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Physical Review A

Non-Markovian analysis of coherence in a driven two-level atom

J. R. Brinati, Salomon Sylvain Mizrahi, Gilberto Aparecido Prataviera

Published Oct 1, 1994• 17 citations
Publisher

Abstract

Recent femtosecond processes [Phys. Rev. Lett. 66, 2464 (1991)] confirm the importance of non-Markovian effects since the dynamical characteristic times of the system become of the same time scale as the reservoir. We analyze here the memory effects in the density-matrix coherence for a driven two-level atom in the stationary regime. The coherence parameter \ensuremath{\eta} is written in terms of the detuning, the strength of the external driving force, and the temperature as the reservoir. We verify that the memory effects, present in the density matrix, are enhanced by the external driving force. We introduce the Shanon information entropy and we verify that, while in the Markovian approximation the entropy decreases monotonically as function of the detuning, in the non-Markovian approach it is characterized by a minimum, which enables us to differentiate between the two processes.

DOI
10.1103/physreva.50.3304

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Recent femtosecond processes [Phys. Rev. Lett. 66, 2464 (1991)] confirm the importance of non-Markovian effects since the dynamical characteristic times of the system become of the same time scale as the reservoir. We analyze here the memory effects in the density-matrix coherence for a driven two-level atom in the stationary regime. The coherence parameter \ensuremath{\eta} is written in terms of the detuning, the strength of the external driving force, and the temperature as the reservoir. We verify that the memory effects, present in the density matrix, are enhanced by the external driving force. We introduce the Shanon information entropy and we verify that, while in the Markovian approximation the entropy decreases monotonically as function of the detuning, in the non-Markovian approach it is characterized by a minimum, which enables us to differentiate between the two processes.
The absorptive part of the susceptibility of a system constituted by a strongly driven two-level atom (interacting with a reservoir) is analyzed when probed by a weak field. Memory effects are taken into account (the correlation time of the reservoir variables is finite) and we verify that they lead to several distinct features for the absorption and stimulated emission of radiation, when compared with those obtained in the Markovian approximation. Moreover, we verify that memory effects lead to a signature, namely, the sensitive dependence of the maxima of the absorption lines on the Rabi frequency at off-resonance pumping. Other characteristics of the spectrum are enhanced, even in the presence of a small but non-null correlation time of the reservoir, for instance, the maxima of the dispersionlike absorption curves at on-resonance pumping. In this work several situations are analyzed for different values of the parameters of the problem: the off-resonance frequencies of the driving field, the Rabi frequency, the memory time, and temperature of the reservoir.
We examine the impact of the non-Markovian environment on the dynamical behavior of the quantum coherence and entropy squeezing considering a two-level atomic system (qubit) immersed in a reservoir with zero-temperature for two types of non-Markovian environments. We consider a cavity little off-resonance with the transition frequency of the qubit and the case of a non-perfect photonic band gap (PBG). We show that the amount of coherence is dependent on the structure of the environment and influenced through the memory effects. We obtain that the delay and revival of the coherence loss might take place by controlling the detuning of the cavity-qubit system. Whereas, a partial coherence trapping occurs in non-ideal PBG and the decrease of the gap width will destroy the coherence. On the other hand, we show the situation for which the squeezing is occurring and enhanced with respect to the main parameters for the system. Finally, we display the monotonic dependence of the quantum coherence and squeezing on the main model parameters.
By considering an exactly solvable model for a driven two non-interacting qubits, each coupled to a bosonic environment with zero temperature, under the non-Markovian dissipative process, we study the variation of coherence and correlations in terms of different physical parameters. We show the influence of the external classical driving field as well as the initial quantum states. Moreover, we highlight the relationship between the coherence, single-qubit population, and correlations according to the physical parameters of the whole system. We reveal that the preservation and enhancement of coherence and correlations may occur by adjusting the strength of the classical driving field, initial states, and non-Markovian effects.
The description of a non-Markovian process is presented and the asymptotic solution of the generalized master equation is obtained for a two-level atom pumped externally by a monochromatic time-dependent electric field. We give the explicit expression for the absorption line-shape function and discuss its features as a function of the temperature and the memory parameter. We verified the following interesting results: (a) At a critical temperature the line shape undergoes a phase transition, the single-bump profile splits into two bumps; (b) for small values of the memory parameter k the linewidths broaden, becoming larger than the ones calculated under the Markovian approximation, and after reaching some maximum value they narrow monotonically with increasing values of k, this behavior being standard for any finite temperature.
We study non-Markovian dynamics of a two level atom using pseudomode method. Because of the memory effect of non-Markovian dynamics, the atom receives back information and excited energy from the reservoir at a later time, which causes more complicated behaviors than Markovian dynamics. With pseudomode method, non-Markovian dynamics of the atom can be mapped into Markovian dynamics of the atom and pseudomode. We show that by using pseudomode method and quantum jump approach for Markovian dynamics, we get a physically intuitive insight into the memory effect of non-Markovian dynamics. It suggests a simple physical meaning of the memory time of a non-Markovian reservoir.
We consider two recently proposed measures of non-Markovianity applied to a particular quantum process describing the dynamics of a driven qubit in a structured reservoir. The motivation for this study is twofold: on one hand, we study the differences and analogies of the non-Markovianity measures, and on the other hand, we investigate the effect of the driving force on the dissipative dynamics of the qubit. In particular we ask if the driving force introduces new channels for energy and/or information transfer between the system and the environment or if it amplifies existing ones. We show under which conditions the presence of the driving force slows down the inevitable loss of quantum properties of the qubit.