  Quantum information technologies, and intelligent learning systems, are both
emergent technologies that will likely have a transforming impact on our
society. The respective underlying fields of research -- quantum information
(QI) versus machine learning (ML) and artificial intelligence (AI) -- have
their own specific challenges, which have hitherto been investigated largely
independently. However, in a growing body of recent work, researchers have been
probing the question to what extent these fields can learn and benefit from
each other. QML explores the interaction between quantum computing and ML,
investigating how results and techniques from one field can be used to solve
the problems of the other. Recently, we have witnessed breakthroughs in both
directions of influence. For instance, quantum computing is finding a vital
application in providing speed-ups in ML, critical in our "big data" world.
Conversely, ML already permeates cutting-edge technologies, and may become
instrumental in advanced quantum technologies. Aside from quantum speed-up in
data analysis, or classical ML optimization used in quantum experiments,
quantum enhancements have also been demonstrated for interactive learning,
highlighting the potential of quantum-enhanced learning agents. Finally, works
exploring the use of AI for the very design of quantum experiments, and for
performing parts of genuine research autonomously, have reported their first
successes. Beyond the topics of mutual enhancement, researchers have also
broached the fundamental issue of quantum generalizations of ML/AI concepts.
This deals with questions of the very meaning of learning and intelligence in a
world that is described by quantum mechanics. In this review, we describe the
main ideas, recent developments, and progress in a broad spectrum of research
investigating machine learning and artificial intelligence in the quantum
domain.
