Quantum computers are expected to enable fast solving of large-scale combinatorial optimization problems. However, their limitations in fidelity and the number of qubits prevent them from handling real-world problems. Recently, a quantum-inspired solver using tensor networks has been proposed, which works on classical computers. Particularly, tensor networks have been applied to constrained combinatorial optimization problems for practical applications. By preparing a specific tensor network to sample states that satisfy constraints, feasible solutions can be searched for without the method of penalty functions. Previous studies have been based on profound physics, such as U(1) gauge schemes and high-dimensional lattice models. In this study, we devise to design feasible tensor networks using elementary mathematics without such a specific knowledge. One approach is to construct tensor networks with nilpotent-matrix manipulation. The second is to algebraically determine tensor parameters. We showed mathematically that such feasible tensor networks can be constructed to accommodate various types of constraints. For the principle verification, we numerically constructed a feasible tensor network for facility location problem, to find much faster construction than conventional methods. Then, by performing imaginary time evolution, feasible solutions were always obtained, ultimately leading to the optimal solution.
Quantum computers are expected to enable fast solving of large-scale combinatorial optimization problems. However, their limitations in fidelity and the number of qubits prevent them from handling real-world problems. Recently, a quantum-inspired solver using tensor networks has been proposed, which works on classical computers. Particularly, tensor networks have been applied to constrained combinatorial optimization problems for practical applications. By preparing a specific tensor network to sample states that satisfy constraints, feasible solutions can be searched for without the method of penalty functions. Previous studies have been based on profound physics, such as U(1) gauge schemes and high-dimensional lattice models. In this study, we devise to design feasible tensor networks using elementary mathematics without such a specific knowledge. One approach is to construct tensor networks with nilpotent-matrix manipulation. The second is to algebraically determine tensor parameters. We showed mathematically that such feasible tensor networks can be constructed to accommodate various types of constraints. For the principle verification, we numerically constructed a feasible tensor network for facility location problem, to find much faster construction than conventional methods. Then, by performing imaginary time evolution, feasible solutions were always obtained, ultimately leading to the optimal solution.
Combinatorial optimization is of general interest for both theoretical study and real-world applications. Fast-developing quantum algorithms provide a different perspective on solving combinatorial optimization problems. In this paper, we propose a quantum-inspired tensor-network-based algorithm for general locally constrained combinatorial optimization problems. Our algorithm constructs a Hamiltonian for the problem of interest, effectively mapping it to a quantum problem, then encodes the constraints directly into a tensor network state and solves the optimal solution by evolving the system to the ground state of the Hamiltonian. We demonstrate our algorithm with the open-pit mining problem, which results in a quadratic asymptotic time complexity. Our numerical results show the effectiveness of this construction and potential applications in further studies for general combinatorial optimization problems.
Abstract Constrained combinatorial optimization problems abound in industry, from portfolio optimization to logistics. One of the major roadblocks in solving these problems is the presence of non-trivial hard constraints which limit the valid search space. In some heuristic solvers, these are typically addressed by introducing certain Lagrange multipliers in the cost function, by relaxing them in some way, or worse yet, by generating many samples and only keeping valid ones, which leads to very expensive and inefficient searches. In this work, we encode arbitrary integer-valued equality constraints of the form A x ⃗ = b ⃗ , directly into U(1) symmetric tensor networks (TNs) and leverage their applicability as quantum-inspired generative models to assist in the search of solutions to combinatorial optimization problems within the generator-enhanced optimization framework of Alcazar et al (2021 arXiv: 2101.06250 ). This allows us to exploit the generalization capabilities of TN generative models while constraining them so that they only output valid samples. Our constrained TN generative model efficiently captures the constraints by reducing number of parameters and computational costs. We find that at tasks with constraints given by arbitrary equalities, symmetric matrix product states outperform their standard unconstrained counterparts at finding novel and better solutions to combinatorial optimization problems.
