  In recent years, natural language processing (NLP) has got great development
with deep learning techniques. In the sub-field of machine translation, a new
approach named Neural Machine Translation (NMT) has emerged and got massive
attention from both academia and industry. However, with a significant number
of researches proposed in the past several years, there is little work in
investigating the development process of this new technology trend. This
literature survey traces back the origin and principal development timeline of
NMT, investigates the important branches, categorizes different research
orientations, and discusses some future research trends in this field.
