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Design, Automation, and Test in Europe

Quantum Circuit Simplification Using Templates

D. Maslov, Cho Eun Young, D. Michael Miller, Gerhard W. Dueck

Published Apr 1, 2005• 114 citations
Publisher

Abstract

Optimal synthesis of quantum circuits is intractable and heuristic methods must be employed. Templates are a general approach to reversible quantum circuit simplification. We consider the use of templates to simplify a quantum circuit initially found by other means. We present and analyze templates in the general case, and then provide particular details for circuits composed of NOT, CNOT and controlled-sqrt-of-NOT gates. We introduce templates for this set of gates and apply them to simplify both known quantum realizations of Toffoli gates and circuits found by earlier heuristic Fredkin and Toffoli gate synthesis algorithms. While the number of templates is quite small, the reduction in quantum cost is often significant.

DOI
10.1109/date.2005.249

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Quantum circuits are often generated by decomposing gates from networks with classical reversible gates. Only in rare cases, the results are minimal. Post-optimization methods, such as template matching, are employed to reduce the quantum costs of circuits. Quantum templates are derived from identity circuits. All minimal realizations, within certain limitations, can be embedded into templates. Due to this property, templates matching has the potential to reduce quantum costs of circuits. However, one of the difficulties in finding templates matches is due to the mobility of the gates within the circuit. Thus far, template matching procedures have employed heuristics to reduce the search space. This article presents an in-depth study of exact template matching with a set of algorithms. A graph structure with the corresponding circuits facilitates the discovery of potential sequences of templates to be matched, and how exact minimization of circuits can be accomplished. The significance of the proposed method is verified in benchmarks optimization.

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Optimal synthesis of quantum circuits is intractable and heuristic methods must be employed. Templates are a general approach to reversible quantum circuit simplification. We consider the use of templates to simplify a quantum circuit initially found by other means. We present and analyze templates in the general case, and then provide particular details for circuits composed of NOT, CNOT and controlled-sqrt-of-NOT gates. We introduce templates for this set of gates and apply them to simplify both known quantum realizations of Toffoli gates and circuits found by earlier heuristic Fredkin and Toffoli gate synthesis algorithms. While the number of templates is quite small, the reduction in quantum cost is often significant.
Quantum circuits are time-dependent diagrams describing the process of quantum computation. Usually, a quantum algorithm must be mapped into a quantum circuit. Optimal synthesis of quantum circuits is intractable, and heuristic methods must be employed. With the use of heuristics, the optimality of circuits is no longer guaranteed. In this paper, we consider a local optimization technique based on templates to simplify and reduce the depth of nonoptimal quantum circuits. We present and analyze templates in the general case and provide particular details for the circuits composed of NOT, CNOT, and controlled-sqrt-of-NOT gates. We apply templates to optimize various common circuits implementing multiple control Toffoli gates and quantum Boolean arithmetic circuits. We also show how templates can be used to compact the number of levels of a quantum circuit. The runtime of our implementation is small, whereas the reduction in the number of quantum gates and number of levels is significant.
Abstract—Reversible logic has applications in quantum com-puting, low power CMOS, nanotechnology, optical computing, and DNA computing. The most common reversible gates are the Toffoli gate (which performs a controlled NOT) and the Fredkin gate (performs a controlled swap of two inputs). Our synthesis algorithm first finds a cascade of Toffoli and Fredkin gates with no backtracking and minimal look-ahead. Next we apply trans-formations that reduce the size of the circuit. Transformations are accomplished via template matching. The basis for a template is a network with m gates that realizes the identity function. If a sequence in the network to be synthesized matches more than half of a template, then a transformation that reduces the gate count can be applied. In this paper we show that Toffoli and Fredkin gates behave in a similar manner. Therefore, some gates in the templates may not need to be specified—they can match a Toffoli or a Fredkin gate. We formalize this by introducing the box gate. All templates with less than six gates are enumerated and classified. We synthesize all three input, three output reversible functions and compare our results to those obtained previously. I.
Reversible logic has applications in quantum computing, low power CMOS, nanotechnology, optical computing, and DNA computing. The most common reversible gates are the Toffoli gate and the Fredkin gate. Our synthesis algorithm first finds a cascade of Toffoli and Fredkin gates with no backtracking and minimal look-ahead. Next we apply transformations that reduce the size of the circuit. Transformations are accomplished via template matching. The basis for a template is a network with m gates that realizes the identity function. If a sequence in the network to be synthesized matches more than half of a template, then a transformation that reduces the gate count can be applied. In this paper we show that Toffoli and Fredkin gates behave in a similar manner. Therefore, some gates in the templates may not need to be specified-they can match a Toffoli or a Fredkin gate. We formalize this by introducing the box gate. All templates with less than six gates are enumerated and classified. We synthesize all three input, three output reversible functions and compare our results to those obtained previously.
An r-valued m-variable reversible logic function maps each of the rm input patters to a unique output pattern. The synthesis problem is to realize a reversible function by a cascade of primitive reversible gates. In this paper, we present a simple heuristic algorithm that exploits the bidirectional synthesis possibility inherent in the reversibility of the specification. The primitive reversible gates considered here are MVL extensions of the well-known binary Toffoli gates. We analyze the structure of the gates that we use and show how these gates can be easily simulated in a quantum technology. We present exhaustive results for the 9! 2-variable 3-valued reversible functions comparing the results of our algorithm to optimal results found by breadth-first search. We also show results for specific ternary examples including showing how the presented technique can be applied to the synthesis of a 3-input, 3-valued adder which is not itself a reversible problem. When the circuit for the full adder is synthesized, we use it as an example of an approach to a circuit simplification, called the templates tool. Formalization and proper investigation of the templates simplification tool proposed in this paper is still under investigation.
Reversible logic computing is an emerging research area due to its high-speed computing abilities and generating the lossless information. Due to the ability of reverse operation and high-speed computing performance, reversible computation is considered as a future alternative to the conventional computing. The rapid progress of reversible computation in the current scenario, the reversible circuit is the way to solve this type of computation. Moreover, the reversible circuit computation is the higher level abstraction of the quantum circuit. The synthesis design of these circuits is an important aspect in the current literature. In this paper, we present our work in the field of synthesis design of a reversible circuit. In this work, we propose a modified transformation-based synthesis design using the FREDKIN/SWAP gate to reduce the quantum cost and minimizing the number of gates. To obtain this synthesis design approach, we have introduced some new templates which are defined by rules of replacing the k-CNOT/FREDKIN/SWAP gates using a template matching approach. Here, we are considering only k-CNOT ( k ≥ 1), FREDKIN and SWAP based gate in our templates. Finally, we provide our experimental results based on different benchmark reversible circuits. The experimental result has shown that the number of quantum cost and gate count are reduced using the FREDKIN/SWAP gate in the transformation-based approach with the help of proposed templates.
Quantum circuits are often generated by decomposing gates from networks with classical reversible gates. Only in rare cases, the results are minimal. Post-optimization methods, such as template matching, are employed to reduce the quantum costs of circuits. Quantum templates are derived from identity circuits. All minimal realizations, within certain limitations, can be embedded into templates. Due to this property, templates matching has the potential to reduce quantum costs of circuits. However, one of the difficulties in finding templates matches is due to the mobility of the gates within the circuit. Thus far, template matching procedures have employed heuristics to reduce the search space. This article presents an in-depth study of exact template matching with a set of algorithms. A graph structure with the corresponding circuits facilitates the discovery of potential sequences of templates to be matched, and how exact minimization of circuits can be accomplished. The significance of the proposed method is verified in benchmarks optimization.