Dynamics of a quantum two-level system under the action of phase-diffusion field

We study a behavior of quantum two-level system, interacting with noisy phase-diffusion field. The dynamics is shown to split into two regimes, determined by the coherence time of the phase-diffusion field. For both regimes we present a model of quantum system behavior and discuss possible applications of the obtained effect for spectroscopy. In particular, the obtained analytical formula for the macroscopic polarization demonstrates that the phase-diffusion field does not affect the absorption line shape, which opens up an intriguing possibility of noisy spectroscopy, based on broadband sources with Lorentzian line shape.     

Exact results on cavity cooling in a system of a two-level atom and a cavity field

Using the algebraic dynamical method, this paper investigates the laser cooling of a moving two-level atom coupled to a cavity field. Analytical solutions of optical forces and the cooling temperatures are obtained. Considering Rb atoms as an example, it finds that the numerical results are relevant to the recent experimental laser cooling investigations.     

Exact solutions for a quantum hard-core system

Exact closed expressions have been obtained for the first three partial waves of the two-particle density matrix for a hard-core pairwise interaction.     

Symmetry theory in a two-level quantum system

We develop the theory of symmetry for a two-level quantum system in oder to illustrate the main ideas of the general theory of symmetry in quantum theory. It is based on the diffeomorphism of the two-dimensional sphere S ² onto the space of states P ¹ and the isomorphism between the groups P(2) and SO ₃ (). In particular, rotational invariance leads to the appearance of the spin1/2 in a natural way.     

Linear feedforward control of two-level quantum system by modulated external field

We propose a model of feedforward (open-loop) optical control of two-level atom in the linear form. This model allows to express the general form of solution for the atomic level populations via the arbitrary shapes of the control signal. Then we make numerical investigations of different shapes for the optical control signal.     

The ac-Stark effect in coherent pulse propagation for a two-level system

When the Maxwell-Bloch equations contain an ac-Stark term, these equations can be transformed by a very simple transformation to Maxwell-Bloch equations without the ac-Stark term. Thus, by using the method of Ablowitz, Kaup, and Newell, the ac-Stark effect in coherent pulse propagation can also be exactly solved.     

Excitation of a two-level system by a chirped laser pulse

The excitation of a two-level system by a chirped laser pulse is analyzed using an analytical approach (in the perturbation-theory limit) that involves the modified rotating-wave approximation and the numerical solution to the Bloch equation. The dependence of the population of the upper energy level on the chirp is studied for various radiation intensities and pulse durations. An exact solution is compared with the result obtained using the modified rotating-wave approximation and the perturbation theory. It is demonstrated that, for a certain range of parameters, the excitation probability of a two-level system can be effectively controlled via a variation in the chirp.     

Functional quantum statistics of light propagation in a two-level system

The functional Fokker-Planck formalism developed in a preceding paper is applied to the problem of a radiation field propagating in a medium, which contains resonant two-level atoms. Besides the electromagnetic field also the medium is described by continuous space dependent fields. We give the masterequation and transform it into ac-number functional differential equation for a characteristic functional. This equation is reduced considerably by the projection onto one dimension and the introduction of the diffusion approximation. It forms a solid basis for the study of all types of light propagation in resonant media including classical and quantum noise. We give an approximate solution of this equation by considering the problem of an externally pumped optical transmission line, in the case that saturation effects are absent. The spectral function of the electric field strength is obtained which describes a statistical mixture of photons with the quasiparticles of the polarization field. It shows the onset of a condensation of the quasiparticles into a single state. Self excitation of the transmission line is obtained at a certain threshold of the atomic inversion. This threshold is characterized by a finite occupation number of one single quasiparticle state. The influence of a finite length of the transmission line is briefly considered.     

A three-process quantum engine cycle consisting of a two-level system

Based on the thermodynamic properties of isoenergetic,adiabatic and isothermal quantum processes,it is shown that it is possible to combine the three processes into a quantum engine cycle.The efficiency of the three-process cycle can be derived and is dependent on the highest and lowest temperatures.The efficiency in some operation regions does not demonstrate a monotonically increasing function of the temperature difference.When the highest temperature of the cycle is larger than the critical temperature,which can be determined by the characteristics of the three-process cycle,a unique region where the efficiency decreased with the increase of the temperature difference exists.     

Exact analytical solution for quantum spins mixing in spin-1 Bose--Einstein condensates

This paper solves exactly a set of fully quantized coupled equations describing the quantum dynamics of quantum spins mixing in spin-1 Bose-Einstein condensates by deriving the exact explicit analytical expressions for the evolution of creation and annihilation operators.     

强热辐射环境中两能级原子量子态保真度

两个两能级原子置于强热辐射场环境中，原子用泡利算符描述，环境用无穷的谐振子热库描述，运用密度矩阵方法，得到两能级原子密度矩阵元演化规律.针对三种不同的初始状态，分析置于热辐射场中原子量子态保真度.结果表明：当两个原子初始处于不同量子叠加态时，量子信息在传输过程中可能发生部分失真，也可能不失真. 关键词： 热辐射场 两能级原子 保真度 消相干     

Exact analytical expression for magnetic field induction of rectangular shape sample

An analytical expression for the magnetic field inside and outside of a magnetized rectangular-shaped sample without unphysical singularities on its boundaries is derived.     

Electromagnetically induced transparency and evolution of a two-level system under chaotic field of arbitrary intensity

The electromagnetically induced transparency (EIT) with a (near-)resonant chaotic (amplitude-phase fluctuating, Gaussian-Markovian) coupling field is studied theoretically. The Fourier transform of the steady-state EIT spectrum, which determines a nonstationary probe absorption, is also considered. This quantity equals the average diagonal element of the (reduced) evolution operator of the coupled transition (the evolution function). The exact solution in the form of a continued fraction is obtained and used to perform numerical calculations. Moreover, a number of approximate analytical results are obtained, which, together with the results of previous publications, describe the EIT and the evolution function in all possible regimes. In particular, in the constructive-interference case the EIT increases with the coupling-field bandwidth ν at sufficiently small ν. For a strong field, the maximum of the transparency as a function of ν is less than that for a monochromatic field of the same average intensity. In contrast, for a weak field, there is a range of ν values, where the field fluctuations do not affect the EIT. The latter result is shown to hold for a broad class of stochastic fields. Received 31 December 2000 and Received in final form 14 May 2001     

Exact and approximate results of non-extensive quantum statistics

We develop an analytical technique to derive explicit forms of thermodynamical quantities within the asymptotic approach to non-extensive quantum distribution functions. Using it, we find an expression for the number of particles in a boson system which we compare with other approximate scheme (i.e. factorization approach), and with the recently obtained exact result. To do this, we investigate the predictions on Bose-Einstein condensation and the blackbody radiation. We find that both approximation techniques give results similar to (up to ) the exact ones, making them a useful tool for computations. Because of the simplicity of the factorization approach formulae, it appears that this is the easiest way to handle with physical systems which might exhibit slight deviations from extensivity. Received 19 August 1999 and Received in final form 1 November 1999