QUANTum Information Circuits
From the point of view of theoretical methods, the group follows a mathematical system theory approach. The classical concepts such as the observability, state/parameter estimation, filtering, controllability and feedback stabilization need to be adapted or even partly re-considered to take into account the particular features of quantum systems. Some of these features are related to the irreversible and partial nature of quantum measurements as well as their back-action on the state of the quantum system. The group also studies the properties of open quantum systems described by stochastic differential equations and hidden Markov chains. Finally, for systems composed of many interacting sub-systems, they explore the particular structure provided by the tensor product of simple models such as spin 1/2 particles and quantum harmonic oscillators. On the experimental side, they develop new quantum information processing devices based on quantum superconducting circuits. Indeed, by combining superconducting circuits in low temperatures and using techniques from microwave measurements, the macroscopic and collective degrees of freedom such as the voltage and the current are forced to behave according to the laws of quantum mechanics.