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arXiv:1701.03541 · Physical Review A

Robust two-level system control by a detuned and chirped laser pulse

Hanlae Jo, Han-gyeol Lee, S. Guérin, Jaewook Ahn

Published Sep 6, 2017• 12 citations
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Abstract

We propose and demonstrate a robust control scheme by an ultrafast nonadiabatic chirped laser pulse, designed for targeting coherent superpositions of two-level systems. Robustness against power fluctuation is proved by our numerical study and a proof-of-principle experiment performed with femtosecond laser interaction on cold atoms. They exhibit for the final driven dynamics a cusp on the Bloch sphere, corresponding to a zero curvature of fidelity. This solution is particularly simple and thus applicable to a wide range of potential applications.

DOI
10.1103/physreva.96.033403

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We propose and demonstrate a robust control scheme by an ultrafast nonadiabatic chirped laser pulse, designed for targeting coherent superpositions of two-level systems. Robustness against power fluctuation is proved by our numerical study and a proof-of-principle experiment performed with femtosecond laser interaction on cold atoms. They exhibit for the final driven dynamics a cusp on the Bloch sphere, corresponding to a zero curvature of fidelity. This solution is particularly simple and thus applicable to a wide range of potential applications.
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The excitation of a two-level system by a chirped laser pulse is analyzed using an analytical approach (in the perturbation-theory limit) that involves the modified rotating-wave approximation and the numerical solution to the Bloch equation. The dependence of the population of the upper energy level on the chirp is studied for various radiation intensities and pulse durations. An exact solution is compared with the result obtained using the modified rotating-wave approximation and the perturbation theory. It is demonstrated that, for a certain range of parameters, the excitation probability of a two-level system can be effectively controlled via a variation in the chirp.
We present a novel scheme for coherent manipulation of populations and robust creation of arbitrary coherent superposition of metastable states of a quantum system with lambda configuration of operating energy levels using laser pulses with a quadratically chirped carrier frequency. The case of a single broadband laser pulse is considered, when the frequency spectrum of the pulse exceeds the frequency distance between the metastable energy levels of the system. The results of the dressed state analysis and numerical simulation are presented, demonstrating reliable and robust creation of an arbitrary coherent superposition of metastable states by changing the parameters of laser pulses.
Quantum control of the four-wave mixing produced by two-color $\ensuremath{\omega}\text{\ensuremath{-}}3\ensuremath{\omega}$ femtosecond laser pulses propagating in a two-level system is accomplished by varying the relative phase between the two laser pulses. The phase sensitivity of the spectral $5\ensuremath{\omega}$ anti-Stokes component is cancelled when the ac Stark shift produced by the $\ensuremath{\omega}$ and $3\ensuremath{\omega}$ pulses is compensated. We apply perturbation theory and provide simple analytical formulas for the phase-sensitivity cancellation in the $5\ensuremath{\omega}$ anti-Stokes component. We demonstrate that this quantum interference effect is robust against chirping of the pulses and show that asymmetric chirping results in phase-sensitive shaping of the four-wave mixing spectra.
We present a simple approach allowing one to obtain analytical expressions for laser pulses that can drive a two-level system in an arbitrarily chosen way. The proposed scheme relates every desired population-evolution path to a single resonant laser pulse. It allows one to drive the system from any initial superposition of the two states to a final state having the desired distribution of the populations. We exemplify the scheme with a concrete example, where the system is driven from a nonstationary superposition of states to one of its eigenstates. We argue that the proposed approach may have interesting applications for designing pulses that can control ultrafast charge-migration processes in molecules. Although focused on laser-driven population control, the results obtained are general and could be applied for designing other types of control fields.