Towards Exponential Quantum Improvements in Solving Cardinality-Constrained Binary Optimization

TL;DR

This paper introduces a quantum algorithm leveraging Grover's search for solving cardinality-constrained binary optimization problems more efficiently, demonstrating exponential improvements and practical hybrid classical-quantum frameworks.

Contribution

The work presents a novel Grover-based quantum algorithm exploiting problem structure and a hybrid ADMM framework for efficient approximate solutions in constrained binary optimization.

Findings

Achieves exponential reduction in Grover iterations for quadratic objectives.

Provides a hybrid quantum-classical framework with provable approximation guarantees.

Demonstrates potential for quantum advantage in constrained binary optimization.

Abstract

Cardinality-constrained binary optimization is a fundamental computational primitive with broad applications in machine learning, finance, and scientific computing. In this work, we introduce a Grover-based quantum algorithm that exploits the structure of the fixed-cardinality feasible subspace under a natural promise on solution existence. For quadratic objectives, our approach achieves ${O}\left(\sqrt{\frac{\binom{n}{k}}{{M}}}\right)$ Grover rotations for any fixed cardinality $k$ and degeneracy of the optima $M$, yielding an exponential reduction in the number of Grover iterations compared with unstructured search over $\{0,1\}^n$. Building on this result, we develop a hybrid classical--quantum framework based on the alternating direction method of multipliers (ADMM) algorithm. The proposed framework is guaranteed to output an $\epsilon$-approximate solution with a consistency tolerance $\epsilon + \delta$ using at most $ {O}\left(\sqrt{\binom{n}{k}}\frac{n^{6}k^{3/2} }{ \sqrt{M}\epsilon^2 \delta }\right)$ queries to a quadratic oracle, together with ${O}\left(\frac{n^{6}k^{3/2}}{\epsilon^2\delta}\right)$ classical overhead. Overall, our method suggests a practical use of quantum resources and demonstrates an exponential improvements over existing Grover-based approaches in certain parameter regimes, thereby paving the way toward quantum advantage in constrained binary optimization.