A Survey on Quantum Reinforcement Learning

With recent advances in the fabrication and control of hardware for quantum information processing, the possibilities of merging quantum computing (QC) with machine learning (ML) have received a huge amount of attention within the growing research community. Hereby, reinforcement learning (RL) is the third paradigm besides supervised and unsupervised learning. In this survey article, we provide an overview over so-called quantum reinforcement learning (QRL) algorithms. We understand these as quantum-assisted approaches, that solve a particular task (be they classical or quantum in nature) by employing quantum resources (either in simulation and/or in experiment). In order to keep this contribution as self-contained as possible, we provide the necessary backgrounds before venturing into the QRL literature. We start out with a brief recap of the essentials of the RL paradigm in the fully classical setting in Sec. 2. Further, in Sec. 3 we provide a quick introduction to QC and variational quantum circuits (VQCs). Readers familiar with either of the topics may safely skip these sections. In Sec. 4 we turn our attention to the emerging field of QRL, starting out with a quick overview of the literature.