Universal single-qubit quantum controller-observers

A feedback loop consists of three dynamical systems: an observer, a controller, and the system to be manipulated. Because of back-action in quantum systems, quantum controllers can act. as quantum observers and vice-versa. We demonstrate that under very general conditions, a single quantum bit can serve as both a full controller and full observer of a quantum system for a feedback loop. By similar techniques, we show that a qubit can be harnessed to serve as a quantum communication channel between two systems, and that multiple systems call be connected together to create all efficient universal quantum computer. We propose experimental realizations of our approach, and explore the implications for controllability, observability, and quantum information processing.