A practically scalable approach to the closest vector problem for sieving via QAOA with fixed angles

The NP-hardness of the closest vector problem (CVP) is an important basis for quantum-secure cryptography, in much the same way that integer factorisation’s conjectured hardness is at the foundation of cryptosystems like RSA. Recent work with heuristic quantum algorithms (Yan et al 2022 arXiv:2212.12372 [quant-ph]) indicates the possibility to find close approximations to (constrained) CVP instances that could be incorporated within fast sieving approaches for factorisation. This work explores both the practicality and scalability of the proposed heuristic approach to explore the potential for a quantum advantage for approximate CVP, without regard for the subsequent factoring claims. We also extend the proposal to include an antecedent ‘pre-training’ scheme to find and fix a set of parameters that generalise well to increasingly large lattices, which both optimises the scalability of the algorithm, and permits direct numerical analyses. Our results further indicate a noteworthy quantum speed-up for lattice problems obeying a certain ‘prime’ structure, approaching fifth order advantage for quantum approximate optimisation algorithm of fixed depth p = 10 compared to classical brute-force, motivating renewed discussions about the necessary lattice dimensions for quantum-secure cryptosystems in the near-term.