“…On the other hand, machine learning algorithms on a quantum processor have been developed since the invention of Harrow-Hassidim-Lloyd (HHL) algorithm, and they are dubbed as "quantum machine learning" [9,10,11,12,13,14,15,16]. In the last few years, there has been surging interest in quantum machine learning leveraging the variational method [17,18,19,20,21,22] because a primitive type of quantum computers is about to be realized in the near future, and such machines may have the potentials to outperform classical computers [23,24]. Those near-term quantum computers are called noisy intermediate-scale quantum (NISQ) devices [25] and consist of hundreds to thousands of physical, non-fault-tolerant qubits.…”