Quantum interactions in a stochastic representation and two-level systems

The interaction between two quantum systems is formulated using a stochastic representation that allows one of them to be replaced by equivalent commutative random sources. The proposed method is applied to two-level systems in contact with a thermal bath. Strong-coupling effects and long-lived fluctuations of the total response of two systems in a common thermal bath are discussed.     

Effect of nuclear quadrupole interactions on the dynamics of two-level systems in glasses

The standard tunneling model describes quite satisfactorily the properties of amorphous solids at temperatures T < 1K in terms of an ensemble of two-level systems including the logarithmic temperature dependence of the dielectric constant. Yet, experiments have shown that at ultralow temperatures T< 5 mK such a temperature behavior breaks down and the dielectric constant becomes temperature independent (plateau effect). In this Letter we suggest an explanation of this behavior exploiting the effect of the nuclear quadrupole interaction on tunneling. We also predict that the application of a sufficiently large magnetic field B> 10T should restore the logarithmic dependence because of the suppression of the nuclear quadrupole interaction.     

Coherent interactions of terahertz strain solitons and electronic two-level systems in photoexcited ruby

We observe coherent interactions between an ultrashort, longitudinal acoustic soliton train and the 29-cm(-1) electronic transition in photoexcited ruby. Propagation of the strain pulses over millimeter distance through an excited zone reveals striking behavior of the induced electronic population, which has been explained by impulsive excitation of the two-level systems, combined with the nonlinear properties of the solitons in the resonant medium. This opens up new possibilities for coherent manipulation of ultrashort acoustic pulses by local electronic centers.     

Solitons in molecular systems with nonlinear nearest-neighbour interactions

The soliton theory of the motion of an extra electron (intramolecular excitation) in one-dimensional molecular structures with a strong electron (exciton)-phonon coupling is developed for a wide class of nonlinear (realistic) nearest-neighbour interactions which provide finite values of the soliton dynamical quantities at velocities less than or equal to that of sound.     

Transient effects of three photon pulses in resonant two-level atom systems

Using statistical quantum electrodynamics techniques, we compute the polarization of a two-level atom system following three pulses. Homogeneous and inhomogeneous broadening and pulse width are incorporated in the formalism. Phase conjugation by three-photon process is investigated.     

Supersymmetry of two-level systems

Physics Institute, Russian Academy of Sciences. Presented at the International Workshop Squeezing, Groups, and Quantum Mechanics, Baku, Azerbaijan, September 16–22, 1991.     

Resonant nonlinear intrachannel interactions in strongly dispersion-managed transmission systems

Nonlinear intrachannel interactions in a transmission system with strong periodic dispersion management are studied. The ghost pulse at the zero bit is shown to grow resonantly as a result of periodic forcing and temporal phase matching with the signal pulses. The growth rate depends on the degree of overlap between the signals. The analysis agrees with direct numerical simulation of the full system. Growth rates for various bit patterns as a function of map strength are obtained.     

Simulation of two-level systems

A macroscopic model of a two-level system that contains only classical components but reproduces quantum states is constructed using numerical methods. A computer analog of the measuring procedure is developed. The analysis of the single-particle state distribution makes it possible to compare the results with quantum mechanical results. In the modeling of a two-particle singlet state, the probability and correlation functions distributions and restrictions on the Bell??s parameters are obtained. The results agree with the quantum-mechanical results.     

非线性两能级系统中的Landau-Zener-Coulomb跃迁

非线性跃迁动力学的研究是量子操控中的一个重要课题,在原子分子光物理、量子信息以及固体物理等领域都有着重要的应用.本文研究了具有粒子间相互作用两能级系统中的Landau-Zener-Coulomb跃迁动力学.结果表明,对于能级的斜率正负不同的情况,粒子间相互作用对系统跃迁动力学的影响完全不同.为正时,粒子间相互作用总是抑制能级间的非绝热跃迁,且相互作用强度越大,两能级间的非绝热跃迁越难发生.而为负时,弱相互作用会促进能级间的非绝热跃迁,在强相互作用情况下,跃迁概率会出现振荡.随着相互作用强度的增大,振荡幅度逐渐减小,能级间的非绝热跃迁受到抑制.     

