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International Conference on Computer Aided Design

A Robust Quantum Layout Synthesis Algorithm with a Qubit Mapping Checker

Tsou-An Wu, Ying-Jie Jiang, Shao‐Yun Fang

Published Oct 30, 2022• 17 citations
Publisher

Abstract

Layout synthesis in quantum circuits maps the logical qubits of a synthesized circuit onto the physical qubits of a hardware device (coupling graph) and complies with the hardware limitations. Existing studies on the problem usually suffer from intractable formulation complexity and thus prohibitively long runtimes. In this paper, we propose an efficient layout synthesizer by developing a satisfiability modulo theories (SMT)-based qubit mapping checker. The proposed qubit mapping checker can efficiently derive a SWAP-free solution if one exists. If no SWAP-free solution exists for a circuit, we propose a divide-and-conquer scheme that utilizes the checker to find SWAP-free sub-solutions for sub-circuits, and the overall solution is found by merging sub-solutions with SWAP insertion. Experimental results show that the proposed optimization flow can achieve more than 3000× runtime speedup over a state-of-the-art work to derive optimal solutions for a set of SWAP-free circuits. Moreover, for the other set of benchmark circuits requiring SWAP gates, our flow achieves more than 800× speedup and obtains near-optimal solutions with only 3% SWAP overhead.

DOI
10.1145/3508352.3549394

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Layout synthesis in quantum circuits maps the logical qubits of a synthesized circuit onto the physical qubits of a hardware device (coupling graph) and complies with the hardware limitations. Existing studies on the problem usually suffer from intractable formulation complexity and thus prohibitively long runtimes. In this paper, we propose an efficient layout synthesizer by developing a satisfiability modulo theories (SMT)-based qubit mapping checker. The proposed qubit mapping checker can efficiently derive a SWAP-free solution if one exists. If no SWAP-free solution exists for a circuit, we propose a divide-and-conquer scheme that utilizes the checker to find SWAP-free sub-solutions for sub-circuits, and the overall solution is found by merging sub-solutions with SWAP insertion. Experimental results show that the proposed optimization flow can achieve more than 3000× runtime speedup over a state-of-the-art work to derive optimal solutions for a set of SWAP-free circuits. Moreover, for the other set of benchmark circuits requiring SWAP gates, our flow achieves more than 800× speedup and obtains near-optimal solutions with only 3% SWAP overhead.
The physical qubits in current quantum computers do not all interact with each other. Therefore, in executing a quantum algorithm on an actual quantum computer, layout synthesis is a crucial step that ensures that the synthesized circuit of the quantum algorithm can run smoothly on the quantum computer. In this paper, we focus on a layout synthesis problem for quantum circuits and improve a prior work, TB-OLSQ, which adopts a transition-based satisfiability modulo theories (SMT) formulation. We present how to modify TB-OLSQ to obtain an accelerated version for runtime reduction. In addition, we extend the accelerated version by considering gate absorption for better solution quality. Our experimental results show that compared with TB-OLSQ, the accelerated version achieves 121X speedup for a set of SWAP-free circuits and 6X speedup for the other set of circuits with no increase in SWAP gates. In addition, the accelerated version with gate absorption helps reduce the number of SWAP gates by 38.9% for the circuits requiring SWAP gates, while it is also 3X faster.
In Layout Synthesis, the logical qubits of a quantum circuit are mapped to the physical qubits of a given quantum hardware platform, taking into account the connectivity of physical qubits. This involves inserting SWAP gates before an operation is applied on distant qubits. Optimal Layout Synthesis is crucial for practical Quantum Computing on current error-prone hardware: Minimizing the number of SWAP gates directly mitigates the error rates when running quantum circuits. In recent years, several approaches have been proposed for minimizing the required SWAP insertions. The proposed exact approaches can only scale to a small number of qubits. In this paper, we provide two encodings for Optimal Layout Synthesis as a classical planning problem. We use optimal classical planners to synthesize the optimal layout for a standard set of benchmarks. Our results show the scalability of our approach compared to previous leading approaches. We can optimally map circuits with 9 qubits onto a 14 qubit platform, which could not be handled before by exact methods.
In Layout Synthesis, the logical qubits of a quantum circuit are mapped to the physical qubits of a given quantum hardware platform, taking into account the connectivity of physical qubits. This involves inserting SWAP gates before an operation is applied on distant qubits. Optimal Layout Synthesis is crucial for practical Quantum Computing on current error-prone hardware: Minimizing the number of SWAP gates directly mitigates the error rates when running quantum circuits. In recent years, several approaches have been proposed for minimizing the required SWAP insertions. The proposed exact approaches can only scale to a small number of qubits. Proving that a number of swap insertions is optimal is much harder than producing near optimal mappings. In this paper, we provide two encodings for Optimal Layout Synthesis as a classical planning problem. We use optimal classical planners to synthesize the optimal layout for a standard set of benchmarks. Our results show the scalability of our approach compared to previous leading approaches. We can optimally map circuits with 9 qubits onto a 14 qubit platform, which could not be handled before by exact methods.
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.
With the announcement of the most advanced silicon spin quantum-bit (qubit) chip by Intel, silicon-based quantum circuit manufacturing technology has shown the superior potential to realize quantum computing to other technologies because of the mature semiconductor manufacturing technologies and the compatibility with electronics. Due to the bottleneck in scalability caused by interconnections in silicon-based quantum circuits, crossbar architectures serve as one of the most promising solutions for circuitry implementation. This paper proposes the first satisfiability modulo theories (SMT) formulation to optimally solve the layout synthesis problem by simultaneously performing scheduling, mapping, and routing for silicon-based quantum systems with a specific crossbar architecture. The experiments demonstrate that the proposed method can effectively generate layout synthesis results with minimal usage of swap and shuttle gates within the shortest circuit depths, and the solutions greatly outperform those derived from a state-of-the-art work.
Due to its effect on the success rate of a quantum circuit, quantum layout synthesis is a crucial step for circuit compilation. As such, having a layout synthesis tool that provides high solution quality is important to maximize circuit performance and fidelity for NISQ application. Previous heuristic approaches have been shown to be far from optimal when evaluated on known-optimal benchmarks. Alternatively, exact layout synthesis tools can generate optimal results with the aid of constraint solvers but generally suffer from scalability issues because of inefficient encodings and slow optimization methods. In this paper, we propose a scalable optimal layout synthesis tool that improves upon previous works, through a more succinct problem formulation as well as better encoding techniques. Additionally, we implement a depth and SWAP count optimization feature that performs iterative refinement under a fixed time budget. Experimental results show that for depth optimization, our tool can achieve a 692× speedup over the state-of-the-art optimal layout synthesis, and for SWAP optimization, we can obtain a 6,957× speedup on average. Compared to a leading heuristic-based synthesizer, for depth optimization, we can solve circuits consisting of 54 program qubits and 1726 gates within 11 hours with an 18× depth reduction and by 12× SWAP count reduction on average.