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IEEE International Midwest Symposium on Circuits and Systems (MWSCAS)

A Novel Classical-Ising Hybrid Annealing Method with QUBO Model Cutting

Yuta Atobe, Masashi Tawada, Nozomu Togawa

Published Aug 11, 2024• 0 citations
Publisher

Abstract

Most quantum annealers, or Ising machines have the hardware limitations in its qubit size and thus the computable problem size is much limited. This paper proposes a novel classical-Ising hybrid annealing method which virtually extends the computable size of Ising machines based on a theoretical analysis. Given a quadratic unconstrained binary optimization (QUBO) model, the proposed method selects the cutting binary variable that is most likely to have the same value as the ground-state solution. Then, we recursively cut the QUBO model based on the cutting binary variable until we obtain sufficiently small-sized subQUBO models. Experimental evaluations demonstrate that the proposed hybrid annealing method can give much better quasi-ground-state solutions than state-of-the-art existing methods for large-sized QUBO models.

DOI
10.1109/mwscas60917.2024.10658806

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Most quantum annealers, or Ising machines have the hardware limitations in its qubit size and thus the computable problem size is much limited. This paper proposes a novel classical-Ising hybrid annealing method which virtually extends the computable size of Ising machines based on a theoretical analysis. Given a quadratic unconstrained binary optimization (QUBO) model, the proposed method selects the cutting binary variable that is most likely to have the same value as the ground-state solution. Then, we recursively cut the QUBO model based on the cutting binary variable until we obtain sufficiently small-sized subQUBO models. Experimental evaluations demonstrate that the proposed hybrid annealing method can give much better quasi-ground-state solutions than state-of-the-art existing methods for large-sized QUBO models.
Ising machines are expected to solve combinatorial optimization problems efficiently by representing them as Ising models or equivalent quadratic unconstrained binary optimization (QUBO) models . However, upper bound exists on the computable problem size due to the hardware limitations of Ising machines. This paper propose a new hybrid annealing method based on partial QUBO extraction, called subQUBO model extraction, with multiple solution instances. For a given QUBO model, the proposed method obtainsNIN_INI​quasi-optimal solutions (quasi-ground-state solutions) in some way using a classical computer. The solutions giving these quasi-optimal solutions are calledsolution instances. We extract a size-limited subQUBO model as follows based on a strong theoretical background: we randomly selectNSN_SNS​(NS<NI)(N_S<N_I)(NS​<NI​)solution instances among them and focus on a particular binary variablexix_ixi​in theNSN_SNS​solution instances. Ifxix_ixi​value is muchvariedoverNSN_SNS​solution instances, it is included in the subQUBO model; otherwise, it is not. We find a (quasi-)ground-state solution of the extracted subQUBO model using an Ising machine and add it as a new solution instance. By repeating this process, we can finally obtain a (quasi-)ground-state solution of the original QUBO model. Experimental evaluations confirm that the proposed method can obtain better quasi-ground-state solution than existing methods for large-sized QUBO models.
We present a heuristic algorithm designed to solve Quadratic Unconstrained Binary Optimization (QUBO) problems efficiently. The algorithm, referred to as IC-D2S, leverages a hybrid approach using Ising and classical machines to address very large problem sizes. Considering the practical limitation on the size of the Ising machine (IM), our algorithm partitions the QUBO problem into a collection of QUBO subproblems (called subQUBOs) and utilizes the IM to solve each subQUBO. Our proposed heuristic algorithm uses a set of control parameters to generate the subQUBOs and explore the search space. Also, it utilizes an annealer based on cosine waveform and applies a mutation operator at each step of the search to diversify the solution space and facilitate the process of finding the global minimum of the problem. We have evaluated the effectiveness of our IC-D2S algorithm on three large-sized problem sets and compared its efficiency in finding the (near-)optimal solution with three QUBO solvers. One of the solvers is a software-based algorithm (D2TS), while the other one (D-Wave) employs a similar approach to ours, utilizing both classical and Ising machines. The results demonstrate that for large-sized problems (≥ 5000) the proposed algorithm identifies superior solutions. Additionally, for smaller-sized problems (= 2500), IC-D2S efficiently finds the optimal solution in a significantly faster manner.
