The very first definition of Machine Learning, coined in 1959 by the pioneering father Arthur Samuel, is as follows: “[the] field of study that gives computers the ability to learn without being explicitly programmed”. Okay, definitions are boring. Let me give an analogy: think of machine ...
In practice, a more relevant scenario to consider instead of arbitrary unitaries is learning a unitary that can be represented by a polynomial-depth quantum circuit. This class of unitaries corresponds to those that can be efficiently implemented on a quantum computer, and it is exponentially smal...