I. Introduction
Quadratic unconstrained binary optimization (QUBO) problems, also known as unconstrained binary quadratic programs, are a major class of combinatorial optimization problems, most of which are NP-hard [1]. QUBO problems appear in a variety of application areas, such as quantum computing [1], [2], graph-cut optimization [3], [4], [5], video segmentation [6], visual recognition [7], data hashing [8], human tracking [9], vehicle routing [10], network flow optimization [11], material composition optimization [12], magnet array optimization [13], model predictive control [14], unit commitment [15], power network reconfiguration [16], scientific computing [17].