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Physical Review A

Quantum fluctuations in unstable dissipative systems

S. T. Gevorkyan, Gagik Yu. Kryuchkyan, Naira T. Muradyan

Published Mar 13, 2000• 31 citations
Publisher

Abstract

We show how quantum instability is displayed in the von Neumann entropy and in the Wigner function. For this purpose, an intracavity second-harmonic generation close to the Hopf bifurcation range is studied. We examine the role of dissipation in unstable dynamics and the formation of the quantum states of the cavity modes, and discuss contrast ensemble behavior with that of individual realization on the basis of a quantum-jump simulation method. Namely, it is found that the Wigner functions for fundamental and second-harmonic modes prepared initially in a vacuum state acquire the three-hump structure due to phase symmetry breaking in the bifurcation range. The time evolution of coherent states leads to long-lived swing of the system between two side humps in a phase space.

DOI
10.1103/physreva.61.043805

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We show how quantum instability is displayed in the von Neumann entropy and in the Wigner function. For this purpose, an intracavity second-harmonic generation close to the Hopf bifurcation range is studied. We examine the role of dissipation in unstable dynamics and the formation of the quantum states of the cavity modes, and discuss contrast ensemble behavior with that of individual realization on the basis of a quantum-jump simulation method. Namely, it is found that the Wigner functions for fundamental and second-harmonic modes prepared initially in a vacuum state acquire the three-hump structure due to phase symmetry breaking in the bifurcation range. The time evolution of coherent states leads to long-lived swing of the system between two side humps in a phase space.
We investigate quantum-mechanical counterpart of a classical instability in a phase space by the numerical method of quantum trajectories with moving basis. As an application the model of coupled two oscillators driven by a monochromatic force in the presence of dissipation (intracavity second harmonic generation) is analyzed. The system of interest is characterized by two bifurcations leading to ranges of instability: the Hopf bifurcation which connects a steady state dynamics of the oscillatory modes to a self-pulsing temporal dynamics and the bifurcation of the period-doubling. The both two regimes are analyzed on the framework of the semiclassical phase trajectories and the Wigner functions of the oscillatory modes in phase space.
For the process of intracavity second-harmonic generation in the region of unstable dynamics, the joint fluctuations of numbers of photons and phases of the fundamental mode and second harmonic are studied in positive P representation. It is shown that above the bifurcation point of the optical system the distribution functions of the phases of the fundamental mode and second harmonic have a two-component structure. The quantum trajectories of the phases of these modes are not localized in the components of their states, which demonstrates the presence of quantum-mechanical interference between these components. In both components of each state the same values of numbers of photons are realized. It is shown that in the case of coherent initial states of both modes the joint distribution functions of the numbers of photons and phases are in the oscillatory regime. With time, the modes pass from a two-component delocalized state to a one-component one and then back to the two-component state. The time of oscillation of the distribution function is of the order of the photon's lifetime in the cavity volume. Self-matching of phases of interacting modes is investigated above the bifurcation point.
The principle of ergodicity of the quantum theory has been used for elaboration of a new technique for numerical simulation of the Wigner function of open dissipative quantum systems. With this purpose the density matrix of a quantum system is represented via averaging over the ensemble of quantum states in time intervals instead of averaging over the ensemble of stochastic variables. It is shown that this approach leads to new approximate expressions for quantum distributions in the phase space, in particular, Wigner functions for systems localized in the region of classical phase trajectories. As an application, the Wigner functions are calculated for the process of intracavity second harmonic generation in the region of Hopf bifurcations.
We study the quantum dynamics of the number of photons of the interacting modes, the dynamics of the quantum entropy, and the Wigner function of the states of the fundamental and the third harmonic modes for the process of intracavity third harmonic generation. It is shown that the quantum dynamics of the system strongly depends on the external resonant perturbation of the fundamental mode and on the coupling coefficient of the interacting modes. In the region of long interaction times, the modes of the field can be both in stable and in unstable states—depending on the above-mentioned quantities. In the paper, we also investigate the dynamics of transition of the system from stable to unstable states.