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IEICE Transactions on Fundamentals of Electronics Communications and Computer Sciences

An Efficient Method to Decompose and Map MPMCT Gates That Accounts for Qubit Placement

Atsushi Matsuo, Wakaki Hattori, Shigeru Yamashita

Published Aug 9, 2022• 3 citations
Publisher

Abstract

Mixed-Polarity Multiple-Control Toffoli (MPMCT) gates are generally used to implement large control logic functions for quantum computation. A logic circuit consisting of MPMCT gates needs to be mapped to a quantum computing device that invariably has a physical limitation, which means we need to (1) decompose the MPMCT gates into one- or two-qubit gates, and then (2) insert SWAP gates so that all the gates can be performed on Nearest Neighbor Architectures (NNAs). Up to date, the above two processes have only been studied independently. In this work, we investigate that the total number of gates in a circuit can be decreased if the above two processes are considered simultaneously as a single step. We developed a method that inserts SWAP gates while decomposing MPMCT gates unlike most of the existing methods. Also, we consider the effect on the latter part of a circuit carefully by considering the qubit placement when decomposing an MPMCT gate. Experimental results demonstrate the effectiveness of our method.

DOI
10.1587/transfun.2022eap1050

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Mixed-Polarity Multiple-Control Toffoli (MPMCT) gates are generally used to implement large control logic functions for quantum computation. A logic circuit consisting of MPMCT gates needs to be mapped to a quantum computing device that invariably has a physical limitation, which means we need to (1) decompose the MPMCT gates into one- or two-qubit gates, and then (2) insert SWAP gates so that all the gates can be performed on Nearest Neighbor Architectures (NNAs). Up to date, the above two processes have only been studied independently. In this work, we investigate that the total number of gates in a circuit can be decreased if the above two processes are considered simultaneously as a single step. We developed a method that inserts SWAP gates while decomposing MPMCT gates unlike most of the existing methods. Also, we consider the effect on the latter part of a circuit carefully by considering the qubit placement when decomposing an MPMCT gate. Experimental results demonstrate the effectiveness of our method.
We usually use Mixed-Polarity Multiple-Control Toffoli (MPMCT) gates to realize large control logic functions for quantum computation. A logic circuit consisting of MPMCT gates needs to be mapped to a quantum computing device that has some physical limitation; (1) we need to decompose MPMCT gates into one or two-qubit gates, and then (2) we need to insert SWAP gates such that all the gates can be performed on Nearest Neighbor Architectures (NNAs). Up to date, the above two processes have been independently studied intensively. This paper points out that we can decrease the total number of the gates in a circuit if the above two processes are considered dynamically as a single step; we propose a method to inserts SWAP gates while decomposing MPMCT gates unlike most of the existing methods. Our additional idea is to consider the effect on the latter part of a circuit carefully by considering the qubit layout when composing an MPMCT gate. We show some experimental results to confirm the effectiveness of our method.
We usually use Mixed-Polarity Multiple-Control Toffoli (MPMCT) gates to realize large control logic functions for quantum computation. A logic circuit consisting of MPMCT gates needs to be mapped to a quantum computing device that has some physical limitation; (1) we need to decompose MPMCT gates into one or two-qubit gates, and then (2) we need to insert SWAP gates such that all the gates can be performed on Nearest Neighbor Architectures (NNAs). Up to date, the above two processes have been independently studied intensively. This paper points out that we can decrease the total number of the gates in a circuit if the above two processes are considered dynamically as a single step; we propose a method to inserts SWAP gates while decomposing MPMCT gates unlike most of the existing methods. Our additional idea is to consider the effect on the latter part of a circuit carefully by considering the qubit layout when composing an MPMCT gate. We show some experimental results to confirm the effectiveness of our method.
To realize fault-tolerant quantum circuits, one of the important task is to reduce the number of T gates (T-count) when we decompose MPMCT (Mixed Polarity Multiple-Control Toffoli) gates into elementary gates. When we have enough auxiliary qubits, we have an efficient decomposition of a single MPMCT gate. However, it has not been studied intensively how to decompose many MPMCT gates efficiently at the same time. Thus, this paper studies what is an efficient decomposition method when we have many MPMCT gates. We consider two situations to reduce T-count when we need to decompose many MPMCT gates. Then, we propose a heuristic method to reorder MPMCT gates if possible so that we can find the two situations as much as possible. Our preliminary experiments show that our heuristic can reduce T-count drastically.
Abstract We propose a method for efficient mixed polarity multiple controlled Toffoli (MPMCT) gate decomposition from the perspective of a cost metric related to Toffoli gates, namely Toffoli-depth. When using the technique presented in a previous study, there is a range in which Toffoli-depth (consequently T-depth) of the implemented circuit increases proportionally as the number of provided (clean) work qubits increases. In other words, using the previous technique may result in more inefficient MPMCT gates even though the number of helpful work qubits has increased. In this work, a technique is devised to provide sufficient help from clean work qubits at the central part of the implemented circuit as many as possible, thereby addressing the issues with the previous technique. Meanwhile, one of the representative algorithms that use MPMCT gates is Grover’s algorithm. We show the implementation results for MPMCT gates according to the number of work qubits, using Grover’s algorithm as an example. It is experimentally demonstrated that T-depth decreases much more quickly when using our method than the previous method.
State-of-the-art studies on quantum layout synthesis have proposed various approaches based on the assumption that the input circuit is only composed of single-qubit and two-qubit gates. This assumption greatly simplifies the layout synthesis problem, and thus they only require ensuring that all controlled-NOT (CNOT) gates satisfy the hardware constraints imposed by a given coupling graph. However, during the design of quantum circuits, multi-controlled Toffoli (MCT) gates are usually used to better characterize the function of the circuits. Directly decomposing them into single and two-qubit gates with a fixed routine ignores the flexibility and optimization opportunity provided by various decomposition (i.e., logic synthesis) possibilities and thus suffers from sub-optimal results. This article proposes a co-optimization approach for quantum logic and layout synthesis. The MCT gates are first decomposed into Toffoli gates, and an efficient qubit mapping checking process is proposed to optimally solve the SWAP-free layout synthesis problem by automatically determining the decomposition result of each Toffoli gate. If a SWAP-free result cannot be found, the proposed algorithm flow is then used to obtain a solution that minimizes the total cost that simultaneously counts the cost caused by the different decomposition methods for Toffoli gates and the cost induced by SWAP gates. Compared with a state-of-the-art method, the proposed approach reduces the number of additional CNOT gates by 16% with around 25X runtime speedup for the cases with SWAP-free solutions, which cannot be obtained without the co-optimization approach.
It is considered to be very important to reduce so-called the T-depth of a target quantum circuit for realizing future fault-tolerant quantum computation. Thus, there have been many researches to consider how to decompose Multiple Controlled Toffoli (MCT) gates with few T-depth; these existing methods do not consider the T-depth of each qubit when they decompose one MCT gate. In contrast to existing methods, this paper proposes to consider the T-depth of each qubit when we decompose an MCT gate; we show that we can decrease the T-depth by considering the T-depth of each qubit and appropriately selecting ancilla qubits and decomposition methods. In addition, to consider the impact of the decomposition of one MCT gate on the decomposition of subsequent gates, our method utilizes beam search to select a possibly best decomposition for each MCT gate. We confirmed that the proposed method can reduce the T-depth by an average of 17.1% compared to existing methods.