Quantum Circuit Learning [1]: Hybrid Framework for machine learning with low-depth quantum circuit. QCL takes input data, iteratively tunes circuit parameters and gives desired output by utilizing gradient-based optimization of parameters similar to backpropagation method.
Unitary Gate: Reversible transformations on quantum states; building blocks of quantum circuit;
[1] K. Mitarai, M. Negoro, M. Kitagawa, and K. Fujii, “Quantum Circuit
Learning,” Physical Review A, vol. 98, p. 032309, Sept. 2018.