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Acta Physica Sinica

Relations of classical-quantum correspondencein Dicke model

Wang Yu-Bo, Song Li-Jun, Dong Yan, Gai Yong-Jie

Published Jan 1, 2011• 7 citations
Publisher

Abstract

Dicke model displays quantum chaotic dynamic properties in the non-rotating wave approximation. On the basis of properties of the classical phase space of Dicke model, we employ the one-to-many notion, namely, evolution from one point on the classical phase space to two initial coherent states. Then we obtain a good quantum phase space, which corresponds to the classical one, by using the method of averaging the statistical entangled values of two initial states in the evolution. The numerical computation shows that classical chaos can promote the origination of bipartite entanglement, and simultaneously, the average entanglement can be regarded as the signature of quantum chaos. A good classica-quantum correspondence can be obtained by using the average entanglement.

DOI
10.7498/aps.60.020302

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In this thesis, we explore the Dicke model, well-known for its explanation of the superradiant phase transition and signatures of Classical and Quantum Chaos. We then propose a variational approach to determine the semi-classical (SC) equations of motion, using coherent states. The SC dynamics exhibits signs of classical chaos both with increasing coupling and as the energy of the system is gradually increased, eventually leading to it completely engulfing the 2 dimensional phase space of the pseudospin, which we examine through Poincare sections. We then use numerical diagonalization to study the Quantum Dicke model - it is observed that at and after exactly the critical value of the coupling, the system shows a net macroscopic photon expectation value, and the pseudospin component in the z direction starts to steadily decline - implying a net fraction of the atoms are shifting to the excited state. We then study the dynamics of the Quantum Dicke Hamiltonian using coherent states and compare the expectation values of the dimensional with the classical trajectories from the semi-classical dynamics for stable and chaotic orbits. We then introduce a linear quench which drives the Dicke model across the Superradiant phase transition by changing the coupling strength. The quenched dynamics is simulated using both semi-classical and quantum dynamics - it is seen that dynamically varying quantities are periodically oscillating around their instantaneous steady-state coupling values.

•May 1, 2016•0 citations
Dicke model displays quantum chaotic dynamic properties in the non-rotating wave approximation. On the basis of properties of the classical phase space of Dicke model, we employ the one-to-many notion, namely, evolution from one point on the classical phase space to two initial coherent states. Then we obtain a good quantum phase space, which corresponds to the classical one, by using the method of averaging the statistical entangled values of two initial states in the evolution. The numerical computation shows that classical chaos can promote the origination of bipartite entanglement, and simultaneously, the average entanglement can be regarded as the signature of quantum chaos. A good classica-quantum correspondence can be obtained by using the average entanglement.
The Dicke model displays quantum chaotic dynamic properties in the without-rotating-wave approximation. We explore the dynamic properties of the single-particle coherence in Dicke model by using the first-order temporal correlation function and numerical simulation. The results reveal that the first-order temporal correlation function decays very rapidly when the initial coherent state is centered in chaotic regions, but rather slowly when the initial coherent state is centered in regular regions. This indicates that the single-particle coherence is highly sensitive to initial states, and the classical chaos suppresses quantum coherence. The mean single particle coherence during the evolution is studied, and a better quantum-classical correspondence is obtained. Finally, the dynamics of single-particle coherence in the whole phase space is investigated, which reveals the chaotic and regular structures of the phase space more clearly.
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.
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.
Now, many different approaches have been presented to study the different semi-classical models derived from the Dicke Hamiltonian, which reflect a fact that the quantum-mechanical spin possesses no direct classical analog. The Hartree-Fock-type approximation is one of the widely used approaches, with which we derive the Heisenberg equations of motion for the system and replace the operators in these equations with the corresponding expectation values. In the present paper, we investigate the role of quantum phase transition in determining the chaotic property of the time-dependent driven Dicke model. The semi-classical Hamiltonian is derived by evaluating the expectation value of the Dicke Hamiltonian in a state, which is a product state of photonic and atomic coherent states. Based on the inverse of the relations between the position-momentum representation and the Bosonic creation-annihilation operators, the Hamiltonian is rewritten in the position-momentum representation and it undergoes a spontaneous symmetry-breaking phase transition, which is directly analogous to the quantum phase transition of the quantum system. In order to depict the Poincaré sections, which are used to analyze the trajectories through the four-dimensional phase space, we give the equations of motion of system from the derivatives of the semi-classical Hamiltonian for a variety of different parameters and initial conditions. According to the Dicke quantum phase transition observed from the experimental setup , we study the effect of a monochromatic non-adiabatic modulation of the atom-field coupling in Dicke model (i.e., the driven Dicke model) on the system chaos by adjusting the pump laser intensity. The change from periodic track to chaotic figure reflects the quantum properties of the system, especially the quantum phase transition point, which is a key position for people to analyse the shift from periodic orbit to chaos. In an undriven case, the system reduces to the standard Dicke model. We discover from the Poincaré sections that the system undergoes a change from the classical periodic orbit to a number of chaotic trajectories and in the superradiant phase area, the whole phase space is completely chaotic. When the static and driving coupling both exist, the system shows rich chaotic motion. The ground state properties are mainly determined by the static coupling, while the orbit of the system is adjusted by the driving coupling. If the static coupling is greater than the critical coupling, the system displays completely chaotic images in the Poincaré sections, and the periodic orbits in the chaos can also be adjusted by the strong driving coupling. While the static coupling is less than the critical coupling, the system can also show the chaotic images by adjusting the driving coupling strength and oscillation frequency. Moreover, by tuning the oscillation frequency, the Poincaré sections may change from the classical orbits to the chaos, and back to the classical orbits in the normal phase of the system.
We study spin squeezing, an intrinsic quantum property, in the Dicke model without the rotating-wave approximation. We show that the spin squeezing can reveal the underlying chaotic and regular structures in phase space given by a Poincaré section, namely, it acts as an indicator of quantum chaos. Spin squeezing vanishes after a very short time for an initial coherent state centered in a chaotic region, whereas it persists over a longer time for the coherent state centered in a regular region of the phase space. We also study the distribution of the mean spin directions when quantum dynamics takes place. Finally, we discuss relations among spin squeezing, bosonic quadrature squeezing, and two-qubit entanglement in the dynamical processes.
In this thesis, we explore the Dicke model, well-known for its explanation of the superradiant phase transition and signatures of Classical and Quantum Chaos. We then propose a variational approach to determine the semi-classical (SC) equations of motion, using coherent states. The SC dynamics exhibits signs of classical chaos both with increasing coupling and as the energy of the system is gradually increased, eventually leading to it completely engulfing the 2 dimensional phase space of the pseudospin, which we examine through Poincare sections. We then use numerical diagonalization to study the Quantum Dicke model - it is observed that at and after exactly the critical value of the coupling, the system shows a net macroscopic photon expectation value, and the pseudospin component in the z direction starts to steadily decline - implying a net fraction of the atoms are shifting to the excited state. We then study the dynamics of the Quantum Dicke Hamiltonian using coherent states and compare the expectation values of the dimensional with the classical trajectories from the semi-classical dynamics for stable and chaotic orbits. We then introduce a linear quench which drives the Dicke model across the Superradiant phase transition by changing the coupling strength. The quenched dynamics is simulated using both semi-classical and quantum dynamics - it is seen that dynamically varying quantities are periodically oscillating around their instantaneous steady-state coupling values.