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The Mathieu–Schrodinger equation, which describes the behavior of a quantum pendulum, depending on the value of the parameter l (pendulum filament length), can have the symmetry of the Klein’s four-group or its invariant subgroups. The paper shows that the mean values of z-components of the angular momentum of nondegenerate quantum states (the symmetry region of the four-group) tend to zero and their root mean square fluctuations are non-zero. Consequently, in this region of parameter values, the fluctuations overlap the mean values of the angular momentum and they become indistinguishable. Therefore, it can be argued that if, with an increase in the parameter, the system goes into a non-degenerate state, then after the inversion of the parameter change and the transition to the region of degenerate states, the initial states will not be restored. This behavior of the average values of angular momenta is caused by the combined actions of two factors: discontinuous change in the system at the points of change of its symmetry and the presence of quantum fluctuations in nondegenerate states.

This study demonstrates how the spin quantum dynamics of a single Fe atom adsorbed on Cu(001) can be controlled and manipulated by the vibrations of a nearby copper tip attached to a nano cantilever by virtue of the dynamic magnetic anisotropy. The magnetic properties of the composite system are obtained from \emph{ab initio} calculations in completely relaxed geometries and turned out to be dependent considerably on the tip-iron distance that changes as the vibrations set in. The level populations, the spin dynamics interrelation with the driving frequency, as well as quantum information related quantities are exposed and analyzed., accepted in New Journal of Physics

The time dependence of the entanglement for two Tavis–Cummings atoms with dynamical Stark shift is studied. It is shown that the influence of the dynamical Stark shift is not equivalent for different Bell states. Namely for the states ϕ = 1 2 ( | 00 〉 ± | 11 〉 ) sudden death of entanglement happens, while for the states ψ = 1 2 ( | 01 〉 ± | 10 〉 ) we still have a rest of entanglement. A direct relation between the sudden death of entanglement and the values of the dynamical Stark shift is shown. The fidelity of the system is evaluated.

We consider the controlled switching of individual spins in a nonlinear, interacting spin chain by means of external magnetic fields. We show analytically and by full numerical simulations that stochastic switching is achievable when the driving fields are such that the underlying semi-classical dynamics is chaotic. On the basis of random matrix theory and the geometry of quantum evolution we confirm the quantum case to follow qualitatively the semi-classical behavior.

One of the main models used to study problems of ferromagnetic resonance is the Landau–Lifshitz phenomenological model. According to this model, the dynamics of the magnetization in a ferromagnet is described by a nonlinear Landau–Lifshitz equation. Because of the nonlinearity of this equation the physical properties described by it are extremely diverse. Depending on the physical situation, a system is characterized by soliton or chaotic solutions. Usually it is assumed that a necessary condition for obtaining a chaotic solution is that the system be acted upon by a random field due to fluctuations of the local magnetization. It is shown here that chaotic dynamics can also be obtained in the case of a regular external force. The conditions for the appearance of Hamiltonian chaos are determined, and numerical estimates are made for concrete substances. A kinetic equation describing the dynamics of the magnetization under conditions of stochasticity is obtained. It is shown that the solution of the kinetic...

The possibility of chaos formation is studied in terms of a generalized Jaynes-Cummings model which is a key model in the quantum electrodynamics of resonators. In particular, the dynamics of a three-level optical atom which is under the action of the resonator field is considered. The specific feature of the considered problem consists in that not all transitions between the atom levels are permitted. This asymmetry of the system accounts for the complexity of the problem and makes it different from the three-level systems studied previously. We consider the most general case, where the interaction of the system with the resonator depends on the system coordinate inside the resonator. It is shown that, contrary to the commonly accepted opinion, the absence of resonance detuning does not guarantee the system state controllability. In the course of evolution the system performs an irreversible transition from the purely quantum-mechanical state to the mixed state. It is shown that the asymmetry of the system levels accounts for the fact that the upper excited level turns out to be the most populated one.

The nonlinear system of Bloch equations describing the dynamics of the magnetization is investigated in the cases of weak and strong nonlinearity. Nonlinearity appears as a consequence of the influence of the oscillator circuit. It is shown that in the case of weak nonlinearity one can use perturbation theory to obtain an analytical solution that agrees well with the results of numerical calculations. In the case of strong nonlinearity the motion of the magnetization can have a stochastic character, similar to a strange attractor. The fractal dimension of the stochastic attractor is estimated.

The influence of the dynamical Stark shift on the thermal entanglement and the efficiency of the quantum Otto cycle is studied for the su(1,1) Tavis-Cummings system. It is shown that the degree of the thermal entanglement becomes larger as the dynamical Stark shift increases. In contrast, the efficiency of the Otto cycle is degraded with an increase of the values of dynamical Stark shift. Expressions for the efficiency coefficient are derived. Using those expressions we identify the maximal efficiency of the quantum Otto cycle from the experimentally measured values of the dynamical Stark shift, Comment: to appear in J.Phys. A

We study the role of thermal fluctuations on the spin dynamics of a thin permalloy film with a focus on the behavior of spin torque and find that the thermally assisted spin torque results in new aspects of the magnetization dynamics. In particular, we uncover the formation of a finite, spin torque-induced, in-plane magnetization component. The orientation of the in-plane magnetization vector depends on the temperature and the spin-torque coupling. We investigate and illustrate that the variation of the temperature leads to a thermally-induced rotation of the in-plane magnetization., Comment: 9 pages, 12 figures