Breakdown of the Quantum Distinction of Regular and Chaotic Classical Dynamics in Dissipative Systems.

Quantum chaos has recently received increasing attention due to its relationship with experimental and theoretical studies of nonequilibrium quantum dynamics, thermalization, and the scrambling of quantum information. In an isolated system, quantum chaos refers to properties of the spectrum that emerge when the classical counterpart of the system is chaotic. However, despite experimental progress leading to longer coherence times, interactions with an environment can never be neglected, which calls for a definition of quantum chaos in dissipative systems.

Analysis of chaos and regularity in the open Dicke model.

We present an analysis of chaos and regularity in the open Dicke model, when dissipation is due to cavity losses. Due to the infinite Liouville space of this model, we also introduce a criterion to numerically find a complex spectrum which approximately represents the system spectrum. The isolated Dicke model has a well-defined classical limit with two degrees of freedom.

Quantum measurement optimization by decomposition of measurements into extremals.

Using the convex structure of positive operator value measurements and several quantities used in quantum metrology, such as quantum Fisher information or the quantum Van Trees information, we present an efficient numerical method to find the best strategy allowed by quantum mechanics to estimate a parameter. This method explores extremal measurements thus providing a significant advantage over previously used methods. We exemplify the method for different cost functions in a qubit and in a harmonic oscillator and find a strong numerical advantage when the desired target error is sufficiently small.

Asymptotic focusing properties of a sinusoidal phase grating.

The lensing behavior of sinusoidal phase gratings is studied in the limit of large phase amplitudes phi. The resulting asymptotic expressions are compared with the exact results and are shown to represent excellent approximations even for moderate values of phi. They are well suited to a quantitative description of focusing imperfections caused by the strong spherical aberrations encountered in this type of microlens array.