  The development of quantum control methods is an essential task for emerging
quantum technologies. In general, the process of optimizing quantum controls
scales very unfavorably in system size due to the exponential growth of the
Hilbert space dimension. Here, I present a scalable subsystem-based method for
quantum optimal control on a large quantum system. The basic idea is to cast
the original problem as a robust control problem on subsystems with requirement
of robustness in respect of inter-subsystem couplings, thus enabling a drastic
reduction of problem size. The method is in particular suitable for target
quantum operations that are local, e.g., elementary quantum gates. As an
illustrative example, I employ it to tackle the demanding task of pulse
searching on a 12-spin coupled system, achieving substantial reduction of
memory and time costs. This work has significant implications for coherent
quantum engineering on near-term intermediate-scale quantum devices.