Relaxation in two-level long-range-memory systems

Recently, new rigorous results concerning integrals of exact memory functions in the time-convolution Generalized Master Equations (GME) for state occupation probabilities, governing relaxation of open quantum systems, have been obtained. They include that a) time integrals of exact memories and b) memories w _ij(t) have tails unobtainable by perturbational arguments which cause that does not exist or is infinite. For a two-level system, a simple model for such memories is considered and solved. It is concluded that GME may yield that with increasing time, the system unphysically more and more deviates from equilibrium, indicating thus instability of the equilibrium distribution. Thus, in contrast to, e.g., the famous Boltzmann equation, the mathematical structure of GME alone does not guarantee the stability of the equilibrium state.     

Phase memory effects in probe-field spectroscopy of two-level systems at low collision frequencies

The absorption (amplification) spectrum of a weak probe field by two-level atoms located in a strong resonant laser field and colliding with buffer-gas atoms is analyzed theoretically. The analysis is performed for low collision frequencies compared to the Doppler absorption linewidth (low gas pressure) and with allowance made for an arbitrary change in the phase of the radiation-induced dipole moment at elastic collisions between gas particles. The phase memory effects have been found to lead to a strong qualitative and quantitative transformation of the probe-field spectrum even at rare collisions, when the well-known Dicke manifestation mechanism of the phase memory effects (the removal of Doppler broadening due to the restriction of the spatial particle motion by collisions) is inoperative. The strong influence of the phase memory effects on the spectral resonances at low gas pressures stems from the fact that the phase-conserving collisions change the velocity dependence of the partial refractive index n(v) (the refractive index for particles moving with velocity v).     

Reservoir effects in two-level models

By use of an effective Hamiltonian method in thermo field dynamics, nonequilibrium phenomena in two-level systems coupled with a reservoir are studied. It is explicitly demonstrated that the system approaches a canonical equilibrium state specified by the temperature of the reservoir and by the Lagrangian multipliers corresponding to the conservation laws preserved in the interaction between the reservoir and the system.     

Berry phase in a generalized nonlinear two-level system

In this paper, we investigate the behaviour of the geometric phase of a more generalized nonlinear system composed of an effective two-level system interacting with a single-mode quantized cavity field. Both the field nonlinearity and the atom--field coupling nonlinearity are considered. We find that the geometric phase depends on whether the index $k$ is an odd number or an even number in the resonant case. In addition, we also find that the geometric phase may be easily observed when the field nonlinearity is not considered. The fractional statistical phenomenon appears in this system if the strong nonlinear atom--field coupling is considered. We have also investigated the geometric phase of an effective two-level system interacting with a two-mode quantized cavity field.     

Berry phase in a generalized nonlinear two-level system

In this paper, we investigate the behaviour of the geometric phase of a more generalized nonlinear system composed of an effective two-level system interacting with a single-mode quantized cavity field. Both the field nonlinearity and the atom-field coupling nonlinearity are considered. We find that the geometric phase depends on whether the index k is an odd number or an even number in the resonant case. In addition, we also find that the geometric phase may be easily observed when the field nonlinearity is not considered. The fractional statistical phenomenon appears in this system if the strong nonlinear atom-field coupling is considered. We have also investigated the geometric phase of an effective two-level system interacting with a two-mode quantized cavity field.     

Possible Minkowskian language in two-level systems

One hundred years ago, in 1908, Hermann Minkowski completed his proof that Maxwell’s equations are covariant under Lorentz transformations. During this process, he introduced a four-dimensional space called the Minkowskian space. In 1949, P.A.M. Dirac showed the Minkowskian space can be handled with the light-cone coordinate system with squeeze transformations. While the squeeze is one of the fundamental mathematical operations in optical sciences, it could serve useful purposes in two-level systems. Some possibilities are considered in this report. It is shown possible to cross the light-cone boundary in optical and two-level systems while it is not possible in Einstein’s theory of relativity.