Dispersive regime of the Jaynes-Cummings and Rabi lattice

Photon-based strongly-correlated lattice models like the Jaynes-Cummings and Rabi lattices differ from their more conventional relatives like the Bose-Hubbard model by the presence of an additional tunable parameter: the frequency detuning between the pseudo-spin degree of freedom and the harmonic mode frequency on each site. Whenever this detuning is large compared to relevant coupling strengths, the system is said to be in the dispersive regime. The physics of this regime is well-understood at the level of a single Jaynes-Cummings or Rabi site. Here, we extend the theoretical description of the dispersive regime to lattices with many sites, for both strong and ultra-strong coupling. We discuss the nature and spatial range of the resulting qubit-qubit and photon-photon coupling, demonstrate the emergence of photon- pairing and squeezing, and illustrate our results by exact diagonalization of the Rabi dimer.Comment: 22 pages, 7 figures, 1 table, Published by NJP, Focus Issues "Focus on Quantum Microwave Field Effects in Superconducting Circuits

Positive- and negative-frequency noise from an ensemble of two-level fluctuators

The analysis of charge noise based on the Bloch-Redfield treatment of an ensemble of dissipative two-level fluctuators generally results in a violation of the fluctuation-dissipation theorem. The standard Markov approximation (when applied to the two-level fluctuators coupled to a bath) can be identified as the main origin of this failure. The resulting decoherence rates only involve the bath response at the fluctuator frequency, and thus completely neglect the effects of frequency broadening. A systematic and computationally convenient way to overcome this issue is to employ the spectator-qubit method: by coupling an auxiliary qubit to the two-level fluctuator ensemble, an analytical approximation for $S({\omega})$ fully consistent with the fluctuation-dissipation theorem can be obtained. We discuss the resulting characteristics of the noise which exhibits distinct behavior over several frequency ranges, including a $1/f$ to $1/f^2$ crossover with a $T^3$ temperature dependence of the crossover frequency.Comment: 15 pages, 6 figure

Can Auditors Be Independent? - Experimental Evidence

The Sarbanes-Oxley Act of 2002 has transformed the institutional environment in the US by making the audit committee responsible for the appointment, compensation and oversight of the auditor. We examine whether this institutional change successfully resolves the alleged problem of an unconscious favoring of the management (Bazerman et al. 1997, 2002, 2006) by changing the effects of auditors’ economic incentives and psychological pressure. In our experimental design, we make use of the particular features of the German institutional setting as it enables us to manipulate the client of the auditor in a realistic and clear-cut way. 72 German auditors with at least two years of job experience participated in our experiment. Following Turner (2001), we distinguish in our analyses between belief tasks (e.g. evidence evaluation) and action tasks (e.g. audit opinion). Our findings imply that certain institutional features seem to be helpful in ensuring auditor independence. First, we find that auditors demonstrate professional scepticism in belief tasks. This seems to counteract any potentially negative effect of the acceptability heuristic in actions tasks. Second, experience helped auditors in coping with psychological pressure. Third, making the auditor accountable to a supervisory board was helpful in reducing the risk that financial considerations would impair auditor independence.

Resummation for Nonequilibrium Perturbation Theory and Application to Open Quantum Lattices

Lattice models of fermions, bosons, and spins have long served to elucidate the essential physics of quantum phase transitions in a variety of systems. Generalizing such models to incorporate driving and dissipation has opened new vistas to investigate nonequilibrium phenomena and dissipative phase transitions in interacting many-body systems. We present a framework for the treatment of such open quantum lattices based on a resummation scheme for the Lindblad perturbation series. Employing a convenient diagrammatic representation, we utilize this method to obtain relevant observables for the open Jaynes-Cummings lattice, a model of special interest for open-system quantum simulation. We demonstrate that the resummation framework allows us to reliably predict observables for both finite and infinite Jaynes-Cummings lattices with different lattice geometries. The resummation of the Lindblad perturbation series can thus serve as a valuable tool in validating open quantum simulators, such as circuit-QED lattices, currently being investigated experimentally.Comment: 15 pages, 9 figure