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The Journal of Physical Chemistry A

A New Control Scheme of Multilevel Quantum System Based on Effective Decomposition by Intense CW Lasers

Michihiko Sugawara, Masuichi Tamaki, Satoshi Yabushita

Published Aug 25, 2007• 6 citations
Publisher

Abstract

We propose a new scheme for quantum dynamics control of multilevel system using intense lasers. To do so, we apply intense CW lasers to create a strongly coupled subsystem with which one can make the complementary space effectively isolated, and we apply the established control schemes to the isolated subsystem. We have also obtained an effective Hamiltonian for the target subsystem with the help of the second-order perturbation theory. Numerical demonstrations on model systems show that the present decomposition scheme effectively works for population dynamics control. It is also found that relaxation processes can be suppressed under the proposed scheme.

DOI
10.1021/jp073268m

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We propose a new scheme for quantum dynamics control of multilevel system using intense lasers. To do so, we apply intense CW lasers to create a strongly coupled subsystem with which one can make the complementary space effectively isolated, and we apply the established control schemes to the isolated subsystem. We have also obtained an effective Hamiltonian for the target subsystem with the help of the second-order perturbation theory. Numerical demonstrations on model systems show that the present decomposition scheme effectively works for population dynamics control. It is also found that relaxation processes can be suppressed under the proposed scheme.
A new quantum control scheme for general multilevel systems using intense laser fields is proposed. In the present scheme, the target subspace consisting of several quantum levels is effectively isolated by applying intense cw lasers with specific conditions. The formulation is carried out using the Green function with the help of projection operator method. Dynamics of the isolated target subspace is governed by an effective Hamiltonian. The developed scheme is applied to the quantum control of dissipative four- and five-level systems. It is clarified that the present method makes it possible not only to manipulate the coherent population dynamics but also to suppress the dissipative dynamics.
A quantum control scheme for multilevel systems utilizing effective decomposition is proposed. In this method, a subsystem chosen for control target is effectively isolated by irradiating intense continuous-wave laser fields onto the complimentary space. By choosing the control target space consisting of small number of eigenstates, effective dressed states for the isolated space are analytically given, which makes it possible to express the performance index defined in the target space as an explicit function of laser parameters. Then, designing the control field can be treated as a static parameter optimization. The present scheme makes it possible not only to optimize the laser parameters but also to investigate the parameter search space, which clarifies the physics of the control process.
Coherent population trapping effect is widely studied for simple three-level systems of different types ((Xi) , (Lambda) , V). For real atoms situation is more complicated. Atomic levels are degenerated one and in the presence of external fields coupling of magnetic sublevels appears due to as radiation, so magnetic fields, leading to the systems of levels of the higher order. Our theory and model of calculation for interaction of multilevel atom with intense radiation field at the presence of external magnetic field have been applied for considering coherent population trapping effect for such systems.
In this thesis, we present work on coherent control of multilevelquantum systems in the strong field limit using shaped ultrafastlaser pulses. In recent years there have been numerous multiphotonabsorption experiments in two,three, and four-level atomic/molecularsystems and many are performed in the limit of weak fields whereperturbation theory is valid. Here, we describe a series ofexperiments aimed at exploring and understanding multiphotontransitions when the exciting field is strong and perturbationtheory breaks down. Our approach to strong field control utilizesboth parameterized scans of various pulse shapes and closed-looplearning control to identify a pulse shape that is optimal forpopulating a target quantum state. With this we will highlight thedifference between sequential population transfer and adiabaticrapid passage in multilevel systems with multiphoton couplingbetween levels. Additionally, we examine strong field control of afour-level atomic interferometer and show how interference in atarget state changes from resonant pathways in the frequency domainto time-domain interference via a singe path.Further, we use shaped femtosecond pulses to demonstrate aphenomenon in which a three-level atom becomes a modulator of anultrafast pulse. The results are based on a pump-probe scheme thatis very similar to Electromagnetically Induced Transparency (EIT).Important dynamics associated with a time-dependent coupling fieldare examined. Lastly, we extend previous work on two-photon drivensuperfluorescence from a shaped ultrafast drive laser and show howstimulated emission near threshold can turn modest coherent controlyields into essentially perfect discrimination between systems wherea control factor of about 104 is achieved between atomic andmolecular species.
The use of laser pulses to alter the internal quantum structure of individual atoms and molecules has applications in quantum information processing, the coherent control of chemical reactions and in quantum-state engineering. This book presents the underlying theory of such quantum-state manipulation for researchers and graduate students. The book provides the equations, and approaches for their solution, which can be applied to complicated multilevel quantum systems. It also gives the background theory for application to isolated atoms or trapped ions, simple molecules and atoms embedded in solids. Particular attention is given to the ways in which quantum changes can be displayed graphically to help readers understand how quantum changes can be controlled.
The interaction of high power laser radiation with the atoms in the external magnetic field leads to the new nonlinear effects due to modifications of populations of the states of the system 'atom plus field.' From one side it can be followed by new peculiarities in the effect of the coherent population trapping and from other -- the effects of the population modifications are important for explanation of regularities of nonlinear magneto-optical effects. The modification of population dynamics due to the parameters of the system -- radiation field intensities, detunings, polarizations, magnetic field strength and orientation, relaxation constants leads to the new features in polarization effects, resonant fluorescence and resonant Raman effects. The new features of the considered theory are that it is built for multilevel atoms and has not restrictions on the probe intensity compared to the pump intensity. The theory and method permit to investigate the populations modifications depending on modifications of the parameters of the system and to find the relation between population dynamics and new nonlinear resonant magneto-optical effects. The considered system consists of atomic gas of low concentration in the constant magnetic field and two monochromatic laser fields of different intensities and polarizations. Analytically the influence of magnetic field on population dynamics of three-level systems have been considered. For real multilevel atom the computer simulations show the dependence of population dynamics on relaxation constants and the differences of this dynamics for different magnetic sublevels of the excited states of the atom. The results agree with known theoretical and experimental works.