Feedback control of quantum system

Seth Lloyd

This paper discusses fully coherent quantum feedback control, in which the sensors, controller, and actuators are quantum systems and interact coherently with the system to be controlled: as a result, the entire feedback loop is coherent. Unlike conventional semiclassical feedback control of quantum systems, feedback control by quantum controllers is not stochastic, preserves the initial state of the controlled system, and can control quantum systems in ways that are not possible using conventional, incoherent feedback control. In particular, the target state to which the quantum controller drives the system can be entangled with another quantum system. This paper investigates quantum controllers and states necessary and sufficient conditions for a Hamiltonian quantum system to be observable and controllable by a quantum controller.

M. R. James

G. Huang

M. R. James

Recent theoretical and experimental advances mean that it is now possible to control physical systems at the quantum level. Indeed, developments in quantum technology provide strong motivation for the feedback control of quantum systems. This paper will discuss both measurement feedback and coherent feedback control for quantum systems. Measurement feedback involves conditioning on the measurement information, and so we describe the Belavkin quantum filter and its use in quantum control. In contrast, coherent feedback does not involve any measurements, and we discuss some recent results on coherent feedback control.

J. W. Clark, Dennis G. Lucarelli, Tzyh‐Jong Tarn

Richard J. Nelson, Yaakov S. Weinstein, David G. Cory +1 more

In the conventional picture of quantum feedback, control sensors make measurements on a quantum system, a classical controller processes the results of the measurements, and semiclassical actuators act back on the system to alter its behavior. We describe and provide an experimental demonstration of an alternative method for quantum feedback control, in which the sensors, controller, and actuators of conventional feedback control are replaced with quantum systems that interact coherently with the system to be controlled. The resulting control system represents a fully coherent quantum feedback loop.