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IEEE International Conference on Quantum Computing and Engineering (QCE)

Decision Diagram vs. State Vector: A Comparative Study on Quantum Computing Simulation Efficiency

Yusuke Kimura, Shaowen Li, Hiroyuki Sato, Masahiro Fujita

Published Sep 15, 2024• 3 citations
Publisher

Abstract

To efficiently develop algorithms for quantum com-puters, a quantum simulator capable of handling many qubits at high speed is essential. However, a state vector-based simulator needs to store a vector of length$2^{N}$for$N$qubits. In today's typical desktop computing environment, a maximum of around 30 qubits can be handled. A decision diagram-based simulator is expected to solve the problem of memory requirement for some set of quantum algorithms. Decision diagram stores state vectors as graphs, which may significantly reduce memory usage in the presence of regularities in the state vector. In this study, we have implemented our decision diagram-based quantum simulator and conducted experiments using various algorithms to compare with the state vector-based simulator. Algorithms such as Grover and Shor show significant speedup and superiority of performance for larger numbers of qubits, whereas random circuits with many parameterized rotation gates show slower performance. As described above, researchers should understand the characteristics of simulators so that quantum algorithms can be developed more efficiently.

DOI
10.1109/qce60285.2024.00095

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To efficiently develop algorithms for quantum com-puters, a quantum simulator capable of handling many qubits at high speed is essential. However, a state vector-based simulator needs to store a vector of length2N2^{N}2NforNNNqubits. In today's typical desktop computing environment, a maximum of around 30 qubits can be handled. A decision diagram-based simulator is expected to solve the problem of memory requirement for some set of quantum algorithms. Decision diagram stores state vectors as graphs, which may significantly reduce memory usage in the presence of regularities in the state vector. In this study, we have implemented our decision diagram-based quantum simulator and conducted experiments using various algorithms to compare with the state vector-based simulator. Algorithms such as Grover and Shor show significant speedup and superiority of performance for larger numbers of qubits, whereas random circuits with many parameterized rotation gates show slower performance. As described above, researchers should understand the characteristics of simulators so that quantum algorithms can be developed more efficiently.
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An N-bit quantum state requires a vector of length 2N, leading to an exponential increase in the required memory with N in conventional statevector-based quantum simulators. A proposed solution to this issue is the decision diagram-based quantum simulator, which can significantly decrease the necessary memory and is expected to operate faster for specific quantum circuits. However, decision diagram-based quantum simulators are not easily parallelizable because data must be manipulated dynamically, and most implementations run on one thread. This paper introduces ring communication-based optimal parallelization and automatic swap insertion techniques for multi-node implementation of decision diagram-based quantum simulators. The ring communication approach is designed so that each node communicates with its neighboring nodes, which can facilitate faster and more parallel communication than broadcasting where one node needs to communicate with all nodes simultaneously. The automatic swap insertion method, an approach to minimize inter-node communication, has been employed in existing multi-node state vector-based simulators, but this paper proposes two methods specifically designed for decision diagram-based quantum simulators. These techniques were implemented and evaluated using the Shor algorithm and random circuits with up to 38 qubits using a maximum of 256 nodes. The experimental results have revealed that multi-node implementation can reduce run-time by up to 26 times. For example, Shor circuits that need 38 qubits can finish simulation in 147 seconds. Additionally, it was shown that ring communication has a higher speed-up effect than broadcast communication, and the importance of selecting the appropriate automatic swap insertion method was revealed.
Quantum-mechanical phenomena are playing an increasing role in information processing as transistor sizes approach the nanometer level, while the securest forms of communication rely on quantum data encoding. When they involve a finite number of basis states, these phenomena can be modeled as quantum circuits, the quantum analogue of conventional or classical logic circuits. Simulation of quantum circuits can therefore be used as a tool to evaluate issues in the design of quantum information processors. Unfortunately, simulating such phenomena efficiently is exceedingly difficult. The matrices representing quantum operators (gates) and vectors modeling quantum states grow exponentially with the number of quantum bits. The information represented by quantum states and operators often exhibits structure that can be exploited when simulating certain classes of quantum circuits. We study the development of simulation methods that run on classical computers and take advantage of such repetitions and redundancies. In particular, we define a new data structure for simulating quantum circuits called the quantum information decision diagram (QuIDD). A QuIDD is a compressed graph representation of a vector or matrix and permits computations to be performed directly on the compressed data. We develop a comprehensive set of algorithms for operating on QuIDDs in both the state-vector and density-matrix formats, and evaluate their complexity. These algorithms have been implemented in a general-purpose simulator program for quantum-mechanical applications called QuIDDPro. Through extensive experiments conducted on representative quantum simulation applications, including Grover's search algorithm, error characterization, and reversible circuits, we demonstrate that QuIDDPro is faster than other existing quantum-mechanical simulators such as the National Institute of Standards and Technology's QCSim program, and is far more memory-efficient. Using QuIDDPro, we explore the advantages of quantum computation over classical computation, simulate quantum errors and error correction, and study the impact of numerical precision on the fidelity of simulations. We also develop several novel algorithms for testing quantum circuit equivalence and compare them empirically. The QuIDDPro software is equipped with a user-friendly interface and is distributed with numerous example scripts. It has been used as a laboratory supplement for quantum computing courses at several universities.
Simulating quantum computation on a classical computer is a difficult problem. The matrices representing quantum gates, and vectors modeling qubit states grow exponentially with the number of qubits. It has been shown experimentally that the QuIDD (Quantum Information Decision Diagram) datastructure greatly facilitates simulations using memory and runtime that are polynomial in the number of qubits. In this paper, we present a complexity analysis which formally describes this class of matrices and vectors. We also present an improved implementation of QuIDDs which can simulate Grover's algorithm for quantum search with the asymptotic runtime complexity of an ideal quantum computer up to negligible overhead.
Despite the recent progress in the physical implementation of quantum computers, a significant amount of research still depends on the use of quantum circuit simulators running on classical hardware. While there are several techniques for quantum circuit simulation, many state-of-the-art simulators rely on an array-based simulation approach. However, this array-based approach has exponential memory complexity with respect to the number of simulated qubits. To address this drawback, complementary approaches based on decision diagrams have been proposed. While these approaches allow simulating circuits that could not be simulated before, they come with their own drawbacks. Unfortunately, no detailed case study has been conducted to date, which compares those complementary approaches and their respective strengths and weaknesses. In this work, we are addressing this by providing a survey on both approaches as well as a detailed case study on their respective performances.
Recent technological advancements show promise in leveraging quantum mechanical phenomena for computation. This brings substantial speed-ups to problems that are once considered to be intractable in the classical world. However, the physical realization of quantum computers is still far away from us, and a majority of research work is done using quantum simulators running on classical computers. Classical computers face a critical obstacle in simulating quantum algorithms. Quantum states reside in a Hilbert space whose size grows exponentially to the number of subsystems, i.e., qubits. As a result, the straightforward statevector approach does not scale due to the exponential growth of the memory requirement. Decision diagrams have gained attention in recent years for representing quantum states and operations in quantum simulations. The main advantage of this approach is its ability to exploit redundancy. However, mainstream quantum simulators still rely on statevectors or tensor networks. We consider the absence of decision diagrams due to the lack of parallelization strategies. This work explores several strategies for parallelizing decision diagram operations, specifically for quantum simulations. We propose optimal parallelization strategies. Based on the experiment results, our parallelization strategy achieves a 2-3 times faster simulation of Grover’s algorithm and random circuits than the state-of-the-art single-thread DD-based simulator DDSIM.