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Physical Review A

Optical control of a spin switch in the weak spin-orbit coupling limit

Ignacio R. Solá, Jesús González‐Vázquez, Vladimir S. Malinovsky

Published Oct 25, 2006• 16 citations
Publisher

Abstract

A method to optically control a dark transition, for instance, the coupling between different spin states, is proposed. The control is achieved by manipulating the direction, amplitude, and duration of dynamic Stark shifts. Laser-driven spin switches can be prepared by conveniently generalizing different optical techniques, such as $\ensuremath{\pi}$-pulse schemes and adiabatic passage schemes. The efficiency and robustness of the schemes is analyzed for both two-level and multilevel systems, implying quantum state selective wave packet transfer between states of different multiplicity.

DOI
10.1103/physreva.74.043418

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A method to optically control a dark transition, for instance, the coupling between different spin states, is proposed. The control is achieved by manipulating the direction, amplitude, and duration of dynamic Stark shifts. Laser-driven spin switches can be prepared by conveniently generalizing different optical techniques, such as \ensuremathπ\ensuremath{\pi}\ensuremathπ-pulse schemes and adiabatic passage schemes. The efficiency and robustness of the schemes is analyzed for both two-level and multilevel systems, implying quantum state selective wave packet transfer between states of different multiplicity.
Stark-chirped rapid adiabatic passage [T. Rickes et al., J. Chem. Phys. 113, 534 (2000); A. A. Rangelov et al. Phys. Rev. A 72, 053403 (2005)] has been proposed as a laser scheme that allows the optical excitation of atoms or molecules with high quantum yields. The laser control proceeds via adiabatic passage assisted by dynamic Stark shifts. We propose several extensions of the scheme with alternative sequences, by frequency tuning or by repeating the pulse sequences, in order to achieve fine state selectivity within multilevel structures and adiabatic passage of quantum superposition states. The efficiency and selectivity of the schemes is compared with that achieved by other optical coherent methods.
Based on the idea of reverse engineering, we design an optimal laser pulse to control strong-field multiphoton atomic transitions. Starting from the time-dependent Schr\"odinger equation of the full system, we adiabatically eliminate the nonessential states and apply the rotating-wave approximation to arrive at an effective two-state representation that involves dynamic Stark shifts and multiphoton coupling. Solving this equation inversely for the field, we obtain an analytical laser pulse shape that is expected to induce the full system's evolution according to user-defined quantum pathways. In our procedure, the amplitude and phase of the laser pulse are engineered such that the dynamically shifted electronic states are resonantly coupled during the action of the pulse at each moment of time. As a result, the driven system evolves from an arbitrary initial population distribution to any desired final quantum state superposition at a predefined rate. The proposed scheme is demonstrated using the example of the $3s\ensuremath{\rightarrow}4s$ two-photon transition of atomic sodium. By solving the time-dependent Schr\"odinger equation of the single-active electron with two different methods, either propagating time-dependent coefficients of many field-free states or directly propagating the three-dimensional electronic wave packet on a grid, we demonstrate the robustness as well as the limitations of the presented reverse engineering scheme.
Spin precession in rubidium atoms is investigated through a pump–probe technique. The excited wave packet corresponds to a precession of spin and orbital angular momentum around the total angular momentum. We show that using shaped laser pulses allows us to control this dynamics. With a Fourier transform limited pulse, the wave packet is initially prepared in the bright state (coupled to the initial state) whereas a pulse presenting a π step in the spectral phase prepares the wave packet in the dark state (uncoupled to the initial state).
By dynamic Stark shift using strong nonresonant pulses, we show that it is in principle possible to prepare arbitrary superposition states of mixed multiplicity. By a proper choice of parameters, the transfer of population is shown to follow the Rabi formula, where the initial and target states are now vibrational states of two light-induced molecular potentials of different multiplicity. Starting from nonstationary wave packets, the spin transfer can proceed via parallel transfer using a single pulse or by sequential transfer using a pulse sequence. A simple model is proposed to analyze the properties of both schemes and the feasibility of their experimental implementation for spin-orbit transitions in Rb2.
By controlling nonresonant dynamic Stark shifts it is possible to effectively decouple the intramolecular couplings of simple molecules. We have illustrated this effect in the 1 (1)Sigma(u)-->1 (3)Pi(u) transition in Rb(2). The laser scheme implies two important control knobs: the laser frequency, which must be chosen to avoid both single and multiphoton resonances and to select different electronic environments for the singlet and triplet states, and the pulse intensity, which must amplify the asymmetry in the dynamic polarizabilities that allows the decoupling, avoiding undesired strong-pulse effects. The mechanism of the scheme implies not only brute-force energy shifts but also light-induced reshaping of the potentials to avoid the undesired crossings. Quantitative aspects of the method are analyzed by using molecular models of increasing complexity for Rb(2).
The theory of Quantum Control is starting to lay bridges with the field of Quantum Information and Quantum Computation. Using key ideas of laser control of the dynamics by means of phase manipulation and adiabatic passage, we review laser schemes that allow entanglement preparation in a two-qubit system. The schemes are based on sequences that use four time-delayed pulses, with or without concerted decay, in or off resonance with the intermediate levels of the qubit space. We show how to control the fidelity and phase of the entanglement, as well as the sensitivity of the preparation to the different pulse parameters. In general the schemes provide an improvement in robustness and in the finesse of the control to phase, with respect to previously proposed schemes based on sequences of π\piπ pulses.