realizing the desired functions e.g. with a minimal number of gates are determined. However, so far exact synthesis considering Toffoli gate circuits with positive control lines only has been considered. In this paper, we are extending the scope of exact synthesis by additionally considering negative control lines in the circuits to be synthesized. For this purpose, we propose and evaluate a SAT-based synthesis method. Our experiments show that incorporating negative control lines leads to smaller circuits with respect to the number of gates. Furthermore, in some cases even the run-time of the synthesis can be improved.

This paper studies synthesis and optimization of reversible circuits composed of Toffoli gates with negative and positive control lines. The proposed synthesis algorithm performs sorting among optimal implementations of certain functions - called as essential functions - to implement any reversible Boolean function. Essential functions comprise very small amount of all functions. For example, to implement 3 bit circuits, 28 essential functions out of all 40320 functions are needed. The proposed optimization algorithm considers both reversible and quantum circuit costs. First, reversible cost is reduced by considering adjacent gate pairs. Then, inner quantum structures of the gates are investigated and quantum optimization is performed. The proposed algorithms are evaluated on benchmark circuits in comparison with the results in the literature.