Minimizing minor embedding energy: an application in quantum annealing

Abstract

A significant challenge in quantum annealing is to map a real-world problem onto a hardware graph of limited connectivity. When the problem graph is not a subgraph of the hardware graph, one might employ minor embedding in which each logical qubit is mapped to a tree of physical qubits. Pairwise interactions between physical qubits in the tree are set to be ferromagnetic with some coupling strength F<0. Here we address the theoretical question of what the best value F should be in order to achieve unbroken trees in the pre-quantum-processing. The sum of |F| for each logical qubit is defined as minor embedding energy, and the best value F is obtained when the minor embedding energy is minimized. We also show that our new analytical lower bound on |F| is a tighter bound than that previously derived by Choi (Quantum Inf Process 7:193–209, 2008). In contrast to Choi’s work, our new method depends more delicately on minor embedding parameters, which leads to a higher computational cost.