Keldysh Field Theory for Driven Open Quantum Systems

Recent experimental developments in diverse areas - ranging from cold atomic gases over light-driven semiconductors to microcavity arrays - move systems into the focus, which are located on the interface of quantum optics, many-body physics and statistical mechanics. They share in common that coherent and driven-dissipative quantum dynamics occur on an equal footing, creating genuine non-equilibrium scenarios without immediate counterpart in condensed matter. This concerns both their non-thermal flux equilibrium states, as well as their many-body time evolution. It is a challenge to theory to identify novel instances of universal emergent macroscopic phenomena, which are tied unambiguously and in an observable way to the microscopic drive conditions. In this review, we discuss some recent results in this direction. Moreover, we provide a systematic introduction to the open system Keldysh functional integral approach, which is the proper technical tool to accomplish a merger of quantum optics and many-body physics, and leverages the power of modern quantum field theory to driven open quantum systems.Comment: 73 pages, 13 figure

Statistical periodicity in driven quantum systems: General formalism and application to noisy Floquet topological chains

Much recent experimental effort has focused on the realization of exotic quantum states and dynamics predicted to occur in periodically driven systems. But how robust are the sought-after features, such as Floquet topological surface states, against unavoidable imperfections in the periodic driving? In this work, we address this question in a broader context and study the dynamics of quantum systems subject to noise with periodically recurring statistics. We show that the stroboscopic time evolution of such systems is described by a noise-averaged Floquet superoperator. The eigenvectors and -values of this superoperator generalize the familiar concepts of Floquet states and quasienergies and allow us to describe decoherence due to noise efficiently. Applying the general formalism to the example of a noisy Floquet topological chain, we re-derive and corroborate our recent findings on the noise-induced decay of topologically protected end states. These results follow directly from an expansion of the end state in eigenvectors of the Floquet superoperator.Comment: 13 pages, 5 figures. This is the final, published versio

Rebuilding the coalition ship at sea : how uncertainty and complexity drive the reform of portfolio design in coalition cabinets

In the government formation process, coalition partners make decisions about the inner workings of their future government. However, whether the initial allocation of competencies has the desired effects is uncertain, and deals may therefore be subject to change when the government is in office. This study analyses the frequency of changes in portfolio design (i.e. the distribution of competencies among government ministries and office holders) for 112 coalition governments in eight West European democracies (1970–2015). Its central argument is that in uncertain and complex bargaining situations, coalition partners have greater difficulty finding mutually beneficial deals, and changes to the initial allocation of payoffs are therefore more likely. The results indicate that preference divergence and strenuous coalition negotiations make portfolio design reforms more likely, but show no consistent effect of the familiarity among government parties. These findings show how the bargaining context during government formation foreshadows coalition governance over the cabinet’s life-cycle