Phase-controlled all-optical switching based on coherent population oscillation in a two-level system

We theoretically propose a scheme of phase-controlled all-optical switching due to the effect of degenerate four-wave mixing (FWM) and coherent population oscillation (CPO) in a two-level system driven by a strong coupling field and two weak symmetrically detuned fields. The results show that the phase of the FWM field can be utilized to switch between constructive and destructive interference, which can lead to the transmission or attenuation of the probe field and thus switch the field on or off. We also find the intensity of the coupling field and the propagation distance have great influence on the performance of the switching. In our scheme, due to the quick response in semiconductor systems, a fast all-optical switching can be realized at low light level.     

Nonlinearly induced relaxation to the ground state in a two-level system

We report the experimental demonstration of a nonlinear process in a two-level system, in which the amplitude of the excited state decays, transferring irreversibly a large fraction of its energy to the ground state, while shedding a part of it into radiation states. The experiments where preformed in a nonlinear optical waveguide, supporting two or three modes. The process is general, and is expected to occur in other nonlinear few level systems such as nonlinear quantum wells and Bose-Einstein condensates.     

Three-body entanglement induced by spontaneous emission in a three two-level atoms system

We study three-body entanglement induced by spontaneous emission in a three two-level atoms system by using the entanglement tensor approach. The results show that the amount of entanglement is strongly dependent on the initial state of the system and the species of atoms. The three-body entanglement is the result of the coherent superposition of the two-body entanglements. The larger the two-body entanglement is, the stronger the three-body entanglement is. On the other hand, if there exists a great difference in three two-body entanglement measures, the three-body entanglement is very weak. We also find that the maximum of the two-body entanglement obtained with nonidentical atoms is greater than that obtained with identical atoms via adjusting the difference in atomic frequency.     

Multiple induced transparency in a hybrid driven cavity optomechanical device with a two-level system

We present a scheme with the multiple-induced transparency windows in a hybrid optomechanical device. By studying the transmission of a probe field through the hybrid device, we show the successive generations of three transparent windows induced by multiple factors including tunneling, optomechanical and qubit-phonon coupling interactions, and analyze the physical mechanism of the induced transparency based on a simplified energy-level diagram of the system. Moreover, the effects of the transition frequency and decay rate of the two-level system on the multiple-induced transparency windows are discussed. We find that the transparency windows can be modulated by the coupling interaction between the qubit and NMR, the decay of qubit and the power of the control field. Therefore, the transmission of the probe field can be coherently adjusted in the hybrid cavity optomechanical device with a two-level system.     

Radiation induced depinning of a charge density wave system

The frequency-dependent response of a pinned charge density wave is considered in terms of forced vibration of an oscillator held in an anharmonic well. It is shown that the effective pinning-frequency can be reduced by applying a d.c. field. If a strong a.c. field, superposed on a d.c. field is applied on such a system “jumps” can be observed in the frequency dependent response of the system. The conditions at which these “jumps” occur are investigated with reference to NbSe₃. The possibility of observing such phenomena in other systems like superionic conductors, non-linear dielectrics like ferroelectrics is pointed out. The characteristics are expressed in terms of some “scaled variables” — in terms of which the characteristics show a universal behaviour.     

Relaxation of a single two-level system

A relaxation problem for a single two-level system coupled to a Bose field is investigated. The stationary state and relaxation time of the system is analysed. The correction to the unperturbed stationary state is derived in the fourth order of the interaction between the system and bosons. The criteria for an application of the perturbation calculation are discussed.Supported in part by the Polish Academy of Sciences under Contract No. MR I-9.The author would like to express his sincere thanks to the members of the Department of Theoretical Physics at the Silesian University for their comments and discussions.     

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.     

Double-quantum nutations in a two-level spin system

We report experimental results of the oscillatory response of a two-level spin system (E'₁ centres in glassy silica) excited by a step-modulated radiation tuned to the double-quantum (DQ) resonance. On the basis of a semiclassical calculation of the transient regime, the observed oscillations are ascribed to the nutations of the DQ transitions.     

Free-induction decay in a fluctuating two-level system

We study theoretically free-induction decay (FID) in an inhomogeneously broadened system of two-level atoms whose transition frequencies fluctuate according to the Ornstein-Uhlenbeck process. A numerical solution of the model shows that at low intensities the saturation behaviour of FID may depend strongly on the statistics of the fluctuations, but at high intensities the fluctuations play a minor role. The results mimic qualitatively the recent experimental data of DeVoe and Brewer, but a full quantitative agrrement with experiments is not reached.     

Decoherence in a system of many two-level atoms

I show that the decoherence in a system of degenerate two-level atoms interacting with a bosonic heat bath is for any number of atoms governed by a generalized Hamming distance (called "decoherence metric") between the superposed quantum states, with a time-dependent metric tensor that is specific for the heat bath. The decoherence metric allows for the complete characterization of the decoherence of all possible superpositions of many-particle states, and can be applied to minimize the overall decoherence in a quantum memory. For qubits which are far apart, the decoherence is given by a function describing single-qubit decoherence times the standard Hamming distance. I apply the theory to cold atoms in an optical lattice interacting with blackbody radiation.     

Strong field effects in the evolution of a two-level system

Evolution of a two-level system (TLS) in a strong resonant quantum field is considered beyond the rotating wave approximation (RWA). An analytical expression for the atomic population inversion that approximates numerical results with high accuracy is deduced. It allows us to describe new qualitative peculiarities of the TLS evolution in the strong field: i) suppression of the ‘collapse-revival’ effect, and ii) change of the spectrum and form of the population oscillations.     

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     

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.     

Absorption spectrum of a two-level system in a squeezed vacuum

The weak probe-field absorption spectrum of a two level atom driven by a coherent pump field in a squeezed vacuum is calculated. When the squeezed light and coherent driving field are tuned to atomic resonance, the absorption spectrum of the probe field can show a linewidth smaller than the linewidth in normal vacuum.     

Meissner screening by induced weak charge in the dense Weinberg-Salam system

The magnetic screening in cold electroweak matter at high density of leptons J_LM_W³/6π² is investigated within the approximation known to guarantee the Bose-Einstein condensation of W^±-mesons. It is shown that the photon in this medium acquires a magnetic mass, the electro-magnetic field becoming short-range. It is found that the Meissner damping of a magnetic field is provided in this approximation first of all by an induced polarization charge of weak nature.     

Subnatural linewidth in a strongly-driven degenerate two-level system

We observe linewidths below the natural linewidth for a probe laser on a degenerate two-level F → F′ transition, when the same transition is driven by a strong control laser. We take advantage of the fact that each level of the transition is made of multiple magnetic sublevels, and use the phenomenon of electromagnetically induced transparency (EIT) or absorption (EIA) in multilevel systems. Optical pumping by the control laser redistributes the population so that only a few sublevels contribute to the probe absorption, an explanation which is verified by a density-matrix analysis of the relevant sublevels. We observe more than a factor of 3 reduction in linewidth in the D₂ line of Rb in room-temperature vapor. Such subnatural features vastly increase the scope of applications of EIT, such as high-resolution spectroscopy and tighter locking of lasers to atomic transitions, since it is not always possible to find a suitable third level.