Reversible single-target gates are a generalization of Toffoli gates which are a helpful formal representation for the description of synthesis algorithms but are too general for an actual implementation based on some technology. There is an exponential lower bound on the number of Toffoli gates required to implement any reversible function, however, there is also a linear upper bound on the number of single-target gates which can be proven using a constructive proof based on a former presented synthesis algorithm. Since single-target gates can be mapped to a cascade of Toffoli gates, this synthesis algorithm provides an interesting framework for reversible circuit complexity. The paper motivates this framework and illustrates first possible applications based on it.
Reversible single-target gates are a generalization of Toffoli gates which are a helpful formal representation for the description of synthesis algorithms but are too general for an actual implementation based on some technology. There is an exponential lower bound on the number of Toffoli gates required to implement any reversible function, however, there is also a linear upper bound on the number of single-target gates which can be proven using a constructive proof based on a former presented synthesis algorithm. Since single-target gates can be mapped to a cascade of Toffoli gates, this synthesis algorithm provides an interesting framework for reversible circuit complexity. The paper motivates this framework and illustrates first possible applications based on it.
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.
An algorithm for reversible logic synthesis is proposed. The task is, for a given n -bit substitution map, to find a sequence of reversible logic gates that implements the map. The gate library adopted in this work consists of multiple-controlled Toffoli gates with m control bits, where m∈{0,…,n−1} . Controlled gates with large m (> 2) are then further decomposed into smaller gates ( m≤2 ). A primary goal in designing the algorithm is to reduce the number of Toffoli gates, which is known to be universal. The main idea is to view an n -bit substitution map as a rank-2 n tensor and to transform it such that the resulting map can be written as a tensor product of a rank-(2 n -2) tensor and the 2× 2 identity matrix. It can then be seen that the transformed map acts nontrivially on n -1 bits only, meaning that the map to be synthesized becomes ( n -1)-bit substitution. This size reduction process is iteratively applied until it reaches a tensor product of only 2× 2 matrices. The time complexity of the algorithm is exponential in n , as most previously known heuristic algorithms for reversible logic synthesis are, but it terminates within reasonable time for not too large n , which may find practical uses. As stated earlier, our primary target is to reduce the number of Toffoli gates in the output circuit. Benchmark results show that the algorithm works well for hard benchmark functions, but it does not seem advantageous when the function is structured. As an application, the algorithm is applied to find reversible circuits for cryptographic substitution boxes, which are often required in quantum cryptanalysis.
We present a new lower bound on the number of gates in reversible logic circuits that represent a given reversible logic function, in which the circuits are assumed to consist of general Toffoli gates and have no redundant input/output lines. We make a theoretical comparison of lower bounds, and prove that the proposed bound is better than the previous one. Moreover, experimental results for lower bounds on randomly-generated reversible logic functions and reversible benchmarks are given. The results also demonstrate that the proposed lower bound is better than the former one.
We propose the theory of Cayley graphs as a framework to analyse gate counts and quantum costs resulting from reversible circuit synthesis. Several methods have been proposed in the reversible logic synthesis literature by considering different libraries whose gates are associated to the generating sets of certain Cayley graphs. In a Cayley graph, the distance between two vertices corresponds to the optimal circuit size. The lower bound for the diameter of Cayley graphs is also a lower bound for the worst case for any algorithm that uses the corresponding gate library. In this paper, we study two Cayley graphs on the Symmetric Group S2n: the first, denoted by In, is defined by a generating set associated to generalized Toffoli gates; and the second, the hypercube Cayley graph Hn, is defined by a generating set associated to multiple-control Toffoli gates. Those two Cayley graphs have degree n2n−1 and order 2n!. Maslov, Dueck and Miller proposed a reversible circuit synthesis that we model by the Cayley graph In. We propose a synthesis algorithm based on the Cayley graph Hn with upper bound of (n−1)2n+1 multiple-control Toffoli gates. In addition, the diameter of the Cayley graph Hn gives a lower bound of n2n−1.
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.
Optimal synthesis of reversible functions is a nontrivial problem. One of the major limiting factors in computing such circuits is the sheer number of reversible functions. Even restricting synthesis to 4-bit reversible functions results in a huge search space (16! ≈ 244functions). The output of such a search alone, counting only the space required to list Toffoli gates for every function, would require over 100 terabytes of storage. In this paper, we present two algorithms: one, that synthesizes an optimal circuit for any 4-bit reversible specification, and another that synthesizes all optimal implementations. We employ several techniques to make the problem tractable. We report results from several experiments, including synthesis of all optimal 4-bit permutations, synthesis of random 4-bit permutations, optimal synthesis of all 4-bit linear reversible circuits, and synthesis of existing benchmark functions; we compose a list of the hardest permutations to synthesize, and show distribution of optimal circuits. We further illustrate that our proposed approach may be extended to accommodate physical constraints via reporting LNN-optimal reversible circuits. Our results have important implications in the design and optimization of reversible and quantum circuits, testing circuit synthesis heuristics, and performing experiments in the area of quantum information processing.