Perturbative evolution of far off-resonance driven two-level systems: Coherent population trapping, localization, and harmonic generation

The time evolution of driven two-level systems in the far off-resonance regime is studied analytically. We obtain a general first-order perturbative expression for the time-dependent density operator which is applicable regardless of the coupling strength value. In the strong field regime, our perturbative expansion remains valid even when the far off-resonance condition is not fulfilled. We find that, in the absence of dissipation, driven two-level systems exhibit coherent population trapping in a certain region of parameter space, a property which, in the particular case of a symmetric double-well potential, implies the well-known localization of the system in one of the two wells. Finally, we show how the high-order harmonic generation that this kind of systems display can be obtained as a straightforward application of our formulation.Comment: 14 pages, LaTeX, 2 figures, acknowledgments adde

Life Stories

Treball Final de Grau en Psicologia. Codi: PS1048. Curs: 2020/2021The main objective of the present study is to evaluate the characteristics of the vital development until the current age of the individual, based on the evolutionary stages of Erikson (2000), we hope that subject M will be in the eighth stage of integrity versus the hopelessness. To conduct the study, an exhaustive documentation was carried out by the evolutionary authors Erik Erikson (2000), Robert J. Havigurst (1972) and Paul Baltes (1987). After this, an interview was conducted that was previously made through the semi-structured interview of Triadó (Villar & Triadó, 2006). The interview consisted of eighty-two questions, although later some more were asked. To carry out the interview, a warm and comfortable place was sought for the subject. With everything ready, a tape recorder was turned on and information was collected that lasted two hours and thirty-eight minutes in the same session. With the information collected, a transcription and subsequent analysis was carried out. With the information collected and analysed, we can affirm that the objective has been achieved. We have been able to observe how the individual evolved through his narration and managed to solve each of the crossroads that Erikson (2000) raised in his evolutionary theory, therefore subject M is in the eighth stage in the positive vertex which refers to integrity

Decoherence reduction via continuous dynamical decoupling: Analytical study of the role of the noise spectrum

We analyze the robust character against non-static noise of clock transitions implemented via a method of continuous dynamical decoupling (CDD) in a hyperfine Zeeman multiplet in ^{87}\textrm{Rb}. The emergence of features specific to the quadratic corrections to the linear Zeeman effect is evaluated. Our analytical approach, which combines methods of stochastic analysis with time-dependent perturbation theory, allows tracing the decoherence process for generic noise sources. Working first with a basic CDD scheme, it is shown that the amplitude and frequency of the (driving) field of control can be appropriately chosen to force the non-static random input to have a (time-dependent) perturbative character. Moreover, in the dressed-state picture, the effect of noise is described in terms of an operative random variable whose properties, dependent on the driving field, can be analytically characterized. In this framework, the relevance of the spectral density of the fluctuations to the performance of the CDD technique is precisely assessed. In particular, the range of noise correlation times where the method of decoherence reduction is still efficient is identified. The results obtained in the basic CDD framework are extrapolated to concatenated schemes. The generality of our approach allows its applicability beyond the specific atomic system considered

Microcanonical versus Canonical Analysis of Protein Folding

The microcanonical analysis is shown to be a powerful tool to characterize the protein folding transition and to neatly distinguish between good and bad folders. An off-lattice model with parameter chosen to represent polymers of these two types is used to illustrate this approach. Both canonical and microcanonical ensembles are employed. The required calculations were performed using parallel tempering Monte Carlo simulations. The most revealing features of the folding transition are related to its first-order-like character, namely, the S-bend pattern in the caloric curve, which gives rise to negative microcanonical specific heats, and the bimodality of the energy distribution function at the transition temperatures. Models for a good folder are shown to be quite robust against perturbations in the interaction potential parameters.Comment: 4 pages, 4 figure