Abstract

Constraint programming (CP) is a paradigm used to model and solve constraint satisfaction and combinatorial optimization problems. In CP, problems are modeled with constraints that describe acceptable solutions and solved with backtracking tree search augmented with logical inference. In this paper, we show how quantum algorithms can accelerate CP, at both the levels of inference and search. Leveraging existing quantum algorithms, we introduce a quantum-accelerated filtering algorithm for thealldifferentglobal constraint and discuss its applicability to a broader family of global constraints with similar structure. We propose frameworks for the integration of quantum filtering algorithms within both classical and quantum backtracking search schemes, including a novel hybrid classical-quantum backtracking search method. This work suggests that CP is a promising candidate application for early fault-tolerant quantum computers and beyond.

Highlights

  • Constraint programming (CP) is a paradigm used to model and solve constraint satisfaction and combinatorial optimization problems [1]

  • In the context of inference, we explore the use of quantum algorithms for graph problems, especially that for finding maximum matchings in graphs [13], to accelerate classical inference algorithms in CP

  • While we argue that our proposals are suitable for early generations of such devices because their hybrid nature allows for putting smaller parts of the problem on the device, we do not expect that the quantum algorithms we discuss will be successfully implementable using NISQ devices

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Summary

Introduction

Constraint programming (CP) is a paradigm used to model and solve constraint satisfaction and combinatorial optimization problems [1]. The encapsulation of combinatorial substructure within CP models provides an elegant mechanism for carving off portions of complex problems into inference subproblems that can be solved by a quantum co-processor. These smaller subproblems require fewer resources, making them promising candidates for the early faulttolerant quantum computers of the future. Our initial explorations indicate the potential for symbiosis between the two paradigms: quantum algorithms can accelerate both inference and search in CP, and CP offers an attractive, modular formalism for tackling hard problems that makes it a promising candidate application for early fault-tolerant quantum computers, and beyond.

Constraint Programming Background
Constraint satisfaction problems
Backtracking search algorithms
Branch-and-infer search
Propagation Function
Branching Operator
Consistency
Global constraints
CP modeling and solving
Related work
Quantum resources and data access
Quantum-accelerated global constraint filtering
The alldifferent constraint
Classical filtering algorithm
Subroutine
Generalizing quantum filtering
Quantum-accelerated branch-and-infer search
Classical backtracking with quantum-accelerated inference
Exact method
Bounded-error and heuristic methods
Quantum-accelerated backtracking
Background
Partially quantum tree search
Quantum-accelerated backtracking with inference
Representation of tree nodes for CP
Implementation of walk operators for CP
Incorporating quantum algorithms for filtering
Conclusions
A Quantum algorithm for strongly connected components
B Other global constraints
C Other CP modeling examples
Rostering problems
Sports tournament scheduling
Quadratic assignment problems
D Wrapping quantum subroutines into a single unitary

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