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Study of the Dicke model: from phase space approach to quantum trajectories

D. M. Oliveira, Felipe Dimer

Published Jan 1, 2008• 1 citation
OpenAlex

Abstract

In this thesis we study the Dicke model outside the rotating wave approximation (RWA), by employing phase space techniques and the quantum trajectory theory. We present a review of the basic models of open systems in quantum optics and present an experimental proposition justifying the model to be studied. We use the phase space approach to study, among other subjects, entanglement, squeezing and fluctuations across a quantum phase transition. Three different phase space representations are used and their strengths and weaknesses compared. The quantum trajectory theory is applied to visualise the global quantum fluctuations and to learn how different measurement schemes will affect the creation of entanglement.

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In this thesis we study the Dicke model outside the rotating wave approximation (RWA), by employing phase space techniques and the quantum trajectory theory. We present a review of the basic models of open systems in quantum optics and present an experimental proposition justifying the model to be studied. We use the phase space approach to study, among other subjects, entanglement, squeezing and fluctuations across a quantum phase transition. Three different phase space representations are used and their strengths and weaknesses compared. The quantum trajectory theory is applied to visualise the global quantum fluctuations and to learn how different measurement schemes will affect the creation of entanglement.
The dynamical properties of quantum entanglement in the Dicke model without rotating-wave approximation are investigated in terms of the reduced-density linear entropy. The characteristic time of decoherence process in the early-time evolution is numerically obtained and it is shown that the characteristic time decreases as the coupling parameter increases. The mean entanglement, which is defined to be averaged over time, is employed to describe the influences of both quantum phase transition and corresponding classical chaos on the behavior of entanglement. For a given energy, initial conditions are taken to be minimum uncertainty wave packets centered at regular and chaotic regions of the classical phase space. It is shown that the entanglement has a distinct change at the quantum phase transition, and that the entanglement for regular initial conditions is smaller than that for chaotic ones in the case of weak coupling, while it fluctuates with small amplitude in strong coupling and for chaotic initial conditions.
Under the condition of classical limit,quantized Dicke model shows the dynamical behavior of quantum chaos.The numerical calculation shows that there are some particular points in the classical phase-space at which the quantum chaos is present.The impact of initial conditions of these particular positions on entanglement of the model was analyzed respectively under RWA(rotating-wave approximation) or non-RWA.The evolutionary characteristics of the overall dynamics were studied in the phase-space for all the phase points considered to be under the initial condition.The conclusion obtained is that classical chaos will accelerate the generation of entanglement.
In the vicinity of ground-state phase transitions quantum correlations can display nonanalytic behavior and critical scaling. This signature of emergent collective effects has been widely investigated within a broad range of equilibrium settings. However, under nonequilibrium conditions, as found in open quantum many-body systems, characterizing quantum correlations near phase transitions is challenging. Moreover, the impact of local and collective dissipative processes on quantum correlations is not broadly understood. Here we consider as a paradigmatic setting the superradiant phase transition of the open quantum Dicke model and characterize quantum and classical correlations across the phase diagram. We develop an approach to quantum fluctuations which allows us to show that local dissipation, which cannot be treated within the commonly employed Holstein-Primakoff approximation, rather unexpectedly leads to an enhancement of collective quantum correlations, and to the emergence of a nonequilibrium superradiant phase in which the bosonic and spin degrees of freedom of the Dicke model are entangled.
We show how the use of variational states to approximate the ground state of a system can be employed to study a multimode Dicke model. One of the main contributions of this work is the introduction of a not very commonly used quantity, the cooperation number, and the study of its influence on the behavior of the system, paying particular attention to the quantum phase transitions and the accuracy of the used approximations. We also show how these phase transitions affect the dependence of the expectation values of some of the observables relevant to the system and the entropy of entanglement with respect to the energy difference between atomic states and the coupling strength between matter and radiation, thus characterizing the transitions in different ways.