I. Introduction
Ising machines, a new type of non-von Neumann computing scheme specialized in solving combinatorial optimization problems, have attracted increasing attention recently. Since the first commercial quantum annealer emerged in 2011 [1], various types of Ising machines have been developed based on electronic and photonic systems as well as quantum systems [2]. A common feature of Ising machines is the minimization of the energy function of an Ising model or its equivalent; i.e., a quadratic unconstrained binary optimization (QUBO) form given by \begin{equation*}H=\displaystyle \sum_{i}\sum_{j}q_{i,j^{X}i^{X}j},\ q_{i,j}\in \mathbb{R},\ x_{i}\in\{0,1\}. \tag{1}\end{equation*}