Constraint satisfaction problems (CSPs) are ubiquitous in various domains, including artificial intelligence, circuit design, and scheduling. Traditional CSP solvers often struggle with problems exhibiting high dimensionality and complex constraints, leading to exponential time complexity in the worst case. This paper proposes a novel approach to CSP solving that draws inspiration from quantum mechanics, specifically superposition and entanglement, through the utilization of tensor networks and quantum-inspired algorithms. We represent CSP variables and constraints using tensor networks, allowing for efficient storage and manipulation of complex relationships. Furthermore, we employ quantum walks, a quantum-inspired search algorithm, to perform constraint propagation and explore the solution space. This approach aims to achieve significant performance gains by effectively exploiting the inherent parallelism and exploration capabilities associated with quantum phenomena. The core claim is that these quantum-inspired techniques can efficiently explore the solution space of CSPs, particularly those with high dimensionality. The mechanism involves tensor network representation and quantum walk-based search. The effectiveness of this method is demonstrated through theoretical analysis and a conceptual framework, paving the way for future research and development in applying quantum-inspired methods to classical problem-solving.
In this paper, we present two tensor network quantum-inspired algorithms to solve the knapsack and the shortest path problems, and enables to solve some of its variations. These methods provide an exact equation which returns the optimal solution of the problems. As in other tensor network algorithms for combinatorial optimization problems, the method is based on imaginary time evolution and the implementation of restrictions in the tensor network. In addition, we introduce the use of symmetries and the reutilization of intermediate calculations, reducing the computational complexity for both problems. To show the efficiency of our implementations, we carry out some performance experiments and compare the results with those obtained by other classical algorithms.
In this study, we introduce a novel family of tensor networks, termed constrained matrix product states (MPS), designed to incorporate exactly arbitrary discrete linear constraints, including inequalities, into sparse block structures. These tensor networks are particularly tailored for modeling distributions with support strictly over the feasible space, offering benefits such as reducing the search space in optimization problems, alleviating overfitting, improving training efficiency, and decreasing model size. Central to our approach is the concept of a quantum region, an extension of quantum numbers traditionally used in U(1) U ( 1 ) symmetric tensor networks, adapted to capture any linear constraint, including the unconstrained scenario. We further develop a novel canonical form for these new MPS, which allow for the merging and factorization of tensor blocks according to quantum region fusion rules and permit optimal truncation schemes. Utilizing this canonical form, we apply an unsupervised training strategy to optimize arbitrary objective functions subject to discrete linear constraints. Our method’s efficacy is demonstrated by solving the quadratic knapsack problem, achieving superior performance compared to a leading nonlinear integer programming solver. Additionally, we analyze the complexity and scalability of our approach, demonstrating its potential in addressing complex constrained combinatorial optimization problems.
This study explores quantum-inspired optimization algorithms (QIOAs) as practical tools for solving constrained combinatorial problems on classical machines. We analyze tasks that can be formulated in Ising or QUBO forms and benchmark three representative approaches: Simulated Bifurcation (SB), Quantum-Inspired Particle Swarm Optimization (QPSO), and a tensor-network (TN) method. A unified evaluation protocol is introduced that captures solution quality, feasibility, runtime, energy consumption, parameter sensitivity, and scalability. To ensure reliable performance, we develop automated penalty-adjustment and schedule-calibration strategies. Experimental results across diverse benchmarks show that SB and TN together yield the best trade-offs between accuracy and runtime: SB is efficient on dense instances with moderate penalties and low energy use, while TN is advantageous for sparse or modular networks, especially on GPUs. QPSO is competitive in smoother landscapes but more sensitive to hyperparameters. By linking performance to structural properties such as spectral gaps and constraint activity, we provide guidance for selecting suitable algorithms. Overall, QIOAs emerge as effective intermediaries between existing classical heuristics and emerging quantum devices, supported by a reproducible, energy-aware evaluation framework.