We present a heuristic algorithm designed to solve Quadratic Unconstrained Binary Optimization (QUBO) problems efficiently. The algorithm, referred to as IC-D2S, leverages a hybrid approach using Ising and classical machines to address very large problem sizes. Considering the practical limitation on the size of the Ising machine(IM), our algorithm partitions the QUBO problem into a collection of QUBO subproblems (called subQUBOs) and utilizes the IM to solve each subQUBO. Our proposed heuristic algorithm uses a set of control parameters to generate the subQUBOs and explore the search space. Also, it utilizes an annealer based on cosine waveform and applies a mutation operator at each step of the search to diversify the solution space and facilitate the process of finding the global minimum of the problem. We have evaluated the effectiveness of our IC-D2S algorithm on three large-sized problem sets and compared its efficiency in finding the (near-)optimal solution with three QUBO solvers. One of the solvers is a software-based algorithm (D2TS), while the other one (D-Wave) employs a similar approach to ours, utilizing both classical and Ising machines. The results demonstrate that for large-sized problems (>= 5000) the proposed algorithm identifies superior solutions. Additionally, for smaller-sized problems (= 2500), IC-D2S efficiently finds the optimal solution in a significantly faster manner.
In combinatorial optimization problems with one-hot constraints, the number of binary variables that have the value of one in the feasible solution becomes constant and small, so it is important to always consider them to escalate the solution searching process and obtain a more converged solution in annealing. In this paper, we propose a method to preferably select binary variables with the value of one in the subQUBO annealing process using Ising machines such as quantum annealers. Evaluation experiments show that the proposed method obtains a solution with lower energy than a conventional subQUBO method and a full QUBO method.
The trip planning problem (TPP) can be formulated as a combinatorial optimization problem that searches for the best route to visit a series of landmarks and hotels. Meanwhile, Ising machines have attracted attention due to their efficiency in solving combinatorial optimization problems. The Ising machines solve the combinatorial optimization problems by transforming the problems into quadratic unconstrained binary optimization (QUBO) models. However, the possible input QUBO size of current Ising machines is quite limited. Thus, it is hard to directly embed a large-scale TPP onto the current Ising machines. In this paper, we propose a novel subQUBO annealing method based on the combined variable selection method to solve the TPP. The proposed method finds a quasi-optimal solution to a large problem by repeatedly partitioning the original QUBO model into small subQUBOs that can be embedded onto the Ising machine. Specifically, to construct a subQUBO, we select variables from the original QUBO model, which have small deviation values. Further, we select variables randomly from the original QUBO model, so as not to fall into the local optimum. We have conducted an evaluation experiment using Ising machines on TPP and confirmed that the proposed method outperforms the state-of-the-art methods in terms of POI satisfaction and POI cost.
Quantum annealing is a method based on simulated annealing where temperature variations are replaced by quantum fluctuations that cause qubit state transitions. ISING problems and Quadratic Unconstrained Binary Optimization problems (QUBO) can be tackled by quantum annealing-based machines. We can link the interconnected qubits of the machine and the binaries of our model: qubits are binary variables and each pair of them linked by a coupler has a strong impact on the equality or the non-equality of the two binaries of the associated pair. Quantum annealing is based on the fact that any system tends to seek its minimum energy state. Starting from qubits in a state of superposition where all the solutions to the problem are fairly represented, the machine will apply a magnetic field by targeting the qubits and couplers in such a way as to make their value energetically favorable in the direction of optimization (minimization). For the coupled qubits, it is the quadratic products of the binary variables that are considered here and the physical system will make it energetically favorable for them to take (or not) the same values. To work on these machines based on quantum annealing in order to solve optimization problems with constraints, the latter must be relaxed while penalizing the Objective function. After presenting the case of the Max-Cut problem, we will look at such constrained problems.