  The rapid development of quantum computer hardware has laid the hardware
foundation for the realization of QNN. Due to quantum properties, QNN shows
higher storage capacity and computational efficiency compared to its classical
counterparts. This article will review the development of QNN in the past six
years from three parts: implementation methods, quantum circuit models, and
difficulties faced. Among them, the first part, the implementation method,
mainly refers to some underlying algorithms and theoretical frameworks for
constructing QNN models, such as VQA. The second part introduces several
quantum circuit models of QNN, including QBM, QCVNN and so on. The third part
describes some of the main difficult problems currently encountered. In short,
this field is still in the exploratory stage, full of magic and practical
significance.
