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arXiv:2004.10892 · Physical Review A

Simple heuristics for efficient parallel tensor contraction and quantum circuit simulation

Roman Schutski, Taras Khakhulin, Ivan Valer'evich Oseledets, Dmitry Kolmakov

Published Dec 28, 2020• 20 citations
arXivHTMLPDF

Abstract

Tensor networks are the main building blocks in various computational sciences, ranging from many-body theory and quantum computing to probability and machine learning. Here we propose a parallel algorithm for the contraction of tensor networks using probabilistic graphical models. Given a tensor network, this algorithm splits the contraction task almost optimally into subtasks, which can be contracted independently in parallel. Our approach is based on the heuristic solution of the $\ensuremath{\mu}$-treewidth deletion problem in graph theory. We apply the resulting algorithm to the simulation of random quantum circuits and discuss the extensions for general tensor network contractions. According to our estimates, the lower bound on the simulation time of 1 million amplitudes of a 49-qubit circuit of depth 40 on the Summit supercomputer is $>5$ seconds.

DOI
10.1103/physreva.102.062614

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Tensor networks are the main building blocks in various computational sciences, ranging from many-body theory and quantum computing to probability and machine learning. Here we propose a parallel algorithm for the contraction of tensor networks using probabilistic graphical models. Given a tensor network, this algorithm splits the contraction task almost optimally into subtasks, which can be contracted independently in parallel. Our approach is based on the heuristic solution of the \ensuremathμ\ensuremath{\mu}\ensuremathμ-treewidth deletion problem in graph theory. We apply the resulting algorithm to the simulation of random quantum circuits and discuss the extensions for general tensor network contractions. According to our estimates, the lower bound on the simulation time of 1 million amplitudes of a 49-qubit circuit of depth 40 on the Summit supercomputer is >5>5>5 seconds.
Abstract We develop an algorithmic framework for contracting tensor networks and demonstrate its power by classically simulating quantum computation of sizes previously deemed out of reach. Our main contribution, index slicing, is a method that efficiently parallelizes the contraction by breaking it down into much smaller and identically structured subtasks, which can then be executed in parallel without dependencies. We benchmark our algorithm on a class of random quantum circuits, achieving greater than 105 times acceleration over the original estimate of the simulation cost. We then demonstrate applications of the simulation framework for aiding the development of quantum algorithms and quantum error correction. As tensor networks are widely used in computational science, our simulation framework may find further applications.
Tensors are a natural generalization of matrices, and tensor networks are a natural generalization of matrix products. Despite the simple definition of tensor networks, they are versatile enough to represent many different kinds of "products" that arise in various theoretical and practical problems. In particular, the powerful computational model of quantum computation can be defined almost entirely in terms of matrix products and tensor products, both of which are special cases of tensor networks. As such, (classical) algorithms for evaluating tensor networks have profound importance in the study of quantum computation. In this thesis, we design and implement a parallel algorithm for tensor network contraction. In addition to finding efficient contraction orders for a tensor network, we also dynamically slice it into multiple sub-tasks with lower space and time costs, in order to evaluate the tensor network in parallel. We refer to such an evaluation strategy as a contraction scheme for the tensor network. In addition, we introduce a local optimization procedure that improves the efficiency of the contraction schemes we find. We also investigate the applications of our parallel tensor network contraction algorithm in quantum computation. The most ready application is the simulation of random quantum supremacy circuits, where we benchmark our algorithm to demonstrate its advantage over other similar tensor network based simulators. Other applications we found include evaluating the energy function of a Quantum Approximate Optimization Algorithm (QAOA), and simulating surface codes under a realistic error model with crosstalk.
Abstract Quantum computing holds significant promise for solving complex problems, but simulating quantum circuits on classical computers remains essential due to the current limitations of quantum hardware. Efficient simulation is crucial for the development and validation of quantum algorithms and quantum computers. This paper explores and compares various strategies to leverage different levels of parallelism to accelerate the contraction of tensor networks representing large quantum circuits. We propose a new parallel multistage algorithm based on communities. The original tensor network is partitioned into several communities, which are then contracted in parallel. The pairs of tensors of the resulting network can be contracted in parallel using a GPU. We use the Girvan–Newman algorithm to obtain the communities and the contraction plans. We compare the new algorithm with two other parallelisation strategies: one based on contracting all the pairs of tensors in the GPU and another one that uses slicing to cut some indexes of the tensor network and then MPI processes to contract the resulting slices in parallel. The new parallel algorithm gets the best results with different well-known quantum circuits with a high degree of entanglement, including random quantum circuits. In conclusion, the results show that the main factor that limits the simulation is the space cost. However, the parallel multistage algorithm manages to reduce the cost of sequential simulation for circuits with a high number of qubits and allows simulating larger circuits.
One of the key problems in tensor network based quantum circuit simulation is the construction of a contraction tree which minimizes the cost of the simulation, where the cost can be expressed in the number of operations as a proxy for the simulation running time. This same problem arises in a variety of application areas, such as combinatorial scientific computing, marginalization in probabilistic graphical models, and solving constraint satisfaction problems. In this paper, we reduce the computationally hard portion of this problem to one of graph linear ordering, and demonstrate how existing approaches in this area can be utilized to achieve results up to several orders of magnitude better than existing state of the art methods for the same running time. To do so, we introduce a novel polynomial time algorithm for constructing an optimal contraction tree from a given order. Furthermore, we introduce a fast and high quality linear ordering solver, and demonstrate its applicability as a heuristic for providing orderings for contraction trees. Finally, we compare our solver with competing methods for constructing contraction trees in quantum circuit simulation on a collection of randomly generated Quantum Approximate Optimization Algorithm Max Cut circuits and show that our method achieves superior results on a majority of tested quantum circuits. Reproducibility: Our source code and data are available at https://github.com/cameton/HPEC2022_ContractionTrees.
Simulation of quantum computing on supercomputers is a significant research topic, which plays a vital role in quantum algorithm verification, error-tolerant verification, and other applications. Tensor-network contraction based on density matrix is an important single-amplitude simulation strategy, but it is hard to execute on the distributed computing systems. In this paper, we studied the problem in detail, and propose a scheme based on cutting edges of undirected graphs. This scheme cuts edges of undirected graphs with large tree width to obtain many undirected subgraphs with small tree width, and these subgraphs contracted on different computing cores. The contraction results of slave cores are summarized in the master node, which is consistent with the original tensor-network contraction. Thus, we can simulate the larger scale quantum circuit than single core. Moreover, it is an NP-hard problem to find the global optimum cutting edges, and we propose a search strategy based on a heuristic algorithm to approach it. In order to verify the effectiveness of our scheme, we conduct tests based on the quantum approximation optimization algorithm (QAOA) and Shor's algorithm, and it can simulate a 120-qubit three-regular QAOA algorithm on a 4096-core supercomputer, which greatly exceeds the simulation scale on a single core of 100 qubits.
The study of quantum circuit simulation using classical computers is a key research topic that helps define the boundary of verifiable quantum advantage, solve quantum many-body problems, and inform development of quantum hardware and software. Tensor networks have become forefront mathematical tools for these tasks. Here we introduce a method to approximately simulate quantum circuits using sparsely-populated tensors. We describe a sparse tensor data structure that can represent quantum states with no underlying symmetry, and outline algorithms to efficiently contract and truncate these tensors. We show that the data structure and contraction algorithm are efficient, leading to expected runtime scalings versus qubit number and circuit depth. Our results motivate future research in optimization of sparse tensor networks for quantum simulation.