Robust control of quantum systems by quantum systems

Quantum systems can be controlled by other quantum in a reversible way, without any information leaking to the outside of system-controller compound. Such coherent control is deterministic, less noisy than measurement-based feedback control, and has potential applications variety technologies, including computation, communication, metrology. Here we introduce protocol, consisting sequence identical interactions with controlling systems, that steers system from an arbitrary initial state towards target state. We determine broad class such channels achieve convergence state, then stabilize as well protect it against noise. Our results imply also weak counter noise if they occur suitably high frequency. present example scheme does not require knowledge encoded controllers, which could result computation. It thus provides mechanism for autonomous, purely closed-loop control.