Quantum measurement and feedback control of nano-mechanical systems and atomic spin-ensembles

Hideo Mabuchi

Kurt A. Jacobs

We give an introduction to feedback control in quantum systems, as well as an overview of the variety of applications which have been explored to date. This introductory review is aimed primarily at control theorists unfamiliar with quantum mechanics, but should also be useful to quantum physicists interested in applications of feedback control. We explain how feedback in quantum systems differs from that in traditional classical systems, and how in certain cases the results from modern optimal control theory can be applied directly to quantum systems. In addition to noise reduction and stabilization, an important application of feedback in quantum systems is adaptive measurement, and we discuss the various applications of adaptive measurements. We finish by describing specific examples of the application of feedback control to cooling and state-preparation in nano-electro-mechanical systems and single trapped atoms.

Ramon van Handel, John K. Stockton, Hideo Mabuchi +1 more

Massimiliano Rossi, David Mason, Junxin Chen +2 more

N. Behbood, Giorgio Colangelo, Ferran Martin Ciurana +3 more

We apply entropy removal by measurement and feedback to a cold atomic spin ensemble. Using quantum nondemolition probing by Faraday rotation measurement, and feedback by weak optical pumping, we drive the initially random collective spin variable F toward the origin F=0. We use input-output relations and ensemble quantum noise models to describe this quantum control process and identify an optimal two-round control procedure. We observe 12 dB of spin noise reduction, or a factor-of-63 reduction in phase-space volume. The method offers a nonthermal route to generation of exotic entangled states in ultracold gases, including macroscopic singlet states and strongly correlated states of quantum lattice gases.

N. Behbood, B. Dubost, M. Napolitano +4 more

We report on an experiment underway for generating singlet states in a cold atomic ensemble. We use quantum non-demolition measurement and feedback control to produce a macroscopic spin state with total spin zero and reduced spin fluctuations. We have developed a new detection system with the capability of real time measurement and feedback control. Simulations indicate that optical pumping is a good candidate for feedback.